Effects of Plan Dimensions, Seismic Zone, Infill on Storey Drifts and Force Response of L-Shaped Reinforced Concrete Buildings

Recently it has become mandatory to design all the civil engineering structures including building frames for the earthquake effects in addition to dead load, live load and wind load effects. The present work deals with the determination of storey drifts and force response of 20-storeyed reinforced concrete L-shaped buildings located in different seismic zones using ETABS 2013 Ultimate 13.2.2. The effects of plan dimensions, severity of seismic zone, infill walls on the storey drifts and force response have been evaluated. It is observed that the absolute maximum storey drift occurs in Zone V and that the effect of presence of infill walls in the analysis is to reduce the storey drifts. The influence of infill wall on the moments in transfer girders and main beams is not insignificant. Both the design ultimate positive and negative moments in transfer girders and main beams decrease in magnitude when the effect of infill wall is considered in the analysis. The response spectrum method predicts lower maximum storey drift in x-direction compared to the equivalent static lateral force method in all the cases. The response spectrum method predicts higher maximum storey drift in y-direction compared to the equivalent static lateral force method in all the cases. Keywords—Reinforced Concrete Buildings; Storey Drift; Force Response; Infill Wall; Seismic Zone


INTRODUCTION
Building Codes specify that the effects due to earthquake load be considered in addition to those due to dead, live load and wind loads. A vast literature on dynamic analysis exists and a few of them are briefly mentioned here. Wakchaure M R and Ped S P [1] studied the effects of infill in high rise buildings. The infill walls were modeled as equivalent single strut by using the FEMA-356 approach. Mohammed Yousuf and P M Shimpale [2] carried out dynamic analysis for G+5 storied buildings located in seismic zone IV. They considered a rectangular symmetrical, C-shape, L-shape and irregular L unsymmetrical buildings for the analysis. The analysis was carried out by using the ETABS 9.5 software. Amin Alavi and P Srivinivasa Rao [3] studied the behavior of the 5-storied buildings located in seismic zone V. The buildings consisted of eight different configurations with re-entrant corners. Himanshu Gaur et al. [4] analyzed the horizontally irregular buildings for their stability using STAAD Pro software. They considered the 20-storeyed buildings of different shapes like L, U and H-shape for the analysis, each shape having different lateral length ratios. M G Shaikh and Hashmi S Shakeeb [5] investigated the seismic performance of L-shaped building with varying bay length and storey height. The buildings were modelled using STAAD Pro V8i software. The results obtained for infill and without infill building models were compared. Ravikumar C M et al. [6] studied the seismic performance of the buildings which are having irregularities in plan with geometric and diaphragm continuity, re-entrant corners, vertical irregularity with setback and also buildings resting on sloping ground. S Mahesh and Dr P B Panduranga Rao [7] studied the behavior of the G+11 storied building of regular and irregular configurations subjected to earthquake and wind load using ETABS and STAAD Pro V8i software. B Srikanth and V Ramesh [8] studied the earthquake response of a 20-storeyed building by seismic coefficient and Response spectrum methods.
Pravin Ashok Shirule and Bharti V Mahajan [9] conducted the parametric studies on G+13 storeyed RC frame building with asymmetric column distribution with and without shear wall by using response spectrum method of analysis.
A E Hassaballa et al. [10] carried out the seismic analysis of a multi-storied RC frame building situated in Khartoum city using STAAD Pro software. Critical damping of 5% was considered in response spectrum method of analysis. Ramesh Konakalla et al. [11] studied the response of the 20-storeyed building by linear static analysis using STAAD Pro software. One regular symmetric model and three vertical irregular models were considered in the analysis. S.S. Patil et al. [12] carried out the response spectrum analysis for G+14 storeyed building situated in the seismic zone IV using STAAD Pro software. The buildings were modeled as RC bare frame, bare frame with bracing and bare frame with shear wall in the analysis. Bracing and shear walls were located at different locations and directions in the building. Haroon Rasheed Tamboli and Umesh.N.karadi [13] performed the seismic analysis on ten storey buildings considering three cases i) bare frame ii) infill frame iii) infill with ground soft storey and using ETABS software. Seismic zone III and 5% damping was considered in the analysis. Infill was modeled as an equivalent diagonal strut in the analysis. Mohit Sharma and Savitha Maru [14] carried out static and dynamic analyses on G+30 storeyed regular building using STAAD Pro software. Seismic zones II and III and medium soil type were considered in the analysis. P.B Prajapathi and Prof.Mayur G. Vanza [15] analysed 10 storeyed RCC residential buildings with different plan configurations and studied the influence of plan irregularity on the building. Static and dynamic analyses were carried out using SAP software. For dynamic analysis, response spectrum method and time history methods were used. Md Irfanullah and Vishwanath. B. Patil [16] studied the behavior of the building when subjected to seismic loading with various arrangements of infill. The building was having five bays in both X and Y directions and situated in seismic zone IV. Models considered for the analysis were i) Bare frame ii) full infill frame iii) infill in all floor except below plinth iv) infill with first floor as soft storey v) Infill with soft storey at first floor and basement vi) Infill with soft storey at first and basement and infill provided in swastika pattern in ground floor. Equivalent static analysis was carried out by using ETABS 9.6 software.

