assuming the full concrete contribution) or FC limit assumption (i.e. assuming .... in c.a. alleggeriti gettati in operaâ, Atti del XIV Congresso CTE, Mantova 7-9, ...
SEVENTH FRAMEWORK PROGRAMME Capacities Specific Programme Research Infrastructures Project No.: 227887
SERIES SEISMIC ENGINEERING RESEARCH INFRASTRUCTURES FOR EUROPEAN SYNERGIES
Work package [WP7 - TA3 Eucentre]
SESYCOWA Seismic behaviour of structural systems composed of cast in situ concrete walls ‐ Final Report ‐
User Group Leader: Prof. S. Ivorra Revision: Final
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ABSTRACT The present document reports the outcomes of an experimental test campaign for the assessment of the seismic behaviour of a full-scale 3-storey structural system composed of cast-in-situ squat sandwich concrete walls. The structural system, which has been developed by the Italian company Nidyon Costruzioni S.p.a. and referred to as Nidyon NYSP, is based on the production and use of prefabricated modular pre-reinforced polystyrene panels (which therefore will be simply referred as modular panels) which act as support for the placing of the structural concrete (typically shotcrete). Modular panels are assembled at the site in order to obtain the so called support wall. Once the support walls are set in place, two layers of concrete (each one of about 40 mm in thickness) are sprayed on each side to obtain the r/c wall. Nidyon NYSP is characterized by limited costs, fast construction, high thermal and acoustic performance and is typically used for the construction of mid-rise residential building. The final structural solution is the result of a comprehensive research work ,started at the beginning of ‘90s, carried out by the University of Bologna aimed at characterizing both the monotonic in plane and out of plane static behaviour and the in-plane cyclic behaviour of single walls (with and without opening) and coupled walls. The results of the work lead to the formulation of analytical relationship (validated through pseudo-static cyclic tests) for the prediction of the seismic behaviour of the system. In order to verify the actual seismic behaviour of the above structural system shacking table tests on a full-scale prototype building, representative of a mid-rise residential building, appears the most meaningful test typology. It should be noted that, while the description of the construction system and the results of the experimental tests are specifically referred to the products of Nidyon Costruzioni S.p.a. (Rimini,
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Italy); nonetheless, the observations and the conclusions may be extended to structural systems having similar characteristics.
Keywords: Sandwich Panel, mid‐rise building, Experimental Analysis, Shaking Table Tests
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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Community’s Seventh Framework Programme [FP7/2007-2013] for access to EUCENTRE Foundation under grant agreement n° 227887 [SERIES].
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REPORT CONTRIBUTORS Universidad de Alicante Salvador Ivorra Belén Ferrer Politecnico di Bari Dora Foti Università di Bologna Tomaso Trombetti Stefano Silvestri Giada Gasparini Ilaria Ricci Alice Catalini Universitatea Technica din Cluj-Napoca Cristina Mihaela Campian Nidyon Costruzioni S.p.a. Daniele Malavolta
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CONTENTS CONTENTS.................................................................................................................................. vii List of Figures ................................................................................................................................ xi List of Tables ............................................................................................................................... xiii 1
The SeSyCoWa Project ...........................................................................................................1 1.1. Background .....................................................................................................................1 1.2. General objectives ..........................................................................................................2 1.3. Specific objectives ..........................................................................................................3
2
The structural system ...............................................................................................................4 2.1. The modular panel ..........................................................................................................4 2.2. The cast in situ sandwich concrete wall .........................................................................5 2.3. The connections between walls and foundations ...........................................................6 2.4. The connections between orthogonal walls ....................................................................7 2.5. The connections between walls and floors .....................................................................7 2.6. The main featureS and capabilities of the structural system ..........................................8
3
The rationale behind the prototype building .........................................................................10 3.1. Main features and facility constraints ...........................................................................10 3.2. The theoretical seismic capacity and the expexted (predicted) mechanisms of failure11 3.2.1. The theoretical seismic capacity ......................................................................11 Parallel wall – In plane first yielding bending strength .................................................... 12 Parallel wall – In plane ultimate bending strength............................................................ 13 Parallel wall – In plane shear strength .............................................................................. 13 Perpendicular wall – Tensile strength ............................................................................... 14 Parallel wall – Sliding base strength ................................................................................. 14 3.2.2. The expected sequence of mechanisms of failure ............................................14
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Building prototype design .....................................................................................................17 4.1. The prototype dimensions ............................................................................................17 4.2. The foundation ..............................................................................................................18 4.3. Dead and live loads.......................................................................................................19 4.4. Prescribed material properties ......................................................................................21 4.5. Reinforcement ..............................................................................................................22 4.5.1. Base reinforcement of the modular panels .......................................................22 4.5.2. Connections and details....................................................................................23 4.6. The predicted seismic capacity based on the prescribed material properties ...............27
5.
Building prototype construction and transportation ..............................................................28 5.1. Foundation anchorage...................................................................................................29 5.2. Structural walls .............................................................................................................30 5.3. Slabs..............................................................................................................................31 5.4. The trasportation of the prototype building ..................................................................33
6.
Effective Material properties .................................................................................................35 6.1. Concrete properties .......................................................................................................35 6.2. Reinforcement properties .............................................................................................37
7.
Test Setup ..............................................................................................................................38 7.1. Eucentre TREESLab facility ........................................................................................38 7.2. Traditional instrumentation ..........................................................................................39 7.2.1. Accelerometers.................................................................................................39 7.2.2. Potentiometers ..................................................................................................39 7.2.3. Deformometers.................................................................................................40 7.2.4. High resolution cameras ...................................................................................41 7.3. Advanced instrumentation: optical deformations monitoring ......................................43
8.
Test input and time schedule .................................................................................................44 8.1. The reference seismic input and its scaling ..................................................................44 8.2. White noise input ..........................................................................................................45 8.3. Time schedule ...............................................................................................................46
9.
Experimental results ..............................................................................................................47 9.1. The spectrograms as obtained from wHitE noise tests .................................................47 viii
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9.2. The accelerograms recorded by the accelerometers .....................................................49 9.3. The deformations recorded by the deformometers .......................................................55 9.4. The cracking pattern .....................................................................................................57 10. The preliminary interpretation of the experimental results ...................................................59 10.1. Disussion on the stiffness of the prototype building ....................................................59 10.2. Disussion on the over-strength exibithed by the building prototype ............................61 10.3. The comparison between the moment at the base and the “internal moment at the base” .............................................................................................................................70 10.4. Concluding remarks ......................................................................................................71 11. Conclusions ...........................................................................................................................73 References ......................................................................................................................................75
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List of Figures Fig. 2.1: (a) The modular panel (b) details of the edge. ............................................................. 5 Fig. 2.2: The construction phases of the structural wall (a) the placement of the modular panel (b) the assembly of the support wall; the positioning of the additional reinforcement 6 Fig. 2.3: Detail of the connection between wall and foundation ............................................... 6 Fig. 2.4: Detail of the connection panels between: (a) two orthogonal walls, (b) three orthogonal walls, (b) four orthogonal walls ................................................................................ 7 Fig. 2.5: Detail of the connection between wall and floor.......................................................... 8 Fig. 3.1: The schematic representation of the prototype building with the indication of the direction of the shaking .............................................................................................................. 12 Fig. 3.2: The assumption adopted to evaluate the PGA which activates each mechanism. . 15 Fig. 3.3: The graphical representation of the sequence of collapse mechanisms: (0) initial stage; (1) mechanism 1; (2) mechanism 2; (3) mechanism 3; (4) mechanism 4; (5) mechanism 5 ................................................................................................................................ 16 Fig. 4.1: (a) first floor architectural plan; (b) first floor structural plan; (c) section A-A; (d) 3-D view........................................................................................................................................ 18 Fig. 4.2: Photo of the foundation system before the prototype building construction. ........ 19 Fig. 4.3: the location of the applied added mass: (a) first and second floor(b) roof ............. 21 Fig. 4.4: (a) Photo of the modular panel; (b) scheme of the modular panel. ......................... 23 Fig. 4.5: Details of the connection between wall and foundation ........................................... 24 Fig. 4.6: Details of the connection between orthogonal walls: (a) scheme; (b) photos ......... 25 Fig. 4.7: Details of the added reinforcement around the openings: (a) door; (b) window ... 26 Fig. 5.1: connections between walls and foundation-(working phase 1) ................................ 29 Fig. 5.2: photos of the assembly of the support walls-(working phase 2) .............................. 30 Fig. 5.3: additional reinforcement added at the sides (a and b) and around the openings (c) (working phase 2) ........................................................................................................................ 30 Fig. 5.4: application of the sprayed concrete ............................................................................ 31 Fig. 5.5: photos of the temporary supporting system for the slab .......................................... 31 Fig. 5.6: (a) photo of the slab panels; (b) photo of the perimeter beams reinforcement ...... 32 Fig. 5.7: (a) slab cross section; (b) photo of the additional reinforcement around the circular opening. ......................................................................................................................... 32 Fig. 5.8: concrete cast.................................................................................................................. 32 Fig. 5.9: transportation scheme. ................................................................................................ 33 Fig. 5.10: (a) perimeter steel beam; (b) hydraulic actuator; (c) post-tensioned steel cable. 34 Fig. 6.1: (a) photo after a compression test; (b) photo after Brasilian test. ........................... 36 Fig. 6.2: stress-strain response from I A concrete cast (mean response over the two specimens) .................................................................................................................................... 36 Fig. 7.1: the location of the accelerometers (blue circles)........................................................ 39 xi
