Pseudodynamic Tests on 1/2.5 scale R/C Frame ...

3rd World Conference on Structural Control Como, Italy, 7-12 Aprile 2002

Pseudodynamic Tests on 1/2.5 scale R/C Frame Equipped with Different Isolation Systems Mauro Dolce1, Donatello Cardone1, Felice C. Ponzo1, Domenico Nigro1 and Mario Nicoletti2 1 2

DiSGG, University of Basilicata, Macchia Romana Campus, 85100, Potenza, Italy. Italian National Seismic Survey, Rome, Italy.

ABSTRACT This paper presents the main aspects of the experimental program of pseudodynamic tests that are being carried out at the Laboratory of Structures of the University of Basilicata (USB), in co-operation with the Italian National Seismic Survey (SSN), to study the base isolation technique for the seismic retrofit of existing R/C framed buildings. The structural model is a 1/2.5-scale three dimensional frame, designed for gravity loads only. The frame has been equipped with different base-isolation systems, based on both current technologies (i.e. rubber bearings and steel energy dissipating components) and innovative ones (i.e. re-centring devices based on shape memory alloys). The design and the technological aspects of the application of base isolation to existing R/C buildings are dealt with in the paper.

1. INTRODUCTION R/C framed structures designed for gravity loads only undergo extensive damage during earthquakes. Actually, they have some inherent lateral strength, which may be sufficient to resist minor or moderate earthquakes, but the lack of proper detailing in the structural member and the disregarding of basic seismic design rules, inevitably, lead to inadequate seismic performance under strong earthquakes. The collapse of R/C framed structures is often the major cause of loss of life and money during an earthquake. As a consequence, the need to seismically retrofit these structures is a primary concern throughout the world. Traditional retrofitting techniques for R/C framed structures are based on the strengthening of structural members, e.g. through jacketing (Bracci et al., 1992), or on the introduction of new very stiff and strong structural members. A different approach is based on the insertion of an isolation system at the base of the structure (Naeim and Kelly, 1999). Seismic isolation, though not always applicable to existing buildings, has great potentials as a

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retrofitting technique, as it can prevent the superstructure from invasive and expensive works, and assure high protection levels to non structural members and content. The experimental tests carried out within MANSIDE (Dolce et al., 2001a) clearly confirmed the effectiveness of seismic isolation in R/C framed structures designed for underestimated horizontal forces. Some numerical studies (Dolce et al., 2000a) also proved the applicability of seismic isolation to structures designed for gravity loads only. New isolation devices, based on innovative materials, such as SMA - Shape Memory Alloys (Duerig, 1990), have been recently examined during experimental tests on reduced-scale structural models (Dolce et al., 2001a) and real buildings (Dolce et al., 2001b), whit excellent results. In order to better investigate the applicability and actual potential of seismic isolation in the retrofit of existing buildings, a comprehensive experimental pseudodynamic test program on large scale R/C framed structures, has been planned at the Laboratory of DiSGG (Department of Structures, Soil Mechanics and Applied Geology) of the University of Basilicata, in co-operation with the Italian National Seismic Survey. The study has two major scopes. The first is that of examining the technological and practical aspects related to the application of base isolation to existing framed structures. The second is that of comparing the effectiveness of different isolation systems, based on currently used technologies (i.e. rubber bearings and steel energy dissipating components) as well as innovative ones (i.e. SMA based re-centering devices). This paper presents an overview of the experimental program, and describes the isolation systems and the associated design criteria.

2. TEST MODEL Figure 1 shows the R/C structural model designed for the tests. It is a 1/2.5-scale three-dimensional frame, with four storeys and two bays in the longitudinal direction, having usual dimensions (3m interstorey height and about 5.3m bay span in the prototype scale). The lateral columns have constant cross section, corresponding to 25x30cm in the prototype scale, while the central column reduces from 35x30cm to 25x30cm in the prototype scale, when passing from the second to the third storey,. Beams have cross section corresponding to 30x50cm in the prototype scale at the first three storeys, and 30x40cm at the fourth storey. The structural models have been designed for gravity loads only, following the pre-1971Italian Code for R/C structures. Consequently, it is very poor in terms of both seismic resistance and ductility. Extreme care was taken in selecting concrete and steel for the model. Typically, a microconcrete exhibits greater deformability under compression and higher resistance under tension with respect to a standard concrete with the same compression resistance (Woo et al., 1988). In order to attenuate the aforesaid aspects, it was necessary to properly choose: (i) the granulometric curve of the aggregrate, (ii) the aggregrate/concrete ratio and (iii) the water/concrete ratio. Several mixes were manufactured and tested. The selected one had 30.1 N/mm2 average compression strength, being 25 N/mm2 the corresponding design value. The steel reinforcement was designed using the Italian pre-1971 R/C regulations on the full-scale prototype, using FeB38k steel (375 N/mm2 yield strength), then reporting to the model scale. 5.2mm diameter ribbed bars were used, having about 459 N/mm2 yield strength. Additional masses are placed on each floor slab, to take into account the non structural dead loads and a certain amount (1/3) of live loads, as well as the contribution due to mass similitude scaling. The additional masses were applied at each storey by means of concrete blocks, to get 52 kN additional weight at the first three storeys, and 26 kN at the fourth storey.

