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1|S . R. Ch o wd h u r y e t . a l . , ICE R IE 2 0 1 3

Proceedings of the International Conference on Engineering Research, Innovation and Education 2013 ICERIE 2013, 11 13 January, SUST, Sylhet, Bangladesh

Preventing Soft Storey Irregularity in RC Frame Buildings

S.R. Chowdhury1,*, W. Hassan2, S. I. Zaber3, R. Mondal4, M. N. Sirajee5 1 2,3,4,5

Associate Professor, Ahsanullah University of Science and Technology Students, Ahsanullah University of Science and Technology, Dhaka, Bangladesh

Keywords: Soft Story, Equivalent Strut, Irregularities, Lateral Displacement, Simulation, Prevention.

Abstract :Soft storey is the one of which the rigidity is lower than any other storey due to the fact that it has not got the walls with the same properties the other ones have. If vertical load bearing structural elements and the partitioning wall continue in all storeys, there is no soft storey in the construction. Soft storey is generally present at the entrance (bottom) floors of the buildings. Because entrance floors of the buildings are utilized as bank branches, storeys, restaurants, offices, car parking and the upper storeys are used as dwellings. Soft storey is an irregularity which affects the behavior of a construction during a quake and also increases the construction costs. Such features are highly undesirable in buildings built in seismically active areas; this has been verified in numerous experiences of strong shaking during the past earthquakes. For this reason, soft storey should be avoided as much as possible. In case it is necessary, irregularities can be eliminated by increasing the lateral rigidity of this storey by putting up additional walls between single structural elements on the soft storey; placing diagonals between the columns and shear walls; increasing the rigidity of the soft storey by increasing beam-column size of the soft storey. To fulfill the above objectives, a 9 storied RC frame building modeled with the finite element software ETABS (under the action of earthquake loads in equivalent static) is analyzed in this study. A comparative study is made implementing the above mentioned approaches on the analytical model to prevent the irregularity resulted due to a soft story at the first floor on the basis of the material cost as well as other structural parameter such as drift.

1. INTRODUCTION Soft storey irregularity is one of the major reasons of the building collapse during earthquake. Many high-rise buildings in urban and semi urban areas in the world today have open first storey as an unavoidable feature. According to BNBC,1993, a soft storey is the one in which the lateral stiffness is less than 70% of that in the storey above or less than 80% of the average stiffness of the three stores above. If vertical load bearing structural elements and the partitioning wall continue in all storeys, there is no soft storey in the construction. Soft storey is generally present at the entrance floors of the buildings. Because entrance floors of the buildings are utilized as bank branches,

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stores, restaurants, offices, car parking and the upper storeys are used as dwellings. Soft storey can form at any level of a multi- storey building to fulfill required necessity. Amin et al., 2011 investigated the effect of soft story for multistoried reinforced concrete building frame considering equivalent static analysis. Soft story level was altered from ground floor to top floor. Mahmud et al., 2011 showed that the effect of soft story is more significant in case of less stiffer Reinforced Concrete frame structures. They analyzed the building frame with ANSYS to predict the inelastic behavior of RC frame building with brick masonry infill. Arlekar et al., 1997 analyzed (linear elastic analysis) the RC frame buildings using ETABS analysis package to highlight the importance of recognizing the presence of soft story in the analysis of building. Dogan et al., 2002 conducted a study to show the effect of soft storey on the behavior of construction in quake regions of Izmit and Duzce. The soft storey effect in any building changes the behavior of frame action due to the relative changes of stiffness and lateral load distribution mechanism and thus may induce changes in phenomenon like lateral displacement and inter-storey drift ratio. The approaches which will be used in this study have already been mentioned in the current state of the art individually to avoid the soft story irregularities. In this study, an attempt is made to implement all these approaches with the help of finite element software ETABS and find out the suitable possible ways to prevent the soft storey irregularities with respect to the total lateral displacement is concerned.

2. BUILDING STUDIED The plan layout of 3D reinforced concrete moment resisting frame building studied here is shown in Fig.1. The building is kept symmetric in both orthogonal directions in plan to avoid torsional response under pure lateral forces. Columns are taken to be square to keep the discussion focused only on the soft story effect, without being distracted by issues like orientation of columns. In this study soft story is generated only at ground level by providing no infill in that level, where the other storeys have proper infill effect. The effect of unreinforced masonry infill is modeled with equivalent strut model as per FEMA-273, 1997. Nine storied building modeled with equivalent strut as shown in Fig.2 (Elevation as well as 3D view) is selected for this study. 70% infill is provided (also shown in Fig. 1) during model generation and the remaining 30% is kept for functional purpose such as doors and windows.