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Fig. 7.2: the location of the potentiometer (red circles) ........................................................... 40 Fig. 7.3: the schematic representation of the four external walls ........................................... 40 Fig. 7.4: The location of the deformometers: (a) wall 2; (4) wall 4; (c) wall 1; (d) wall 3 .... 42 Fig. 7.5: the location of the cameras inside the building at the bottom storey ...................... 43 Fig. 7.6: the schematic representation of the advanced optical monitoring system ............. 43 Fig. 8.1: the reference seismic input: (a) un-scaled accelerogram; (b) pseudo-acceleration spectrum; (c)pseudo-velocity spectrum; (d) displacement spectrum; (e) Fourier transform; (f) power spectrum ...................................................................................................................... 45 Fig. 9.1: The spectrograms as obtained from: (a) WN1; (b) WN2; (c) WN3; (d) WN4; (e) WN5 0.10 g; (f) WN5 0.20 g; (g) WN5 0.30 g; (h) WN6; (i) WN7 .......................................... 48 Fig. 9.2: recorded accelerograms as obtained from seismic test 1: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile ........................................................................................................................................... 50 Fig. 9.3: recorded accelerograms as obtained from seismic test 2: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile ........................................................................................................................................... 51 Fig. 9.4: recorded accelerograms as obtained from seismic test 3: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile ........................................................................................................................................... 52 Fig. 9.5: recorded accelerograms as obtained from seismic test 4: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile ........................................................................................................................................... 53 Fig. 9.6: recorded accelerograms as obtained from seismic test 5: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile ........................................................................................................................................... 54 Fig. 9.7: deformations recorded by some deformometers placed at wall 1 (a) external surface; (b) internal surface ....................................................................................................... 56 Fig. 9.8: deformations recorded by some deformometers placed at wall 3 (a) external surface; (b) internal surface ....................................................................................................... 57 Fig. 9.9: cracking patterns after WN 6: (a) third floor; (b) second floor; (a) first floor ...... 58 Fig. 9.10: details of the cracks after ST 6: (a) second floor; (b) first floor ............................ 58 Fig. 10.1: experimental frequencies evaluated for each WN test ........................................... 60 Fig. 10.2: time history responses for ST1: total force Ftot(t); base shear Vbase(t); base moment Mbase(t) ........................................................................................................................... 63 Fig. 10.3: time history responses for ST2: total force Ftot(t); base shear Vbase(t); base moment Mbase(t) ........................................................................................................................... 64 Fig. 10.4: time history responses for ST3: total force Ftot(t); base shear Vbase(t); base moment Mbase(t) ........................................................................................................................... 65 Fig. 10.5: time history responses for ST4: total force Ftot(t); base shear Vbase(t); base moment Mbase(t) ........................................................................................................................... 66 Fig. 10.6: time history responses for ST5: total force Ftot(t); base shear Vbase(t); base moment Mbase(t) ........................................................................................................................... 67 Fig. 10.7: The section properties of the Nidyon panel as defined in Membrane 2000 .......... 69 Fig. 10.8: shear stress vs shear strain response of the Nidyon panel (Membrane 2000). ..... 69 Fig. 10.9: time history responses for ST5: total force Ftot(t); base shear Vbase(t); base moment Mbase(t) ........................................................................................................................... 71
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List of Tables Table 3.1: The sequence of expected mechanisms of failure. ...................................................... 15 Table 4.1: dead and live loads applied at each floor ..................................................................... 20 Table 4.2: Prescribed materials properties .................................................................................... 22 Table 4.3: The sequence of the expected mechanisms of failure with the corresponding estimated PGA............................................................................................................................................... 27 Table 5.1: The sequence of the expected mechanisms of failure with the corresponding estimated PGA............................................................................................................................................... 28 Table 6.1: Summary of compression tests on cylinders. .............................................................. 35 Table 6.2: Summary of test results on smooth bars ...................................................................... 37 Table 6.3: Summary of test results on deformed bars .................................................................. 37 Table 8.1: The performed seismic tests ........................................................................................ 44 Table 8.2: The performed seismic tests ........................................................................................ 46 Table 8.3: The shaking Table PGA of each seismic test .............................................................. 46 Table 9.1: The shaking Table PGA of each seismic test .............................................................. 49 Table 9.2: The peak accelerations of each seismic test ................................................................ 55 Table 10.1: The natural frequencies as obtained from finite elements models ............................ 61 Table 10.2:Maximum total horizontal force Ftot, base shear Vbase and base moment Mbase as obtained from each seismic test .................................................................................................... 62 Table 10.3:Maximum total horizontal force Ftot, base shear Vbase and base moment Mbase as obtained from each seismic test. ................................................................................................... 68
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1 1.1.
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The SeSyCoWa Project BACKGROUND
Several different construction techniques characterized by limited costs, fast construction, high thermal and acoustic efficiency have been proposed in the last years for intensive large-scale programs for mid-rise residential buildings at a worldwide level ([7], [8]). Among these, the solution based on the use of cast in situ squat concrete walls, made of lightweight material (e.g. polystyrene) as a support for structural concrete, seems to be promising. It is clear that this system typology may allow to obtain high structural, thermal and acoustic performance, since the reinforced concrete guarantees high load bearing capacity, while the lightweight material ensure thermal and acoustic insulation. Moreover, when those structural walls (sometimes also referred to as panels) are used to build mid-rise residential buildings, they may provide (if correctly designed) the structure with a so called cellular structure (i.e. structure characterized by bundled-tube behaviour), characterized by intrinsic superior performances with respect to the seismic loading. In this regards, the scientific literature on this field is mainly focused upon slender cantilever walls (for sake of clearness, the interested reader may refer to [9] and [10]), while less research effort has been devoted to squat walls despite the fact they already showed valuable strength resources towards large earthquakes (as for example, in Montenegro, [11] and in Cile, [12]). The scientific literature on the seismic behaviour of sandwich r/c walls appear quite poor provided that the “sandwich walls” typology is characterised by basic characteristics - like thickness, reinforcement ratios, construction details, - which are often far from those of traditional r/c walls.
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GENERAL OBJECTIVES
This document provides detailed documentation of the design, testing, and results of the research project named "Seismic Behaviour of Structural Systems composed of Cast in Situ Concrete Walls", referred to as SeSyCoWa, a joint effort between the University of Alicante, Spain (Prof. S. Ivorra) University of Bologna, Italy (Prof. Trombetti), Polytechnic of Bari (Prof. D. Foti) and University of Cluj-Napoca, Romania (Prof. C. Campian). Funded by European Union (EU) the project seeks to establish the seismic performance of modern mid-rise reinforced concrete building composed of precast structural r/c panels produced by the Italian Company Nidyon Costruzioni S.p.a. The structural solution, referred to as Nidyon NYSP, is the result of a comprehensive research program which has been developed since the end of the 1990’s (and currently under further development) through a number of experimental tests (reports of all the experimental tests are available online on the “Nydion Costruzioni s.p.a” web site [1]) which include: -
materials tests,
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uniaxial compression tests (with and without eccentricity),
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diagonal compression tests,
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slip tests (in order to evaluate the capacity of the transversal connections),
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out of plane tests,
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connections tests (orthogonal walls and foundations),
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in plane reversed cyclic tests on single panels (with and without opening),
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in plane reversed cyclic test on a full scale H shaped structure,
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dynamic tests with vibrodyne.
The large amount of data obtained by the aforementioned experimental campaigns allowed to appropriately characterize the in-plane and out-of plane behaviour of the structural system aimed at providing engineering design procedures consistent with the current code provisions [2]. As far as the seismic performance of the structural system is concerned, based on the results of inplane lateral reversed cyclic tests performed on both full-scale wall specimens (with and without opening) and full scale H-shaped structure, analytical formulations for the evaluation of the seismic capacity (in terms of stiffness, strength and ductility) of the structural system, have been developed. The use of such design formulas (based vertical and horizontal reinforcement design, connections design, details design and so on) leading to a specific seismic behaviour through the
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satisfaction of prescribed mechanisms of collapse (i.e. capacity design requirements) have been proposed. Based on the developed formulations, a three-storey full scale building has been conceived and designed in order to provide a specific seismic behaviour. The main objective of this report is to assess the global seismic behaviour of the building under dynamic earthquake excitation.
1.3.
SPECIFIC OBJECTIVES
Provided that the general objective of the research work is the assessment of the global seismic behaviour of the building under dynamic earthquake excitation, the following specific aspects are to be investigated: Global differences between the pseudo-static and dynamic earthquake behaviour (general objective) The wall stiffness under earthquake excitation; The wall strength under earthquake excitation; The cellular behaviour under earthquake excitation; The satisfaction of the prescribed mechanisms of collapse under earthquake excitation.
Furthermore, the damage progression is evaluated by mean of white noise tests at different amplitude performed before and after some shaking table tests. The following chapters are organized as follows: chapter 2 and 3 describe the design phase of the prototype building with particular attention devoted to the rationale leading to the specific building prototype; chapter 4 and 5 deal with the construction phases and material properties; chapters 6 to 9 provide the description of the test set-up, the test schedule and the description of the experimental results.
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The structural system
This section provides the main characteristics of the structural system developed by the Italian company “Nidyon Costruzioni S.p.a”, which is referred to as Nidyon Nysp.
2.1.
THE MODULAR PANEL
The basic element of the Nydion Nysp structural system is a prefabricated modular prereinforced polystyrene panels (which will be referred to as modular panels). The modular panel, which acts as support for the placing of the structural concrete, has a length of 1120 mm and a variable height equal to the building inter-storey height (Figure 2.1). It is composed of a single expanded polystyrene sheet (with thickness between 60÷160 mm in order to accomplish the thermal and acoustic requirements) inserted between two grids of electro-welded steel wire mesh. From Figure 2.1 it can be noted that the polystyrene sheet has an undulated profile in order to increase the adherence with the concrete. The wire meshes are connected by metallic ties (having a diameter of 3 mm and typically placed in quantity of 40÷50 for m2). At the edges the wire meshes are overlapped of about 100 mm in order to guarantee the necessary anchorage length. The steel mesh is typically characterized by 2.5 mm diameter and it is spaced at 50 mm (i.e.
2.5/50 mm).
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(a)
(b)
Fig. 2.1: (a) The modular panel (b) details of the edge. 2.2.
THE CAST IN SITU SANDWICH CONCRETE WALL
In order to realize the structural wall, modular panels are positioned one beside each other to obtain the so-called support wall of the desired dimensions (in accordance with the architectonic design of the building structure). Fig. 1.2 shows the placement of the modular panels. Additional reinforcements (typically 1+112 bars and 8/50 cm U-shaped bars) are added: (i) above, below and on the sides of the openings (window/door) in order to provide the necessary over-strength, (ii) at the lateral edges of the support walls in order to provide the necessary over-strength in areas characterized by localization of stresses. Once the support walls are set in place, two layers of shotcrete (each one of about 40 mm in thickness) are applied in order to obtain the r/c wall. In detail shotcrete is typically applied in two phases: (i) a first layer is sprayed so that the mesh is cover, then, after the first one is hardened, (ii) a second layer is sprayed, in order to reach the desired thickness. Finally, the two external surfaces are smoothed with surface filler. The quantity of the base reinforcement provided by the wire mesh (i.e. 2.5/50x50 mm), together with the typical total thickness of the two concrete layers (40+40 mm), leads to a reinforcement ratio equal to 0.00245 (without any additional bars).