DOLCE, CARDONE, PONZO, NIGRO AND NICOLETTI

Frame X-direction

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External frame Y-direction

Internal frame Y-direction

Figure 1. Structural model selected for the tests

3. ISOLATION SYSTEM The isolation systems implemented and tested in this study are based on the coupling between steel-PTFE sliding bearings and three different dissipating and/or re-centring devices.

E poxy glu e

(b)

(a )

S Is M oAlaisola to retorin LM F RIusbber o la toisola r e etor la s to m er ic o

IsUo-shaped la t o re steel c o n pla e letes m e n t i i a c c ia io S teel-teflon sliding bearings S litte a c c ia io -t e flo n (c)

Figure 2. (a) Global view of the model equipped with the stiffening/isolation system, (b) close up view of the bearing/restraining device and (c) arrangement of the isolation systems at the base of the model

Originally, the R/C model was realized without basement. The columns of the first storey

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were then anchored to the bearing/restraining system shown in figure 2(b), which is made up of (i) a steel socket, (ii) a series of steel plates with oval-shaped holes and (iii) one steel-PTFE sliding bearing. The R/C columns were inserted in the steel socket, previously filled with epoxy glue. After glue hardening, steel dowel bars were transversely inserted in the columns, from one to the opposite side, in order to apply a beneficial lateral confinement, such as to obtain an effective joint restrain at the base of the column of the first storey. The steel plates below the socket, connect the base of the columns with the beams of the base grid, consisting of two adjacent C-sections having flexural stiffness about three times that of each R/C column, and with the steel plates of the cross bracings. The role of the stiffening system shown in figure 2(c) is that of simulating the in-plane stiffness of a real floor. At the bottom there are placed six lubricated steel-PTFE sliders, supporting the weight of the superstructure, while allowing for large horizontal displacements.

4. DESIGN OF THE ISOLATION SYSTEMS Three different isolation systems have been considered. For all of them the function of supporting the weight of the superstructure is played by sliders, while three alternative dissipating and/or re-centring devices characterize the seismic behaviour of each isolation system. Their design was based on the results of a pushover analysis, carried out with the finite element program DRAIN-3DX (Prakash et al., 1994). Its fibre element (element type 15) was used to model beams and columns of the R/C frame. The constitutive laws of concrete and steel were fitted to the corresponding experimental behaviour. A triangular distribution was assumed for the seismic forces, as shown in figure 3(a). Gravity load effects were included in the analyses. Figure 3(b) shows the base shear vs. top floor displacement relationship obtained from pushover analysis on the fixed-base model. As can be seen, the ultimate strength of the frame turns out to be equal to about 21 KN, i.e. about 0.08W, being W = 265 kN the total weight of the model. Base shear (kN)

F

25 20 15 10 5

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0 0

(a)

10

20

30

40

50

Top floor displacement (mm)

Figure 3. (a) Distribution of seismic forces for pushover analysis and (b) top floor displacement vs. base shear relationship

The design of each isolation system was aimed at guaranteeing that the maximum base shear produced by the earthquake at 0.3g PGA (design seismic intensity) was compatible with the ultimate strength of the frame provided by pushover analysis.

DOLCE, CARDONE, PONZO, NIGRO AND NICOLETTI

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The kernel components of the steel-based isolation device consist of U-shaped steel plates, as shown in figure 4(a), plastically deformed through in a combined roller bending and torsion mechanism. The steel components have been arranged inside the device according to the configuration shown in figure 4(b). The dimensions and number of steel components have been selected so that the plastic force of the device plus the friction force of the sliding bearings (about 3% the weight of the structure) was less than the ultimate strength of the R/C frame, reduced by a safety coefficient equal to 1.15. The geometric characteristics of the U-shaped stripes, were determined with the design formula reported in (Dolce et al., 1996). Two groups of four U-shaped plates, having (see figure 4(a)) b = 45mm, h = 3mm, R = 24mm, L = 100mm and H = 54mm, are used. About 5 kN plastic force is expected for each group.