Fig 1: Plan at a typical storey of the example building considered in the study In this study linear elastic analysis are performed with the help of finite element software, ETABS using equivalent static method. Earthquake and wind load are selected in form of lateral load as per rules of Bangladesh National Building Code, BNBC, 1993. Different building models (cases) are used to represent various ways to prevent soft story irregularities as shown in Fig. 3. All these buildings models are generated using equivalent strut to represent masonry infill at every floor (where masonry infill is used) according to FEMA-273, 1997. Strut is placed along one diagonal direction as lateral loading in X direction (Fig. 2 (a)) is considered. Descriptions of different building cases are described below: Case 1: Building has no infill walls. Building has modeled as bare frame; Case 2: Building has modeled considering soft storey at ground floor; Case 3: Building has modeled without soft storey; Case 4: Building has modeled by increasing the stiffness of the column (559mm*457.32mm) only at the soft storey level than those


in the upper storeys (457.32mm*457.32mm) to reduce the stiffness irregularities between soft storey and the story above; Case 5: Building has modeled by increasing the stiffness of the beam (559mm*254mm) only at the soft

(a) (b) Fig 2: A nine storied building frame with soft story at ground floor (a) Elevation and (b) 3D view from ETABS storey level than those in upper storeys (457.32mm*254.32mm) to reduce the stiffness irregularities between soft storey and the story above; Case 6: Building has modeled by providing shear wall at four corners only at the soft storey level; Case 7: Building has modeled by providing steel bracing (I-section) at periphery only at the soft storey level.

3. MODELING OF BUILDINGS Generally masonry infill is modeled using equivalent strut method. The approaches proposed by Paulay and Pristlay, 1992 lead to a simplification in the infilled frame analysis by replacing the masonry infill with an equivalent compressive strut. They assumed constant values for the strut width, between 12.5 to 25 percent of the diagonal dimension of the infill. Shahrin and Hossain, 2011 used the complex expression proposed by Smith and Carter, 1969 and Mainstone, 1974 to estimate the equivalent strut width. RC frames with unreinforced masonry walls can be modeled as equivalent braced frames with infill walls replaced by equivalent diagonal strut which can be used in rigorous nonlinear pushover analysis. FEMA-273 adopted Mainstone, 1974 suggested model to calculate to the strut area which was followed by Amin et al., 2011 and also used in this study. According to FEMA-273, the elastic in-plane stiffness of a solid unreinforced masonry infill panel prior to cracking shall be represented with an equivalent diagonal compression strut of width, w, given by Eq. 1. The equivalent strut shall have the same thickness and modulus of elasticity as the infill panel it represents. (1)

and a= equivalent strut width of infill, in; hcol=column height between centerlines of beams, in; hinf=height of infill panel, in; Efe = expected modulus of elasticity of frame material , psi; Icol= moment of inertia of column, in4;


rinf=diagonal length of infill panel, in; tinf=thickness of infill panel & equivalent strut, in and θ=angle whose tangent is the infill height-to-length aspect ratio, radians θ=tan-1( ) where Linf=length of infill panel, in.

Fig 3: Elevation of different building models (cases) considered in this study 10

Storey Level

8 6

case 2 (strut ) case 2 (brick) case 1 case 3

4 2 0

Fig 4: Geometric Characteristics specified by FEMA 356

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Displacement (mm) Fig 5: Total Lateral Displacement Profile for different building models

In this study, for modeling plane frame, the following material properties and geometrical properties have been used for beams, columns, masonry infill. Values of elastic moduli of concrete and masonry are taken as 24820Mpa and 700Mpa respectively. Poison‟s ratios for concrete and masonry are taken as 0.2 and 0.3 respectively. The unit weights of concrete and masonry are taken as 23.56 KN/m3 and 18.88 KN/m3. The live load on floor has been taken 2KN/m2. The cross-sectional area of column and beam was taken 45.7 cm × 45.7 cm and 45.7cm × 25.4 cm respectively. Infill walls and slabs are modeled as 25.4 cm and 14 cm thick respectively for all models. 210 km/hr.

4. RESULTS AND DISCUSSIONS


Storey Level

Storey Level

Lateral loading only in short direction are considered for results and discussion as lateral displacement are critical in that direction. Fig. 5 shows the variation of lateral displacement with story level for different building model. Case 1 shows the maximum displacement at higher story level as it represents the bare frame. Case 2 modeled with brick wall (here only „brick‟ can be told as a short name) only changing modulus of elasticity, unit weight and poison‟s ratio shows more displacement at story levels above 4 than that of case2 modeled with equivalent strut. 10 10 case 2 with case 2 with E=700 Mpa 8 8 E=700 Mpa case 2 with E=2200 Mpa 6 6 case 2 with E=2200 4 4 Mpa 2

2

0

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0 1 2 3 4 5 6 7 8 9 10 11 12 13 Displacement (mm)

Displacement (mm)

(a) Brick

(b) Strut

Fig 6: Variation of Lateral Displacement with Story height for different E using (a) Masonry (b) Strut Model

Top Displacement (mm)