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(a)
(b)
(c)
Fig. 2.2: The construction phases of the structural wall (a) the placement of the modular panel (b) the assembly of the support wall; the positioning of the additional reinforcement 2.3.
THE CONNECTIONS BETWEEN WALLS AND FOUNDATIONS
The foundation system of the buildings realizing according to the above described structural solution is typically composed of traditional beams located at the bottom of each structural wall (beams grid) in the case of stiff soil; for soil characterized by low mechanical properties foundation slabs or slabs upon piles are typically adopted. The connection between walls and foundations are steel runners designed in order to accomplish with capacity design criteria (typically 1+18/50cm or 8/30cm made up of B450C steel). Fig.1 3 provides a typical detail between wall and foundation.
Fig. 2.3: Detail of the connection between wall and foundation
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2.4.
THE CONNECTIONS BETWEEN ORTHOGONAL WALLS
Orthogonal walls are connected to each other through specifically designed panels referred to as connections panels. The connection panels are designed in order to ensure a so called box behavior under seismic actions. In other words, the connections have to be designed in order to ensure the complete transmission of the actions (i.e. shear, bending, and eventually any axial force) between orthogonal walls. Fig. 2.4 provides details for typical connection panels.
2.5.
THE CONNECTIONS BETWEEN WALLS AND FLOORS
In order to ensure a building cellular behaviour, it is fundamental that (i) the floors have high inplane stiffness; (ii) the connections between walls and floors are able to ensure the complete transmission of the actions. In more detail: the horizontal forces induced by the seismic effects are transmitted to the so called parallel walls (assuming that the direction of the shaking is parallel to the direction of some walls) as in-plane shear actions, the horizontal forces induced by the seismic effects are transmitted to the so called orthogonal walls (i.e. those orthogonal to the parallel walls) as axial actions (i.e. pull-andpush mechanism), provided that global box behavior is ensured.
(a)
(b)
(c)
Fig. 2.4: Detail of the connection panels between: (a) two orthogonal walls, (b) three orthogonal walls, (b) four orthogonal walls
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Fig. 2.5: Detail of the connection between wall and floor In order to provide the above behaviour, specifically designed connections panels (which make use of special r/c beams placed on the top of the walls) are used, while the floors are typically realized with traditional (solid or hollow) concrete slabs. Fig. 2.5 provides details of a typical connection between floor and wall. It should be noted that, typically, the special r/c beams have the same thickness of the wall below.
2.6. THE MAIN FEATURES AND CAPABILITIES OF THE STRUCTURAL SYSTEM Referring to the above discussion, the main characteristics and capabilities showed by the described structural system can be summarized as follows: The structural wall may be identified as a sandwich r/c panel with higher thermal and acoustic insulation properties; If the external walls are connected in order to ensure the complete transmission of forces and the connections are designed in order to satisfy capacity design criteria , the global behavior of the structure under seismic action is a box behaviour; If also internal structural walls (designed with the same criteria of the external walls) are contemplated, the system is characterized by a so called bundled-tube behaviour (i.e. cellular behaviour) [10], [13]. This behaviour leads to high strength resources (which permits to not use the post-elastic behaviour and the ductility properties) and high torsional stiffness; the bundled-tube behaviour ensures that the horizontal actions generate only significant in-plane actions (shear actions in the parallel walls and axial actions in the perpendicular ones);
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The structural system is typically used for the realization of low-rise residential buildings in which the structural walls are characterized by heights comparable to lengths. The walls behave as squat walls. As far as typical cases (i.e. low or mid-rise buildings) are concerned, in addition to what highlighted before, some additional specific observations may be given: Walls typically exhibit low values of vertical stresses Walls are typically characterized by light amount of vertical reinforcement; Walls are typically characterized by the same amount of vertical and horizontal reinforcement (which ensure the satisfaction of the shear-flexure hierarchy); Connections between orthogonal walls and between walls and foundation are designed in order to satisfy capacity design criteria.
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The rationale behind the prototype building
In light of the main features and capabilities of the structural system as discussed in the previous chapter, the present chapter deal with the rationale behind the prototype building aimed at meeting the fundamental characteristics of the building model with the facility constrains. In detail: first the main features of the building model (according to characteristics of the structural system discussed in the previous chapter) are highlighted; then the specific requirements of the facility are listed.
3.1.
MAIN FEATURES AND FACILITY CONSTRAINTS
The main features of the building model (as resulted from the observations of section 2.6) are the following ones: The building model should be representative of common mid-rise buildings (i.e. it should be composed of squat walls); The structural walls should be composed of equal reinforcement ratios in the horizontal and vertical direction; Dead and live loads should be applied in order to provide limited axial stresses to the walls (typical condition of mid-rise buildings); The design should be performed in order to provide the building with a global box behaviour (i.e. special attention need to be devoted to the connections and details); The design should be performed in order to provide the building with a selected seismic behaviour (i.e. capacity design requirements).
The main constraints of the facility are the following ones: Geometrical constraints related to the shaking table dimensions; this constraints affect the building dimensions in plane 10
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Height of the entrance door of the EUCENTRE TREES Lab. This constraint depends on the fact that the building prototype is built outside of the Lab and need to be transported inside the Lab through the entrance door; Maximum shaking table capacity. This constraint limits both the strength of the system (i.e. the maximum base shear) and the height of the system (i.e. the maximum base moment).
3.2. THE THEORETICAL SEISMIC CAPACITY AND THE EXPEXTED (PREDICTED) MECHANISMS OF FAILURE This section presents the analytical formulations which have been developed for the prediction of the theoretical seismic capacity of the prototype building (the interested reader may find all the details in (Ricci 2012) and in (Palermo et al. 2012)). Then, based on the presented analytical formulations, the expected (predicted) sequence of mechanisms of failure is provided.
3.2.1. The theoretical seismic capacity According to the schematic representation of the building prototype (Figure 3.1) as subjected to the uniaxial earthquake input, the following nomenclature is introduced: Parallel wall indicates the wall whose length is parallel to the direction of the shaking; Orthogonal wall indicates the wall whose length is orthogonal to the direction of the shaking.
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Fig. 3.1: The schematic representation of the prototype building with the indication of the direction of the shaking Parallel wall – In plane first yielding bending strength The in plane first yielding bending moment of the single parallel wall can be evaluated with the following expression:
by y1 h y y1 b h y y1 M y1 f ym 2 y y1 2 2 3
2
f ym h y y1 As ,c f ym h 2c n 6 3
(2.1)
Where: b is the thickness of the wall; h / / is the width of the wall, N Ed is the applied axial load, n E s / E c is the Young’s modulus homogenization coefficient, f ym is the average yielding strength of steel, As ,/ / bh is the geometrical reinforcement ratio, As,// is the area of the vertical reinforcements,
As , c
is the area of the additional tensile bars, c is the rebar cover, y y1 is
the position of the neutral axis with respect to the tensile fiber:
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y y1
h N Ed n f yd b
2 N Ed n 2 h h 1 n f b yd 1 n
(2.2)
Parallel wall – In plane ultimate bending strength The in plane ultimate bending moment of the single parallel wall can be evaluated with the following expression: h y M u f ym byu , sb u , sb fcmb0.8 h yu , sb 0.1h 0.4 yu ,sb As ,c f ym h 2c 2 2
Where: N Ed
f cm bh is the dimensionless axial stress, m f ym
(2.3)
f cm is the mechanical
amount of reinforcement, f cm is the mean concrete compression strength, yu , sb is the position of the neutral axis with respect to the tensile fiber (evaluated considering the stress block diagram): 0.8 yu , sb 0.8 m
h
(2.4)
Parallel wall – In plane shear strength The shear strength of the single parallel wall can be evaluated with the following expression: TRd min TRcd ;TScd Asw TRsd 0.9d s f ym cot cot sin cot cot TRcd 0.9dbc f 'cm 1 cot 2
(2.5)
where: TRsd in the steel shear strength, TRcd is the concrete shear strength, s 10 cm is the step of the horizontal reinforcement, is the angle of the concrete truss, is the angle of the horizontal reinforcement, d is the height of the wall section, Asw is the shear area, f 'cm 0.5 f cm is the concrete diagonal truss compressive strength; c is a coefficient equal to: c 1 cp / f cm and cp N Ed b h is the average compressive stress in the section due to the applied axial load N ED .
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As a results of a previous research work, an alternative analytical expression of the shear strength vs lateral displacement behavior, based on the Refined Compression Field Theory, has been developed. All the details may be found in (Palermo et al. 2012).
Perpendicular wall – Tensile strength The tensile strength of the single perpendicular wall (neglecting the concrete tensile strength) can be evaluated with the following expression: N Rd ,wall As , f ym
(2.6)
where As , is the vertical reinforcement area.
Parallel wall – Sliding base strength The sliding base strength of the structure can be evaluated with the following expression: S Rd ,/ / wall N Ed As , runners //
f ym
(3.7)
3
where: 0.5 is the fiction coefficient, As , riprese // is the area of the runners.
3.2.2. The expected sequence of mechanisms of failure Based on the analytical formulations provided in the previous section, the prototype building has been designed in order to experience the sequence of mechanisms of failure (through the satisfaction of capacity design requirements and hierarchy of strength criteria) represented in Figure 2.2 and summarized in Table 3.1 in terms of PGA (SA) which activates the mechanism. The PGA related to a certain value of the base shear or moment are obtained assuming a distribution of the so called equivalent- static forces (i.e. the horizontal forces due to the seismic effects) proportional to the product of the storey mass and the storey height (leading to a triangular distribution for the case of equal storey mass). The dynamic amplification factor is assumed equal to F0=2.5.
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Fig. 3.2: The assumption adopted to evaluate the PGA which activates each mechanism. Table 3.1: The sequence of expected mechanisms of failure. Mechanism number 1 2 3 4 5
Description of mechanism
PGA (SA)
yielding of the orthogonal walls yielding yielding of the parallel walls Flexural failure of the parallel walls Shear failure of the parallel walls Shear failure at the base connections
PGA1 PGA2 PGA3 PGA4 PGA5
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(0)
(1)
(3)
(2)
(4)
(5)
Fig. 3.3: The graphical representation of the sequence of collapse mechanisms: (0) initial stage; (1) mechanism 1; (2) mechanism 2; (3) mechanism 3; (4) mechanism 4; (5) mechanism 5
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4. Building prototype design 4.1.