(b)

(a)

Figure 4. (a) Kernel component of the steel isolation device and (b) arrangement inside the device

This type of isolation device exploits the superelastic properties of Ni-Ti SMA wires subjected to tension loading-unloading cycles. The device used in this study represents an evolution of that described in (Dolce et al., 2000b), already tested in previous experimental campaigns (Dolce et al., 2001a, Dolce et al., 2001b). Figure 5 shows the typical experimental force vs. displacement diagram. As can be seen, the device exhibits a strong supplemental re-centring capacity which counteracts the friction resistance of the sliders. The design procedure was similar to that described for the steel isolation system. Eight 1.84mm diameter wire loops, pre-strained at about 2%, are used for each device. The force exerted by the device is expected to vary from 3.25 kN, at small amplitudes, up to 6.5 kN at 100mm. 12 8

(kN)

4 0 -4 -8

(b) -1 2 -150 -1 00

(a)

-50

0

50

1 00

15 0

(mm )

Figure 5. (a) SMA-based isolation device and (b) typical mechanical behaviour

The rubber devices have been designed according to an iterative procedure based on the use of the elastic response spectra relevant to different equivalent viscous damping values. Firstly, the maximum acceptable structural acceleration (a’eff) has been evaluated, as the ratio between the pushover resistance (Fpushover), affected by a proper safety coefficient (SF = 1.15),

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and the mass of the model (M). Then, the stiffness (Krub) of the rubber devices compatible with the previous acceleration value has been found. In the proposed design procedure, the equivalent viscous damping (ξtot) takes into account the energy dissipation capacity (W) of both rubber and sliding bearings.

5. CONCLUSION An experimental program of pseudodynamic tests on a 1/2.5-scale R/C framed structure is in progress at the Laboratory of Structures of the University of Basilicata. The structural model was designed for gravity loads only and then seismically retrofitted using three different isolation systems, based on both currently used materials (i.e. rubber and steel) and innovative ones (i.e. shape memory alloys). The experimental results will produce important information on the effectiveness and applicability of base isolation, as a retrofitting techniques for existing R/C buildings designed for gravity loads only, and of the actual behavior of different seismic isolation systems

REFERENCES Bracci J.M., Reinhorn A.M. and Mander J.B. (1992), Evaluation of Seismic Retrofit of Reinforced Concrete Frame Structures, Technical Report NCEER-92-0031, NCEE, SUNY/Buffalo. Dolce M., Filardi B., Marnetto R. and Nigro D. (1996) Experimental Tests and Applications of a New Biaxial Elastoplastic Device for the Passive Control of Structures, Proc. 4th World congress on joint sealants and bearing systems for concrete structures, California. Dolce M., Masi A., Telesca F. R. (2000a) Applicabilità dell’isolamento alla base per l’adeguamento sismico di edifici esistenti in c.a. (in italian). Atti del Workshop su ‘Protezione Sismica dell’Edilizia Esistente e di Nuova Edificazione Attraverso Sistemi Innovativi’, Napoli. Dolce M., Cardone D. and Marnetto R. (2000b) Implementation and Testing of Passive Control Devices Based on Shape Memory Alloys, Earthquake Engineering and Structural Dynamics, 29 (7): 945-968. Dolce M., Cardone D. and Ponzo F.C. (2001a) Comparison of Different Passive Control Systems for R/C Frames through Shaking Table Tests, Proc. 5th World Congress on Joints, Bearings and Seismic Systems for Concrete Structures, Roma. Dolce M., Cardone D., Ponzo F.C., Bixio A.R. and Nigro D. (2001b) The Behaviour of SMA Isolation Systems during the Full-Scale Release Tests of the Rapolla’s Building, Proc. 5th World Congress on Joints, Bearings and Seismic Systems for Concrete Structures, Roma. Dolce M., Cardone D., Ponzo F.C., Nigro D. and Nicoletti M. (2001c). Confronto sperimentale di diversi sistemi di isolamento sismico su telaio in c.a. in scala 1:2.5 (in italian), Atti del X Convegno ANIDIS ‘L’ingegneria Sismica in Italia’, Potenza. Duerig T.W., Melton K.N., Stoeckel D. and Wayman C.M. 1990, Engineering aspects of shape memory alloys, Butterworth-Heinemann Ltd, London. Naeim F. and Kelly J.M. (1999) Design of Seismic Isolated Structures, John Wiley & Sons Ltd. Prakash V., Powell G.H., and Campbell S. (1994), DRAIN-3DX, Report No. UCB/SEMM-94/07, Department of Civil Engineering, University of California at Berkeley. Woo K., El-Atter A. and White R.N. (1988) Small-scale modelling techniques for reinforced concrete structures subjected to seismic loads. Technical Report NCRRR88-0041. State University of New York at Buffalo.