Details of Buildings, Loads and Load Combinations Considered
L-shaped Reinforced Concrete Buildings of 20 storeys having soft storey, floating columns and transfer girders with and without infill are analyzed for all loading combinations specified by IS Codes using ETABS software. The effects of the following parameters: 1) L 1 /L 2 ratio (L 1 and L 2 are defined later), 2) Location of building and the corresponding seismic zone, 3) Infill walls or No infill walls on (a) storey drifts and (b) maximum ultimate forces and moments in the main beams and transfer girders are evaluated by performing the stiffness analysis using ETABS Version 2013 Ultimate 13.2.2 software. In the present work, L-shaped reinforced concrete buildings having a foundation depth of 2.0 m below existing ground level, plinth height = 0.5 m and 20 storeys each of 3 m height located in seismic zones II, III, IV and V (Infill and No infill) are considered. In all the cases the first storey (ground floor) is a soft storey. The floating columns start from the top of the 15th floor and extend up to the roof. These are marked as FC in Fig. 1. The other columns shown in Fig. 1 extend up to the roof starting from footing top (regular columns). The floating columns are supported by transfer girders (marked as TB1 and TB2) spanning between regular columns. The dimensions L 1 and L 2 are as defined in Fig. 1. The sizes of the beams and columns are given in Table 1. All the slabs including the roof are of 150 mm thickness. M50 grade concrete is used for all slabs, beams and columns.  The load combinations used for the serviceability limit state are shown in Table 3. The effect due to seismic loading is evaluated using (i) Equivalent Static Lateral Force Method and (ii) Response Spectrum Method separately. The more critical value obtained from these two methods is considered in the design. The effect of the infill wall is accounted in the analysis by treating it as a diagonal strut in accordance with the recommendations of FEMA 356.

Storey Drifts (a) Design Storey Drifts in X-Direction (No Infill)
The storey drifts in x-direction for L-shaped buildings with no infill are given in Table 4 for various values of L 1 /L 2 ratio and zones II and III and in Table 5 for various values of L 1 /L 2 ratio and zones IV and V. Each storey drift entry in the table represents the maximum value obtained by considering all load combinations specified by the relevant IS Codes (called design storey drift). The following observations are made from Table 4 for L-Shaped Buildings in Zone II (No Infill)  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.18.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no.4.  As L 1 /L 2 ratio increases the maximum design storey drift also increases. When L 1 /L 2 ratio =1.0, the value is 1.08 mm.
The following observations are made from Table 4 for L-Shaped Buildings in Zone III (No Infill):  For L 1 /L 2 ratio = 0.25, 0.50 and 0.75, maximum design storey drift occurs at floor no.18.  For L 1 /L 2 ratio = 1.0, maximum design storey drift occurs at floor no. 4.  As L 1 /L 2 ratio increases the maximum design storey drift also increases. When L 1 /L 2 ratio =1.0, the value is 1.08 mm. The following observations are made from Table 5 for L-Shaped Buildings in Zone IV (No Infill):  For all L 1 /L 2 ratios, maximum design storey drift occurs at floor no.18.  As L 1 /L 2 ratio increases the maximum design storey drift also increases. When L 1 /L 2 ratio =1.0, the value is 1.51 mm.
The following observations are made from Table 5 for L-Shaped Buildings in Zone V (No Infill):  For all L 1 /L 2 ratios, maximum design storey drift occurs at floor no.18.
 As L 1 /L 2 ratio increases the maximum design storey drift also increases. When L 1 /L 2 ratio =1.0, the value is 2.25 mm.