THE PROTOTYPE DIMENSIONS
In order to meet the requirements of the facilities with the target model building (see section above on the rationale behind the prototype building) a full scale three-storey prototype building has been designed. The dimensions in plan are equal to 4.10m x 5.50 m. The building height is equal to 8.25 m, while the inter-storey height is equal to 2.75 m. Figure 2.3 gives the plan, a longitudinal section and the 3-D view of the prototype building.
(a)
(b)
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(c)
(d)
Fig. 4.1: (a) first floor architectural plan; (b) first floor structural plan; (c) section A-A; (d) 3-D view.
The length of the parallel walls and orthogonal walls is equal to 5.22 m and 4.12 m, respectively. The two parallel walls present two openings (windows) of equal area, while the two orthogonal walls present a door of equal area. One parallel side presents a balcony at both first and second stories, in order to represent a typical mod- rise residential building.
4.2.
THE FOUNDATION
The foundation system building has not been specifically designed for the building prototype: an existing foundation has been used. It is composed of four r/c beams with a length equal to 6.40m for the two beams corresponding to the parallel side (rectangular cross section equal to 79 cm x 40 cm) and 5.10 m for the two beams corresponding to the orthogonal side (rectangular cross section equal to 65 cm x 40 cm). Figure 4.1 provides a photo of the foundation before the prototype building construction.
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Fig. 4.2: Photo of the foundation system before the prototype building construction. 4.3.
DEAD AND LIVE LOADS
The applied dead and live loads (in addition to the self-weight of the structural elements) are chosen in order to provide the prototype model a global weight representative of the so called “seismic weight” of a real residential building characterised by the same the floor area of the prototype building. In detail the applied loads are equal to: Dead loads: 164 kg/m2 at the first and second floor (corresponding to the sum of the pavement weight and the weight of the typical additional permanent loads, e.g. infills, installations,…); 225 kg/m2 at the roof floor (corresponding to the sum of the insulation layer and the weight of the typical additional permanent loads). Live load: 60 kg/m2 as prescribed by the Italian seismic Code (NTC 2008) It should be noted that the loads were not applied to the balconies in order to not increase the little eccentricities due to the presence of the balcony only in one side. Table 2.2 provides a summary of the loads applied at each floor. Figure 2.3 gives the location of thee added mass simulating the dead and live loads.
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Table 4.1: dead and live loads applied at each floor Roof floor ( Aroof 21 m2 , Wroof Aroof qtot 15.7 t )
Slab: 2.1 t Joists+full bands 2.8 t Pots EPS 0.034 t Support EPS 0.01 t qd: 235 kg/m2 q l: 285 kg/m2 qtot:
520 kg/m2
2 First floor ( A2nd 19 m , W2 nd A2 nd qtot 15.6 t )
Slab: 2.38 t Joists+full bands 3.83 t Pots EPS 0.034 t Support EPS 0.01 t qd: 329 kg/m2 q l: 224 kg/m2
qtot: 5553 kg/m2 Balcony ( Abalcony 1.93 m 2 )
Slab: 0.241 t Joists+full bands 0.162 t Pots EPS 0.048 t Support EPS 0.001 t qd: 212 kg/m2 2 First floor ( A1st 19 m , W1st A1st qtot 15.6 t )
Slab: 2.38 t Joists+full bands 3.83 t Pots EPS 0.034 t Support EPS 0.01 t qd: 329 kg/m2 q l: 224 kg/m2
qtot: Balcony( Abalcony 1.86 m2 )
5553 kg/m2
Slab: 0.233 t Joists+full bands 0.162 t Pots EPS 0.046 t Support EPS 0.001 t qd: 215 kg/m2 Self-weigth 33.08t 20
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(a)
(b)
Fig. 4.3: the location of the applied added mass: (a) first and second floor(b) roof Based on what summarized in Table 2.2 the global weights of the prototype structures are: Wd,sup=51.3 t
is the seismic weight (structural elements plus dead loads) of the superstructure;
Wd=65 t
is the seismic weight (structural elements plus dead loads) of the structure;
4.4.
Wd+l,sup=65 t
is the global seismic weight of the superstructure
Wd+l=80 t
is the global seismic weight of the structure
PRESCRIBED MATERIAL PROPERTIES
The material properties which are prescribed for the prototype building are those typical adopted for mid-rise buildings. In detail: The concrete to be adopted for the structural walls is a C25/30 (according to the Italian building code, NTC2008) and should be applied with the so called “spritz beton” technique. In detail, the specific material is referred to as “RR 32” and is furnished by the Italian Company Fassabortolo. The concrete to be adopted for the floors is a traditional C25/30 concrete. 21
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The steel to be adopted for the wire mesh (smooth bars) is galvanized steel with low carbon content classified as “C7D” (according to UNI EN 10016-2); The steel for the additional reinforcement (standard deformed bars) is a regular B450C (according to the Italian building code, NTC2008); Table 4.2 gives the expected (mean) design strength and Young’s modulus adopted for the materials.
Table 4.2: Prescribed materials properties Material Sprayed concrete Regular concrete Wire mesh Deformed bars
4.5.
Expected strength (MPa) 30 30 500 500
Expected Young’s modulus (MPa) 30588 30588 210000 210000
REINFORCEMENT
4.5.1. Base reinforcement of the modular panels The amount of the base reinforcement which has been used for the modular panels is equal to one half of that typically used for real building. The choice is imposed by the strength limited imposed by the shaking table (see section 2.6). In detail the mesh grid is characterised by a 2.5 mm diameter spaced at 10 cm (i.e. 2.5/10). Transversal connectors have a diameter of 3 mm and are applied in a quantity equal on average to 47/m2. Fig. 4.1 provides details of a modular panel.
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(a)
(b) Fig. 4.4: (a) Photo of the modular panel; (b) scheme of the modular panel.
4.5.2. Connections and details The connections between walls and foundations are realized through runners (8/30, anchorage length equal to 60 cm). Figure 4.2 provides details of the connection between walls and foundations. The connections between orthogonal walls are realized through: (a) double amount of base reinforcement (i.e. 2.5/5 for the first 20 cm from the corner); (b) horizontal U shaped bars
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(8/30); (c) vertical 16 bars in order to support the horizontal U shaped bars. Figure 4.3 and 4.4 provide details of the connections between orthogonal walls. The additional reinforcement around the opening is composed of: (a) double amount of base reinforcement (i.e. 2.5/5 for the first 20 cm from the corner); (b) diagonal mesh grid inclined at 45° with respect to the base grid (c) horizontal U shaped bars (8/30); vertical 12 bars. Figure 4.5 provide details of added reinforcement around the openings.
Fig. 4.5: Details of the connection between wall and foundation
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(a)
(b) Fig. 4.6: Details of the connection between orthogonal walls: (a) scheme; (b) photos
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(a)
(b) Fig. 4.7: Details of the added reinforcement around the openings: (a) door; (b) window
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4.6. THE PREDICTED SEISMIC CAPACITY BASED ON THE PRESCRIBED MATERIAL PROPERTIES Based on the prescribed material properties summarized in section 4.3 (i.e. expected values of strength and modulus) and on the analytical formulations and assumptions discussed in section 3.1.1 and 3.1.2, Table 4.1 provides the expected predicted values of PGA corresponding to each mechanism of failure.
Table 4.3: The sequence of the expected mechanisms of failure with the corresponding estimated PGA. Mechanism number 1 2 3 4 5
Description of mechanism
PGA (SA)
yielding of the orthogonal walls yielding of the parallel walls Flexural failure of the parallel walls Shear failure of the parallel walls Shear failure of the connections at the base
PGA1=0.29g PGA2=0.53g PGA3=0.60g PGA4=0.73g PGA5=0.91g
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5. Building prototype construction and transportation This chapter deals with the description of the construction of the prototype building. The prototype building was built outside of the EUCENTRE Lab in approximately one month, between April and May 2011. The time schedule of the construction phases is provided in Table 5.1. After phase 1 (related to the placement of foundation anchorage), each subsequent working phases is composed of the following main activities: Assembly of wall modular panel; Placement of additional reinforcement; Walls concrete cast; Assembly of the slab modular panels; Placement of additional reinforcement; Slab concrete cast;
Table 5.1: The sequence of the expected mechanisms of failure with the corresponding estimated PGA. Day(s) 19/04/2011 20/04/2011
Working phase
1-foundation 21/04/2011 02/05/2011 03-04/05/2011 05-09/05/2011 10/05/2011 10/05/2011
2-first floor
Activity Beginning of construction Placement of foundation anchorages Placement of foundation anchorages Assembly of wall modular panels Placement of additional reinforcement for the wall Walls concrete cast Assembly of the slab modular panels Placement of additional reinforcement for the slab 28
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11/05/2011 12/05/2011
Slab concrete cast Assembly of wall modular panels Placement of additional reinforcement for the wall Walls concrete cast
13/05/2011 14/05/2011 2-first floor 15/05/2011
Assembly of the slab modular panels Placement of additional reinforcement for the slab Slab concrete cast Assembly of wall modular panels Placement of additional reinforcement for the wall Walls concrete cast
25/05/2011 26/05/2011 27/05/2011 28/05/2011 29/05/2011 3-first floor 30/05/2011 31/05/2011 01/06/2011 5.1.
Assembly of the slab modular panels Placement of additional reinforcement for the slab Slab concrete cast
FOUNDATION ANCHORAGE
In order to connect the existing foundation to the building appropriate connections were designed (see section 4.1.2). Figure 5.1 provides photos of the working phases related to the placement of the connections between walls and foundation.
Fig. 5.1: connections between walls and foundation-(working phase 1)
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5.2.
STRUCTURAL WALLS
Modular panel were assembled in order to obtain the desired support wall, starting from one building corner (Figure 5.2). Once the support walls are completed, the required additional reinforcement was placed at the edges and around the openings (Figure 5.3). After that all the additional reinforcement was positioned, the concrete were applied through the so called “spritz-beton” technique. Figure 5.4 provides photos related to the concrete cast.