Storey Level

Representing masonry wall with only change some basic properties seems overestimate lateral displacement at higher story levels. Case 2 modeled with strut (soft story at ground level) concede more displacement than case 3 (without any soft story). Fig. 6 shows a comparative view of using masonry infill of different elastic moduli for both building models. Fig. 6(a) and 6(b) show case 2 modeled with brick and case 2 modeled with strut respectively. In both cases lateral displacement is reduced with increasing modulus of elasticity of masonry infill. 16 10 70% Infill 9 Strut 15 8 variation in infill 55% infill 7 14 Strut 6 80% infill 5 13 Strut 4 No infill 3 12 2 11 1 0 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 40 50 60 70 80 90 100 Displacement (mm) Percentage of infill (a) (b) Fig 7: Lateral Displacement Profile for different percentage of Infill in Soft Story. Increasing the percentage of infill in the soft story (only at ground floor) reduces the lateral displacement which can be easily understood from Fig. 7. In Fig. 7(a) „No Infill‟ means there is no masonry infill in the ground floor representing soft story at ground floor. The higher the percentage of infill the lower the lateral displacement which is clearly depicted from Fig. 7(b). Fig. 8 shows the comparative scenario among different approaches of preventing soft story irregularities. Case 6 (incorporating share walls at four corners) proofs the least lateral displacement at all story levels. Bare frame model (case 1) is included in Fig. 8 just to show the comparison of each approach with respect to bare frame.


Increasing column stiffness (case 4) than beam stiffness (case 5) at soft story level is effective approach in reducing soft story irregularities. Case 7(Introducing steel bracing at periphery) is proved the second best approach among the approaches taken in this study to prevent soft story irregularities. 10

case 1 case 4 case 6 case 7 case 5

8 Storey Level

6

4 2 0 0

1

2

3

4

5

6 7 8 9 10 11 12 13 14 15 16 Displacement (mm) Fig 8: Comparison of different methods of preventing soft story irregularities.

3. CONCLUSION It is concluded from this study that lateral displacement is reduced with increasing modulus of elasticity of masonry infill. Equivalent diagonal struts are provided as suggested in FEMA-273, in place of masonry to generate infill effect. Increasing the percentage of infill in the soft story reduces the lateral displacement. The higher the percentage of infill the lower the lateral displacement. Incorporating share walls at four corners proofs the best approach in reducing soft story irregularities as far as total lateral displacement is concerned. Increasing column stiffness than beam stiffness at soft story level is effective approach for this purpose. Introducing steel bracing at periphery is also found an effective approach among the approaches taken in this study to prevent soft story irregularities. More general comments can be furnished if a nonlinear procedure commonly referred to as pushover analysis could be possible to apply. Same study could be performed for more number of storeys, changing the width of infill, putting flexible material between columns and walls on the storey atop the soft storey, considering soil-foundation-structure interaction and using multi-modal dynamic analysis.

4. REFERENCES Amin, M.R., Hasan, P. and Islam, B.K.M.A. (2011) „Effect of soft storey on multistoried reinforced concrete building frame‟, 4th Annual Paper Meet and 1st Civil Engineering Congress, December 22-24, Dhaka, Bangladesh,pp. 267-278. Arlekar J.N., Jain S.K. and Murty C.V.R. (1997) „Seismic Response of RC Frame Buildings with Soft First Storeys‟ Proceedings of the CBRI Golden Jubilee Conference on Natural Hazards in Urban Habitat, New Delhi BNBC (1993), “Bangladesh national building code”, Housing and Building Research Institute and Bangladesh Standards and Testing Institutions, Dhaka FEMA 273(1997), NEHRP Guidelines for the seismic rehabilitation of buildings, Federal emergency management agency, Washington D.C. USA. Decanini, L., Mollaioli, F., Mura A., Saragoni R. (2004) „Seismic Performance of Masonry Infilled R/C Frames‟, 13th World Conference on Earthquake Engineering, Vancouver, BC, Canada, August 1-6, paper No.165 Dogan, M., Kirac, N., Gonen, H. (2002) „Soft Story Behavior in an Earthquake and samples of Izmit-Duzce‟ Ecas Civil Engineering Symposium ,14 October, Middle East Technical University, Ankara, Turkey. Mainstone, R.J.(1974) „Supplementary notes on the stiffness and strength of infilled frames. Proc. of Institution of Civil Engineers supplement IV, pp. 57-90. Mahmud K, Sakib N. and Rahman M.R (2011) „ Effect od Soft Story in Reinforced Concrete Frame Structures‟, Proceedings of the Conference in Engineering Research, Innovation and Education, CERIE, 11-13 January, Sylhet. Pauley, T. and Priestley, M.J.N. (1992), „Seismic design of reinforced and masonry buildings‟, USA:Wiley Interscience Inc.


Shahrin, R.and Hossain, T.R.(2011) „Seismic performance evaluation of residential building in Dhaka city by using pushover analysis‟, 4th Annual Paper Meet and 1st Civil Engineering Congress, December 22-24, Dhaka, Bangladesh,pp. 279-286. Smith, B.S. and Coull, A. (1991) Tall building structures: analysis and design Singapore: John wiley and sons Inc. Smith, S.B. and Carter C. (1969) „A Method of Analysis for Infill Frames‟, Proceedings of the institution of Civil Engineers, part 2, vol.44,pp. 31-48