(b) Design Storey Drifts in Y-Direction (No Infill)
The storey drifts in y-direction for L-shaped buildings with no infill are given in Table 6 for various values of L 1 /L 2 ratio and zones II and III and in Table 7 for various values of L 1 /L 2 ratio and zones IV and V. Each storey drift entry in the table represents the maximum value obtained by considering all load combinations specified by the relevant IS Codes. The following observations are made from Table 6 for L-Shaped Buildings in Zone II (No Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.6.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no. 5.  As L 1 /L 2 ratio increases the maximum design storey drift decreases. When L 1 /L 2 ratio =0.25, the value is 2.52 mm.
The following observations are made from Table 6 for L-Shaped Buildings in Zone III (No Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.6.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no. 5.  As L 1 /L 2 ratio increases the maximum design storey drift decreases. When L 1 /L 2 ratio = 0.25, the value is 2.52 mm. The following observations are made from Table 7 for L-Shaped Buildings in Zone IV (No Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.6.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no.5.  As L 1 /L 2 ratio increases the maximum design storey drift decreases. When L 1 /L 2 ratio =0.25, the value is 2.23 mm.
The following observations are made from Table 7 for L-Shaped Buildings in Zone V (No Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.5 and 6.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no.5  As L 1 /L 2 ratio increases the maximum design storey drift decreases. When L 1 /L 2 ratio =0.25, the value is 3.03 mm.
The values of maximum design storey drifts for all L 1 /L 2 ratios and zones are listed separately for X-direction and Ydirection in Table 8.  From Table 8, it can be observed that:  The maximum design storey drift in x-direction, for any given zone, increases with L 1 /L 2 ratio.  The maximum design storey drift in y-direction, for any given zone, decreases with L 1 /L 2 ratio.  The absolute maximum (maximum of maximums) design storey drift in x-or y-direction occurs in zone V.
 The maximum design storey drift in y-direction is greater than that in x-direction for the same zone and L 1 /L 2 ratio in majority of cases.
(c) Design Storey Drifts in X-Direction (Infill) The storey drifts in x-direction for L-shaped buildings with infill are given in Table 9 for various values of L 1 /L 2 ratio and zones II and III and in Table 10 for various values of L 1 /L 2 ratio and zones IV and V. Each storey drift entry in the table represents the maximum value obtained by considering all load combinations specified by the relevant IS Codes. The following observations are made from Table 9 for L-Shaped Buildings in Zone II (Infill):  For all the L 1 /L 2 ratios, maximum design storey drift occurs at floor no.4.  As L 1 /L 2 ratio increases the maximum design storey drift increases. When L 1 /L 2 ratio =1.0, the value is 1.04 mm.
The following observations are made from Table 9 for L-Shaped Buildings in Zone III (Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.18  For L 1 /L 2 ratio =0.50, 0.75 and 1.0, maximum design storey drift occurs at floor no.4.  As L 1 /L 2 ratio increases the maximum design storey drift also increases. When L 1 /L 2 ratio =1.0, the value is 1.06 mm.  The following observations are made from Table 10 for L-Shaped Buildings in Zone IV (Infill):  For L 1 /L 2 ratio = 0.25, 0.50 and 0.75, maximum design storey drift occurs at floor no.18.  For L 1 /L 2 ratio = 1.0, maximum design storey drift occurs at floor no.4.  As L 1 /L 2 ratio increases the maximum design storey drift also increases. When L 1 /L 2 ratio =1.0, the value is 0.92mm.
The following observations are made from Table 10   For L 1 /L 2 ratio = 1.0, maximum design storey drift occurs at floor no.4.  As L 1 /L 2 ratio increases the maximum design storey drift also increases. When L 1 /L 2 ratio =1.0, the value is 1.25mm.