Fig. 5.2: photos of the assembly of the support walls-(working phase 2)
(a)
(b)
(c)
Fig. 5.3: additional reinforcement added at the sides (a and b) and around the openings (c) (working phase 2)
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Fig. 5.4: application of the sprayed concrete 5.3.
SLABS
Before the placement of the slab modular panels, a support grid of timber beams were installed on the top of temporary steel shores (Figure 5.5). Modular polystyrene panels (acting as support for the concrete cast) were then assembled. Along the perimeter of the slab, the appropriate bars were located in order to realize the perimeter beams. Then the slab reinforcement (base reinforcement plus additional reinforcement at the edges and around the circular slab opening for the stair case) was positioned. The cross section of the slab is schematized in Figure 5.6. Once all the reinforcement bars were positioned, regular concrete was cast (Figure 5.7).
Fig. 5.5: photos of the temporary supporting system for the slab
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(a)
(b)
Fig. 5.6: (a) photo of the slab panels; (b) photo of the perimeter beams reinforcement
(a)
(b)
Fig. 5.7: (a) slab cross section; (b) photo of the additional reinforcement around the circular opening.
Fig. 5.8: concrete cast
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5.4.
THE TRASPORTATION OF THE PROTOTYPE BUILDING
As mentioned before (at the beginning of section 5) the prototype building was built outside of EUCENTRE Lab, in front of the main entrance door. The transportation of the building prototype inside the laboratory and its positioning on the shaking table was carried out following the scheme represented in 5.8. In detail: Lifting of the prototype through the action of four hydraulic actuators (Position 1) and positioning on an appropriate system of sledges. Transportation of the system from Position 1 (outside the Lab) to Position 2 (inside the Lab); Transportation of the system from Position 2 to Position 3 (shaking table); Details of the transportation system (i.e. perimeter steel beams, hydraulic actuators, posttensioned steel cables) are provided in Figure 5.9.
Fig. 5.9: transportation scheme.
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(a)
(b)
(c)
Fig. 5.10: (a) perimeter steel beam; (b) hydraulic actuator; (c) post-tensioned steel cable.
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6. Effective Material properties After the construction of the prototype building, material tests were performed on both concrete and steel bars (wire mesh and deformed bars). In the following subsection results of the performed material tests are provided.
6.1.
CONCRETE PROPERTIES
In order to characterize the effective concrete strength 18 cylindrical specimens (2 specimens for each concrete cast) were subjected to compression tests. Test results are summarized in Table 6.1. Also brasilian tests on two cylindrical specimens were performed in order to evaluate the tensile strength. Test results are summarized in table 6.2. The mean compressive strength as obtained from cubic tests was equal to approximately 30 MPa. The mean tensile strength as obtained from cylindrical tests was equal to 1.4 Mpa. The mean elastic modulus (secant modulus), as obtained from compression tests was approximately equal to 25000 MPa. A specific stress-strain response (IA) is represented in Figure 6.1. Photos of specimens after tests are displayed in Figure 6.1.
Table 6.1: Summary of compression tests on cylinders. Cast
Ø [mm]
Area [mm2]
h [mm]
h/Ø
M [kg]
Max load [kg]
strength [Mpa]
IA II A III A VA IB II B III B IV B VB
154 156 136 135 135 135 154 136 155
18627 19113 14527 14314 14314 14314 18627 14527 18869
388 389 339 338 338 340 387 340 390
2,5 2,5 2,5 2,5 2,5 2,5 2,5 2,5 2,5
15,105 15,400 10,165 10,190 10,140 10,110 15,275 10,600 15,365
60800 71500 59200 58600 52900 20000 61700 53400 63300
32 37 40 40 36
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32 36 32
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(a)
(b)
Fig. 6.1: (a) photo after a compression test; (b) photo after Brasilian test.
Fig. 6.2: stress-strain response from I A concrete cast (mean response over the two specimens)
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6.2.
REINFORCEMENT PROPERTIES
For each kind of smooth and deformed bar (i.e. each diameter) three specimens were tested. Tables 6.3 and 6.4 give a summary of the test results. Mean yielding strengths are equal to 458 MPa and 543 MPa for smooth and deformed bars, respectively.
Table 6.2: Summary of test results on smooth bars
Table 6.3: Summary of test results on deformed bars
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7. Test Setup Test setup has been designed in order to provide a comprehensive understanding of the seismic response of the prototype building, with especial attention devoted on the dynamic response of the walls. In more detail, the test setup is designed in order to provide measures of: (i) table and structure accelerations at different locations; (ii) building deformations; (iii) building damage. For this purpose the following instrumentation was installed: Traditional instrumentation: (a) mono-directional and bi-directional accelerometers which are located at the shaking table, foundation and structure; (b) potentiometers to monitor the relative displacement between the shaking table and the foundation; (b) strain gauges which are located on the two orthogonal walls; (c) internal (i.e. inside the building) and external cameras. Advanced instrumentation composed of an optical system monitoring one of the two longitudinal walls (i.e. the wall with the balconies, wall 4 with reference to Figure 7.3).
7.1.
EUCENTRE TREESLAB FACILITY
TREESLab is the experimental laboratory of the Foundation EUCENTRE designed with advanced technologies for structural and seismic simulation. TREESLab can perform several tests among which the most relevant are: shaking table tests on large/full scale prototypes (i.e. buildings, bridge piers, laminar boxes), bearings and isolation devices, structural components. The uniaxial shaking table has a platform size equal to 5.6m x 7.0m and is powered through a single actuator. The total dynamic stroke is equal to 1000 mm (+/- 500 mm), the maximum table velocity is equal to 1.80 m/sec and the maximum table acceleration is approximately equal to 5.0g. The maximum static force is approximately equal to 2100 kN, while the maximum dynamic force is approximately equal to 1700 kN.
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7.2.
TRADITIONAL INSTRUMENTATION
7.2.1. Accelerometers In order to provide measures of the accelerations at different locations of the structures 9 monodirectional accelerometers and 3 bi-directional accelerometers were located at the shaking table, at the foundation and at the three floors of the structures, as displayed in Figure 7.1. In detail: One mono-directional accelerometer (i.e. n. 0 as indicated in Figure 7.1) was located at the shaking table; Two mono-directional (i.e. n. 1 and 2 as indicated in Figure 7.1) accelerometers were located at the foundation; One mono-directional and two bi-directional (i.e. n. 3 to 14 as indicated in Figure 7.1) accelerometers were located at each storey;
Fig. 7.1: the location of the accelerometers (blue circles) 7.2.2. Potentiometers Four potentiometers were installed in order to monitor the relative displacement between the shaking table and the foundation. Figure 7.2 displays the location of the potentiometers. In detail: Potentiometer n. 16 and 17 are oriented along the x direction; Potentiometer n. 18 and 19 are oriented along the y direction;
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Fig. 7.2: the location of the potentiometer (red circles) 7.2.3. Deformometers All the external walls (referred to as wall 1, 2, 3 and 4, see Figure 7.3) were instrumented with deformometers in order to provide measures of deformation. In total 64 deformometers were installed: 20 deformometers with 100 mm base length; 2 deformometers (200 mm base length); 42 deformometers (50 mm base length); 2 deformometers (200 mm base length).
Fig. 7.3: the schematic representation of the four external walls
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In more details: Wall 2 was equipped with: 2 deformometers (250 mm base length) in order to measure the slip at the base (n. 31 and 32); 2 deformometers (100 mm base length) to measure the rocking (n. 30 and 33); 5 deformometers (50 mm base length) placed at 60 cm from the base (i.e. at the end of the connections between the wall and the foundation, n. 34-38); 5 deformometers (50 mm base length) placed around the window (n. 39-43); 6 deformometers (100 mm base length) to measure the shear deformation (n. 44-47 and 5253); 2 deformometers (50 mm base length) between the first and second floor to measure the flexural deformation (n. 48 and 51); 2 deformometers (50 mm base length) between the first and second floor to measure slip (n. 49 and 50). Additional deformometers were placed inside at the internal surface of the wall. Wall 4 (the one which is monitored also with the optical system) is equipped with: 2 deformometers (100 mm base length) to measure the relative displacement between wall 4 and the 2 orthogonal walls (n. 83 and 84). Wall 1 and wall 2 were equipped with the same deformometers: 3 deformometers (50 mm base length) at the base to measure the rocking (20-23 or 54-56); 3 deformometers (50 mm base length) placed at 60 cm from the base (24-26, or 57-59); 3 deformometers (50 mm base length) at the window’s height. Additional deformometers were placed at the internal surface of the walls.
7.2.4. High resolution cameras During the tests the structure was monitored by seven high resolution cameras. Four cameras were placed outside of the building and each of them recorded one of the four sides of the building. The remaining three cameras were placed inside the building at the ground floor (Figure 7.4) in order to record specific portions of wall: between the openings of walls 2 and 4 (cameras n. 2 and 3) and the corner between wall one and wall four.
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(a)
(b)
(c) (d) Fig. 7.4: The location of the deformometers: (a) wall 2; (4) wall 4; (c) wall 1; (d) wall 3
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Fig. 7.5: the location of the cameras inside the building at the bottom storey 7.3. ADVANCED INSTRUMENTATION: OPTICAL DEFORMATIONS MONITORING The parallel wall characterized by the presence of the balcony (i.e. Wall 1 referring to Figure 7.3) was monitored by an advanced optical monitoring system. The system is composed of 10 high resolution cameras (with a resolution of 60 Hz). Each camera recorded a rectangular wall surface with dimensions equal to 1.50m x 2.m. All rectangular surfaces were subdivided into a grid with dimensions equal to 20 x 25 cm. Each side of the grid had attached a marker (1 cm x 1 cm). Figure 7.5 provides the location of the markers.
Fig. 7.6: the schematic representation of the advanced optical monitoring system
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8. Test input and time schedule The present chapter provides details on the tests carried out on the prototype building. First, details on the input, with reference to both seismic tests and white noise tests, are provided. Then the text matrix (i.e. the chronology of the performed tests) is given.
8.1.
THE REFERENCE SEISMIC INPUT AND ITS SCALING
The reference seismic input is the accelerogram recorded during the 1979 Montenegro earthquake by the accelerometric station located at Ulcinj (Hotel Albatros, X component). The un-scaled record is characterized by a Peak Ground Acceleration (PGA) equal to 0.305g. Figure 7.4 gives the recorded accelerogram and the corresponding displacement, pseudo-velocity and pseudo-acceleration spectra. The Fourier transform (amplitude spectrum) and the power spectrum of the signal are also displayed in Figure 7.4 The reference accelerogram was scaled in amplitude to increasing values of PGA from 0.05 g up to 1.2 g. Table 8.1 provides a summary of the performed seismic tests (ST).
Table 8.1: The performed seismic tests Test ST1 ST 2 ST 3 ST 4 ST 5 ST 6
Description Seismic test 0.05 g Seismic test 0.15 g Seismic test 0.50 g Seismic test 1.00 g Seismic test 1.20 g Seismic test 1.20 g
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 8.1: the reference seismic input: (a) un-scaled accelerogram; (b) pseudo-acceleration spectrum; (c)pseudo-velocity spectrum; (d) displacement spectrum; (e) Fourier transform; (f) power spectrum 8.2.
WHITE NOISE INPUT
White noise (WN) tests were performed before and after each seismic test in order to estimate the variation in the dynamic properties of the prototype systems. The white noise signal is automatically produced by the shaking table control system. Three different white noise input amplitudes (in terms of shaking table PGA) were applied: 0.1g; 0.3 g and 0.5 g. Table 8.2 provides a summary of the performed white noise tests. 45
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Table 8.2: The performed seismic tests Test WN 1 WN 2 WN 3 WN 4 WN 5 WN 6 WN 7 WN 8
8.3.
Description White noise 0.05 g White noise 0.10 g White noise 0.10 g White noise 0.10 g White noise series with amplitude increasing from 0.05 g to 0.30 g White noise 0.50 g White noise 0.30 g White noise 0.30 g
TIME SCHEDULE
The time schedule of the whole test program is provided in Table 8.3.
Table 8.3: The shaking Table PGA of each seismic test Test WN1 ST1 WN2 ST2 WN3 ST3 WN4 ST4 WN5 WN6 ST5 WN7 ST6 WN 8
Date Test description 06/12/2011 White noise 0.05 g 06/12/2011 Seismic test 0.05 g 06/12/2011 White noise 0.10 g 06/12/2011 Seismic test 0.15 g 07/12/2011 White noise 0.10 g 07/12/2011 Seismic test 0.50 g 07/12/2011 White noise 0.10 g 07/12/2011 Seismic test 1.00 g 07/12/2011 White noise series with amplitude increasing from 0.05 g to 0.30 g 13/12/2011 White noise a 0.50 g 13/12/2011 Seismic test 1.20 g 16/12/2011 White noise 0.30 g 16/12/2011 Seismic test 1.20 g 16/12/2011 White noise 0.30 g
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9. Experimental results The present chapter presents the experimental results as obtained from both seismic and white noise tests. In detail, the results are presents as follows: Section 9.1 reports the spectrograms as obtained from white noise tests; Section 9.2 reports the accelerograms as recorded by the accelerometers; Section 9.3 reports the deformations as recorded by the deformometers; Sections 9.4 reports photos of the crack patters. A preliminary interpretation of the results is provided in chapter 10.
9.1.
THE SPECTROGRAMS AS OBTAINED FROM WHITE NOISE TESTS
The spectrograms as obtained from each white noise input are displayed in Figure 9.1. The spectrogram is a contour representation of the amplitude of the transfer function. In detail: the horizontal axis reports the frequencies, the vertical axis reports the time, while the colours give the amplitudes of the transfer function. For a white noise input, the peaks of the amplitude of the transfer function allow to evaluate the experimental natural frequencies of the structure. The experimental natural frequencies as determined from each white noise test are summarized in Table 9.1
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0.018 0.016
250
time [s]
0.014 200
0.012 0.01
150 0.008 100
0.006 0.004
50 0.002 0
2
4
6
8
10 freq [Hz]
12
14
16
18
20
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i) Fig. 9.1: The spectrograms as obtained from: (a) WN1; (b) WN2; (c) WN3; (d) WN4; (e) WN5 0.10 g; (f) WN5 0.20 g; (g) WN5 0.30 g; (h) WN6; (i) WN7
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Table 9.1: The shaking Table PGA of each seismic test
9.2.
Test
Experimental frequencies (Hz)
WN 1
10.00
WN 2
10.00
WN 3
10.00
WN 4
10.00
WN 5
10.00
WN 6
8.60
WN 7
8.20
WN 8
8.20
THE ACCELEROGRAMS RECORDED BY THE ACCELEROMETERS
Figures 9.2 to 9.6 report, for the seismic tests ST1,ST2,ST3,ST4,ST5, the accelerograms recorded at the shaking table (feedback), at the foundation and at all floors. Also the peak floor acceleration profile as obtained from each seismic test is provided. Table 9.1 gives a summary of the Peak Acceleration recorded at the shaking table (referred to as ST-PA), at the foundation (referred to as F-PA), at the first floor (1-PA), at the second floor (2-PA), at the third floor (3PA). It should be noted that the dynamic amplifications (i.e. the ratios between the maximum accelerations at the base and at the upper storeys) are significantly less than those typically assumed in an anti-seismic design (i.e. F0=2.5). Further discussion on the consequence of these results are given in chapter 10.
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Feedback - 0.05g
Foundation - 0.05g
0.04
0.04
0.02
0.02
0
0 a [g]
0.06
a [g]
0.06
-0.02
-0.02
-0.04
-0.04
-0.06
-0.06
-0.08 0
10
20
30
40 t [s]
50
60
70
-0.08 0
80
10
20
30
(a)
40 t [s]
50
60
70
80
60
70
80
(b)
First Storey - 0.05g
Second Storey - 0.05g
0.08
0.08
0.06
0.06
0.04 0.04 0.02 a [g]
a [g]
0.02 0
0 -0.02 -0.02 -0.04 -0.04
-0.06 0
-0.06
10
20
30
40 t [s]
50
60
70
-0.08 0
80
(c)
10
20
30
40 t [s]
50
(d)
Third Storey - 0.05g 0.08 0.06 0.04 0.02
a [g]
0 -0.02 -0.04 -0.06 -0.08 -0.1 0
10
20
30
40 t [s]
50
60
70
80
(e) (f) Fig. 9.2: recorded accelerograms as obtained from seismic test 1: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile .
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Foundation - 0.15g 0.2
0.15
0.15
0.1
0.1
0.05
0.05 a [g]
a [g]
Feedback - 0.15g 0.2
0
-0.05
0
-0.05
-0.1
-0.1
-0.15
-0.15
-0.2 0
10
20
30
40
50
60
70
80
-0.2 0
90
10
20
30
40
t [s]
50
60
70
80
90
60
70
80
90
t [s]
(a)
(b)
First Storey - 0.15g
Second Storey - 0.15g
0.2
0.25 0.2
0.15
0.15 0.1 0.1 0.05 a [g]
a [g]
0.05
0
0 -0.05
-0.05
-0.1 -0.1 -0.15 -0.15
-0.2 0
-0.2
10
20
30
40
50
60
70
80
90
-0.25 0
10
20
30
40
t [s]
50 t [s]
(c)
(d)
Third Storey - 0.15g 0.3
0.2
0.1
a [g]
0
-0.1
-0.2
-0.3
-0.4 0
10
20
30
40
50
60
70
80
90
t [s]
(e) (f) Fig. 9.3: recorded accelerograms as obtained from seismic test 2: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile
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Feedback - 0.5g
Foundation - 0.5g
0.4
0.4
0.2
0.2
0
0 a [g]
0.6
a [g]
0.6
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8 0
10
20
30
40
50
60
70
80
-0.8 0
90
10
20
30
40
t [s]
(a) 0.6
0.4
0.4
0.2
0.2 a [g]
a [g]
0.6
0
-0.2
-0.4
-0.4
-0.6
-0.6
30
80
90
60
70
80
90
0
-0.2
20
70
Second Storey - 0.5g 0.8
10
60
(b)
First Storey - 0.5g 0.8
-0.8 0
50 t [s]
40
50
60
70
80
90
-0.8 0
10
20
30
40
t [s]
50 t [s]
(c)
(d)
Third Storey - 0.5g 0.8 0.6 0.4 0.2
a [g]
0 -0.2 -0.4 -0.6 -0.8 -1 0
10
20
30
40
50
60
70
80
90
t [s]
(e)
(f)
Fig. 9.4: recorded accelerograms as obtained from seismic test 3: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile
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Foundation - 1g 1.5
1
1
0.5
0.5
a [g]
a [g]
Feedback - 1g 1.5
0
0
-0.5
-0.5
-1
-1
-1.5 0
10
20
30
40
50 t [s]
60
70
80
90
-1.5 0
100
10
20
30
(a)
40
50 t [s]
60
70
80
90
100
70
80
90
100
(b)
First Storey - 1g
Second Storey - 1g
1.5
1.5
1
1
0.5 0.5
a [g]
a [g]
0 0
-0.5 -0.5 -1 -1
-1.5 0
-1.5
10
20
30
40
50 t [s]
60
70
80
90
-2 0
100
(c)
10
20
30
40
50 t [s]
60
(d)
Third Storey - 1g 2 1.5 1 0.5
a [g]
0 -0.5 -1 -1.5 -2 -2.5 0
10
20
30
40
50 t [s]
60
70
80
90
100
(e) Fig. 9.5: recorded accelerograms as obtained from seismic test 4: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile
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Feedback - 1.2g
Foundation - 1.2g
1
1
0.5
0.5
0
0 a [g]
1.5
a [g]
1.5
-0.5
-0.5
-1
-1
-1.5
-1.5
-2 0
10
20
30
40
50 t [s]
60
70
80
90
-2 0
100
10
20
30
40
(a)
50 t [s]
60
70
80
90
100
70
80
90
100
(b)
First Storey - 1.2g
Second Storey - 1.2g
1.5
2
1
1.5
1 0.5 0.5 a [g]
a [g]
0 0
-0.5 -0.5 -1 -1 -1.5
-2 0
-1.5
10
20
30
40
50 t [s]
60
70
80
90
100
(c)
-2 0
10
20
30
40
50 t [s]
60
(d)
Third Storey - 1.2g 2.5 2 1.5 1
a [g]
0.5 0 -0.5 -1 -1.5 -2 -2.5 0
10
20
30
40
50 t [s]
60
70
80
90
100
(e) (f) Fig. 9.6: recorded accelerograms as obtained from seismic test 5: (a) shaking table; (b) foundation; (c) first storey; (d) second storey; (e) third storey; (f) peak floor acceleration profile
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Table 9.2: The peak accelerations of each seismic test Input PGA [g] 0.050 0.150 0.500 1.000 1.200
Test 1 2 3 4 5 9.3.
ST-PA [g] 0.065 0.176 0.628 1.280 1.650
F-PA [g] 0.066 0.175 0.620 1.280 1.640
1-PA [g] 0.063 0.177 0.695 1.19 1.610
2-PA [g] 0.073 0.219 0.716 1.55 1.830
3-PA [g] 0.087 0.296 0.836 2.04 2.470
THE DEFORMATIONS RECORDED BY THE DEFORMOMETERS
Figures 9.7 and 9.8 provide the deformations recorded by some deformometers for the seismic test ST4 (PGA equal to 1.0g). In detail: Channels n. 24, 25 and 26 and 63, 64 and 65 for wall 1. Channels n. 57,58 and 59 and 74, 75 and 76 for wall 3. It can be noted that the maximum recorded deformations are approximately equal to 0.2·10-3
c [%]
5
x 10
ch.24
-4
0
-5 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
60
70
80
90
100
t [s]
c [%]
2
x 10
ch.25
-4
0
-2 0
10
20
30
40
50
t [s]
c [%]
2
x 10
ch.26
-4
0
-2 0
10
20
30
40
50
t [s]
(a)
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c [%]
2
x 10
ch.63
-4
0
-2 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
60
70
80
90
100
t [s]
c [%]
2
x 10
ch.64
-4
0
-2 0
10
20
30
40
50
t [s] 10
x 10
ch.65
-5
c [%]
5 0 -5 0
10
20
30
40
50
t [s]
(b) Fig. 9.7: deformations recorded by some deformometers placed at wall 1 (a) external surface; (b) internal surface
c [%]
2
x 10
ch.57
-4
0
-2 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
60
70
80
90
100
t [s]
c [%]
2
x 10
ch.58
-4
0
-2 0
10
20
30
40
50
t [s]
c [%]
2
x 10
ch.59
-4
0
-2 0
10
20
30
40
50
t [s]
(a)
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c
[%]
4
x 10
ch.74
-4
2 0 -2 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
60
70
80
90
100
t [s]
c
[%]
2
x 10
ch.75
-4
0 -2 -4 0
10
20
30
40
50
t [s] x 10
ch.76
-4
0
c
[%]
2
-2 0
10
20
30
40
50
t [s]
(b) Fig. 9.8: deformations recorded by some deformometers placed at wall 3 (a) external surface; (b) internal surface 9.4.
THE CRACKING PATTERN
No visible cracks were observed after the first four seismic tests (i.e. PGA equal to 0.05, 0.15, 0.50 and 1.0 g). First visible cracks appeared after WN 6 (i.e. the white noise test performed after seismic test 4 with a PGA equal to 1.0g). Figure 9.9 displays photos of the cracking patterns as observed after WN 6. Cracks are depicted in red. Figure 9.10 displays some cracks details as observed after ST 6 (PGA equal to 1.20g): new cracks are depicted in black.
(a)
(b)
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(c)
(d)
(e) Fig. 9.9: cracking patterns after WN 6: (a) third floor; (b) second floor; (a) first floor
Fig. 9.10: details of the cracks after ST 6: (a) second floor; (b) first floor
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10. The preliminary interpretation of the experimental results This chapter provides a preliminary interpretation of the experimental results. The fundamental result appeared from the experimental tests is that the prototype building showed essentially no visible damages (i.e. no visible concrete cracks) up to a PGA equal to 1.0 g. This is an unexpected response, in the light of the expected (theoretical) building prototype seismic capacity, as discussed in sections 3.2 and 4.2. In this chapter possible reasons leading to this unexpected behaviour are discussed. Based on the results of the white noise tests, a dynamic identification of the prototype building is conducted, aimed at provide an interpretation of the evolution of the experimental natural frequencies as obtained from the spectrograms. Then a discussion on the solicitations experienced by the prototype building during the seismic tests is developed. Finally a possible interpretation of the observed over-strength is provided. A meaningful discussion on the preliminary interpretation of the experimental results can be found in (Ricci 2012).
10.1. DISUSSION ON THE STIFFNESS OF THE PROTOTYPE BUILDING As discussed in section 9.1 the results of the white noise tests allow to evaluate the experimental natural frequencies of the prototype building. For sake of clearness Figure 10.1 provides an histogram of the experimental frequency as obtained from each white noise test. Inspection of the graph clearly reveals that the natural frequencies remain constant (approximately 10 Hz) for the first six tests. After WN7 test it decreased to 8.6 Hz. Finally, after the last white noise test, it further decreased to 8.2 Hz. In order to provide a structural interpretation of these results a finite element model of the prototype building has been realized using the commercial software SAP2000 v.14. Two increasing in complexity models are developed:
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Model A: each wall is modelled with one-layer shell elements with thickness equal to the sum of the thicknesses of the two r/c layers. Model B: each wall is modelled with the so called shell-layered elements in order to accurately model the geometry of the cross section (the two r/c layers and the internal polystyrene layer). No significant differences in terms of global stiffness are observed for the two models; in other words, natural frequencies analyses performed on the two models lead to equivalent results, if the same concrete Young’s modulus is adopted. 12 10 f [HZ]
8 6 4 2 0 1
2
3
4
5
6
7
8
WN
Fig. 10.1: experimental frequencies evaluated for each WN test In order to estimate the damage conditions in terms of cracking corresponding to the experimental measured frequencies, two different limit Young’s modulus have been assumed for the concrete corresponding to the limit cases of: Uncracked condition (UC): Young’s modulus equal to the experimentally measured modulus Ec,UC=25000 MPa (section 6.1); Fully cracked condition (FC): Young’s modulus equal to Ec,FC= 0.11·Ec,u (based on the results of the previous research works on single walls) leading to Ec,FC=2750 MPa Table 10.1 provides the natural frequencies as obtained from the finite elements models assuming the two limit conditions of above. The experimental initial fundamental frequency (10 Hz) is quite close to that obtained from the finite element model considering uncracked condition. In detail, the value of the concrete Young’s modulus leading the initial experimental 60
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frequency (10 Hz), based on the results of natural frequencies analyses performed on finite element model, is equal to approximately 15000 MPa ( approximately 0.60 of the experimental measured concrete modulus). Moreover, the value of the concrete Young’s modulus leading the initial experimental frequency (8.2 Hz), based on the results of natural frequencies analyses performed on finite element model, is equal to approximately 10200 MPa (approximately 0.40 of the experimental measured concrete modulus). As a summary: (a) an equivalent elastic modulus equal to 0.60 Ec,UC or equal to 5.45 Ec,FC is necessary in order to match the initial experimentally measured frequency; (b) an equivalent elastic modulus equal to 0.40 Ec,UC or equal to 3.7 Ec,FC is necessary in order to match the final experimentally measured frequency. In the light of the above considerations it seems that the building exhibited an initial response very close to the theoretical uncracked response, which was never observed during the previous reversed cyclic tests performed on single walls and on a simple structure, which were used to develop the analytical formulations of the seismic capacity of the structural system. An insight on the interpretation of test results obtained from reversed cyclic tests can be found in (Palermo et al. 2012). The observed differences between the dynamic and static response may depend on the specific concrete which has been used (composed by high percentage of small aggregates with respect to the standard concrete typically used) or on the possible effect related to different type of loading (dynamic loading vs quasi static loading), e.g. strain-rate effects.
Table 10.1: The natural frequencies as obtained from finite elements models Condition UC FC
Fundamental natural frequency [Hz] 12.8 4.23
10.2. DISUSSION ON THE OVER-STRENGTH EXIBITHED BY THE BUILDING PROTOTYPE Based on the recorded accelerograms the time history of the following quantities has been evaluated: Total dynamic horizontal forces applied at the shaking table Ftot(t);
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Building prototype base shear Vbase(t); Building prototype base moment Mbase(t); Below stated are the formula used to evaluate each quantity: Ftot t
3
m a t m
table a feedback
i i
t m foundation a foundation t
(10.1)
i 1
Vbase t
3
m a t
(10.2)
i i
i 1
M base t
3
m a t h i i
i
(10.3)
i 1
where mi
is the i-th storey mass at the i-th storey
ai t
is the i-th floor acceleration at the i-th floor
mtable
is the mass of the shaking table;
a feedback t
is the shaking table acceleration;
m foundation
is the mass of the foundation;
a foundation t
is the acceleration of the foundation;
hi
i-th storey height (measured from the foundation)
The maximum values of each quantity are summarized in Table 10.2.
Table 10.2:Maximum total horizontal force Ftot, base shear Vbase and base moment Mbase as obtained from each seismic test Test ST1 ST2 ST3 ST4 ST5
Ftot [kN] 70 190 720 1240 1700
Vbase [kN] 45 130 450 820 1070
Mbase [kN m] 210 630 2100 4000 5300
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60
40
F [kN]
20
0
-20
-40
-60
-80 0
10
20
30
40
50
60
70
80
50
60
70
80
50
60
70
80
t [s]
(a) 50
40
30
20
F [kN]
10
0
-10
-20
-30
-40
-50 0
10
20
30
40
t [s]
(b) 20
15
10
M [t m]
5
0
-5
-10
-15
-20
-25 0
10
20
30
40
t [s]
(c) Fig. 10.2: time history responses for ST1: total force Ftot(t); base shear Vbase(t); base moment Mbase(t)
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(
)
200
150
100
F [kN]
50
0
-50
-100
-150
-200 0
10
20
30
40
50
60
70
80
90
50
60
70
80
90
50
60
70
80
90
t [s]
(a) 150
100
F [kN]
50
0
-50
-100
-150 0
10
20
30
40
t [s]
(b)
80
60
40
M [t m]
20
0
-20
-40
-60 0
10
20
30
40
t [s]
(c) Fig. 10.3: time history responses for ST2: total force Ftot(t); base shear Vbase(t); base moment Mbase(t)
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800
600
400
F [kN]
200
0
-200
-400
-600
-800 0
10
20
30
40
50
60
70
80
90
50
60
70
80
90
50
60
70
80
90
t [s]
(a) 500
400
300
200
F [kN]
100
0
-100
-200
-300
-400
-500 0
10
20
30
40
t [s]
(b)
250
200
150
100
M [t m]
50
0
-50
-100
-150
-200
-250 0
10
20
30
40
t [s]
(c) Fig. 10.4: time history responses for ST3: total force Ftot(t); base shear Vbase(t); base moment Mbase(t)
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1500
1000
F [kN]
500
0
-500
-1000
-1500 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
60
70
80
90
100
t [s]
(a) 800
600
400
200
F [kN]
0
-200
-400
-600
-800
-1000 0
10
20
30
40
50
t [s]
(b)
400
300
200
M [t m]
100
0
-100
-200
-300
-400
-500 0
10
20
30
40
50
t [s]
(c) Fig. 10.5: time history responses for ST4: total force Ftot(t); base shear Vbase(t); base moment Mbase(t)
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1500
1000
500
F [kN]
0
-500
-1000
-1500
-2000 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
60
70
80
90
100
t [s]
(a) 1000
500
F [kN]
0
-500
-1000
-1500 0
10
20
30
40
50
t [s]
(b) 600
400
M [t m]
200
0
-200
-400
-600 0
10
20
30
40
50
t [s]
(c) Fig. 10.6: time history responses for ST5: total force Ftot(t); base shear Vbase(t); base moment Mbase(t)
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Based on the analytical formulations presented in section 3.2 and assuming a distribution of the equivalent horizontal seismic forces proportional to the product of the story mass and the storey height (almost triangular) and a dynamic amplification factor equal to 1.3 (according to what experimentally observed, see section 9.1.), Table 10.3 gives the summary of the predicted values of base shear and base moment leading to the activation of the different stages (i.e. yielding or failure). In the last column of the table it is indicated if the theoretical values are reached during the tests.
Table 10.3:Maximum total horizontal force Ftot, base shear Vbase and base moment Mbase as obtained from each seismic test. Mechanism Orthogonal walls yielding Parallel walls yielding Parallel walls Flexural failure Parallel walls Shear failure Failure at the connection between wall and foundation
Vbase [kN] 480
Mbase [kN m] 2620
If reached during seismic tests Yes (ST4,ST5)
870
4760
Yes (ST5)
990
5410
Yes (ST5)
1200
6560
No
1510
8260
No
Inspection of Table 10.3 reveals that according to the assumption of above during tests 5 the parallel walls should reach the condition of flexural failure, while no visible damages were observed during the test. This is a further confirmation of the already observed result that the prototype building showed an unexpected seismic over-strength. A preliminary assessment of the observed over-strength can be done by evaluating the shear strength prior the concrete cracking (i.e. assuming a tensile strength of the concrete) according to the Modified Compression Field Theory, MCFT (Vecchio and Collins 1986). Figure 10.4 displays the shear stress vs the shear strain response for a the single panel, evaluated according to MCFT using the software Membrane 2000.
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Fig. 10.7: The section properties of the Nidyon panel as defined in Membrane 2000
Shear-xy 4.2
Shear Stress (MPa)
3.5
2.8
2.1
1.4
0.7
0.0 0.0
5.0
10.0
15.0
20.0
25.0
30.0
xy (mm/m)
Fig. 10.8: shear stress vs shear strain response of the Nidyon panel (Membrane 2000). It can be noted that, according to MCFT, the panel should exhibit a shear stress resistance at concrete cracking which is significantly higher than the ultimate shear stress resistance. This shear strength was not considered in the design phases because of it was not “observed” during the previous pseudo-static reversed cyclic tests.
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10.3. THE COMPARISON BETWEEN THE MOMENT AT THE BASE AND THE “INTERNAL MOMENT AT THE BASE” In this section the time history of the moment at the base, calculated as discussed in the previous section, is compared with the time history of the “internal moment” at the base calculated based on the deformations recorded by the deformometers placed at the base of the orthogonal walls. The calculation of the “internal moment” is conducted based on two limit conditions: Uncracked condition (UC); Fully cracked condition (FC); Figure 10.9 compares the time history of the base moment, calculated as discussed in the previous section, with the time history of the internal moment based on the UC and FC limit assumptions, respectively. The comparison are related to the results of ST 4 (PGA=1.0 g) The internal moment are calculated according to the following relationship: M int t W Ec (t )
(10.3)
Where: W
is the section strength modulus calculated according to UC limit assumption (i.e. assuming the full concrete contribution) or FC limit assumption (i.e. assuming concrete in cracked conditions).
Ec
is the experimental measured concrete Young’s modulus
(t)
is the deformation recorded by the deformometer at the base of Wall 1
The comparison of the graphs displayed in Figure 10.9 clearly highlights that the internal moment is higher than the external moment, assuming the uncracked limit condition. In detail maximum internal moment, Mint,max is approximately equal to 3·Mbase,max, where Mbase,max is the maximum base moment (approximately 4000 kN m, see table 10.1). The comparison of the graphs displayed in Figure 10.9 clearly highlights that the internal moment is sensibly less than the external moment, assuming the fully cracked limit condition. In detail the maximum internal moment, Mint,max , is approximately equal to 0.1·Mbase,max. This further confirms what already observed before: the prototype building condition is close to theoretical uncracked condition, while it is far from the fully cracked theoretical condition under a base acceleration of 1.0 g .
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(a)
(b)
(c)
(d)
Fig. 10.9: time history responses for ST5: total force Ftot(t); base shear Vbase(t); base moment Mbase(t) 10.4. CONCLUDING REMARKS The following concluding remarks appear from the preliminary interpretation of the experimental results: the prototype building exhibited an high seismic performance: it was able to sustain increasing levels of the seismic input up to 1.2 g PGA without visible damages; the prototype building seismic response was very close to that of the “theoretical” response of the uncracked structure up to approximately 1.0 g PGA (in terms of both natural frequencies and deformation response); the prototype building exhibited unexpected “over-strengths” which did not allow to observe the expected mechanisms of failure, whose analytical predictions were partially-
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based on the results of pseudo-static reversed cyclic tests performed on both full scale single panels (with and without openings) and on simple H-shaped building structure; the results of the previous pseudo-static reversed cyclic tests did not show any “concrete contribution” to the shear strength of the panels (i.e. the contribution due to the concrete tensile strength). Therefore this contribution has been neglected in the design phase; if the shear strength of the wall is evaluated according to the Modified Compression Field theory (i.e. accounting for the tensile strength contribution) a significantly higher strength (superior than the maximum shear applied during all the seismic tests) is obtained prior to the concrete cracking. the comparison between the time-history of the base moment (i.e. the base moment due to the applied “external” loads, obtained based on the recorded accelerations) and the internal base moment (i.e. the base moment due to the internal forces, obtained based on the recorded deformation) also confirmed that the response of the prototype building is close to that of the “theoretical” uncracked model.
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11. Conclusions The present document provides a detailed summary of the research project named "Seismic Behaviour of Structural Systems composed of Cast in Situ Concrete Walls", referred to as SeSyCoWa, aimed at assessing the seismic behaviour of a modern mid-rise reinforced concrete building composed of precast structural r/c panels. The report gives details on the design phase (i.e. both the rationale leading to the prototype building and the design of the specific prototype building are encompassed), building construction, testing, experimental results and preliminary interpretation of the experimental results. The SeSyCoWa research project is the last phase of a comprehensive research program which has been developed over the past 20 years through a number of experimental including: materials tests, uniaxial compression tests, diagonal compression tests, slip tests, out of plane tests, connections tests, in plane reversed cyclic tests, dynamic tests with vibrodyne. In details the results of the reversed cyclic tests allowed to assess the seismic behaviour of the structural system in terms of stiffness, strength, ductility and dissipative capacities. Based on those results, analytical formulations for the seismic design of mid-rise building structures were developed. In light of all the above, the main goal of the SeSyCoWa research project was to verify the predicted seismic capacity of the building trough shaking table tests. For this purpose, a 3-storey full scale prototype building has been tested at the Eucentre TRESS Lab. Both white noise tests and seismic tests (i.e. shaking table test with a base input consisting in a natural recorded accelerogram) were performed. The preliminary interpretation of the experimental results leads to the following fundamental observations: the prototype building exhibited a high seismic performance: it was able to sustain increasing levels of the seismic input up to 1.2 g PGA without visible damages;
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the prototype building seismic response was very close to that of the “theoretical” response of the uncracked structure up to approximately 1.0 g PGA (in terms of both natural frequencies and deformation response); the prototype building exhibited unexpected “over-strength” which did not allow to observe the expected mechanisms of failure, whose analytical predictions were partiallybased on the results of the pseudo-static reversed cyclic tests performed on both full scale single panels (with and without openings) and on simple H-shaped building structure; the results of the previous pseudo-static reversed cyclic tests did not show any “concrete contribution” to the shear strength of the panels (i.e. the contribution due to the concrete tensile strength). Therefore this contribution has been neglected in the design phase; if the shear strength of the wall is evaluated according to the Modified Compression Field theory (i.e. accounting for the tensile strength contribution) a significantly higher strength (superior than the maximum shear applied during all the seismic tests) is obtained prior to the concrete cracking. the comparison between the time-history of the base moment (i.e. the base moment due to the applied “external” loads, obtained based on the recorded accelerations) and the internal base moment (i.e. the base moment due to the internal forces, obtained based on the recorded deformation) also confirmed that the response of the prototype building is close to that of the “theoretical” uncracked model. Based on all the above observations it clearly appears that a structural system composed of r/c sandwich panels, which is designed according to current seismic design requirements, is able to withstand very high seismic shaking (more than 1.0 g) without exhibiting visible damages, thanks to the development of the so called “box behaviour” which induces only (significantly) in plane actions (shear and moments), well sustained by the sandwich structural r/c walls. Furthermore, it should be noted that the specific prototype building showed “unexpected” overstrengths, with respect to those exhibited during previous pseudo-static reversed cyclic tests. A possible explanation of the observed unexpected response is related to the different behaviour of the concrete observed during the pseudo-static tests and dynamic tests: while during the pseudostatic tests the concrete behaves essentially in cracked conditions (from the beginning of the tests), during the shaking table tests no visible cracks were observed suggesting a concrete uncracked behaviour. On this regard, if the strengths of the walls is computed accounting for the concrete
tensile
contributions
the
observed
“over-strength”
can
be
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