(d) Design Storey Drifts in Y-Direction (Infill)
The storey drifts in y-direction for L-shaped buildings with infill are given in Table 11 for various values of L 1 /L 2 ratio and zones II and III and in Table 12 for various values of L 1 /L 2 ratio and zones IV and V. Each storey drift entry in the table represents the maximum value obtained by considering all load combinations specified by the relevant IS Codes. The following observations are made from Table 11 for L-Shaped Buildings in Zone II (No Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.5 and 6.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no.5  As L 1 /L 2 ratio increases the maximum design storey drift decreases. When L 1 /L 2 ratio =0.25, the value is 2.35mm.
The following observations are made from Table 11 for L-Shaped Buildings in Zone III (Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.5 and 6.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no.5  As L 1 /L 2 ratio increases the maximum design storey drift decreases. When L 1 /L 2 ratio =0.25, the value is 2.35 mm. The following observations are made from Table 12 for L-Shaped Buildings in Zone IV (Infill):  For L 1 /L 2 ratio = 0.25, maximum design storey drift occurs at floor no.5 and 6.  For other values of L 1 /L 2 ratio, maximum design storey drift occurs at floor no.5  As L 1 /L 2 ratio increases the maximum design storey drift decreases. When L 1 /L 2 ratio =0.25, the value is 2.08 mm.
The following observations are made from Table 12 for L-Shaped Buildings in Zone V (Infill):  For all L 1 /L 2 ratios, maximum design storey drift occurs at floor no.5.  As L 1 /L 2 ratio increases the maximum design storey drift also decreases. When L 1 /L 2 ratio =0.25, the value is 2.84 mm.
The values of maximum design storey drifts for all L 1 /L 2 ratios and zones are listed separately for X-direction and Ydirection in Table 13. From Table 13, it can be observed that:  The maximum design storey drift in x-direction, for any given zone except zone V, increases with L 1 /L 2 ratio. In the case of zone V, the maximum design storey drift increases initially, becomes maximum at L 1 /L 2 ratio = 0.75 and later decreases.  The maximum design storey drift in y-direction, for any given zone except zone III, decreases with L 1 /L 2 ratio. In the case of zone III, the maximum design storey drift decreases initially, becomes minimum at L 1 /L 2 ratio = 0.75 and later increases.  The absolute maximum (maximum of maximums) design storey drift in x-or y-direction occurs in zone III at L 1 /L 2 ratio = 1.0.
 The maximum design storey drift in y-direction is greater than that in x-direction for the same zone and L 1 /L 2 ratio.

Variation of Design Ultimate Positive Moment and Design Ultimate Negative Moment in Transfer Girders TB1 and TB2
The design ultimate positive and negative moments in transfer girders are given in Tables 14 and 15.  From the results obtained, the following are observed in regard to transfer girders:  The variation with L 1 /L 2 ratio is insignificant.  The variation with zone is also insignificant.  The influence of infill wall on the moments is significant. Both the design ultimate positive and negative moments decrease in magnitude when the effect of infill wall is considered in the analysis as indicated by Tables 14 and 15.

Variation of Design Ultimate Positive Moment and Design Ultimate Negative Moment in Main Beams
The design ultimate positive and negative moments in main beams are given in Tables 16 and 17.  From the results obtained, the following are observed in regard to main beams:  The variation with L 1 /L 2 ratio is not significant.  The variation with zone is also not significant.
 The influence of infill wall on the moments is significant. Both the design ultimate positive and negative moments decrease in magnitude when the effect of infill wall is considered in the analysis as indicated by Tables 16 and 17.

Comparative Study of Equivalent Static Lateral Force
Method and Response Spectrum Method

Loading Combinations Considered
For the purpose of comparing the two methods, the load combinations shown in Table 18 are considered.

Maximum Storey Drifts in X-Direction
The maximum values of storey drift in x-direction for various values of L 1 /L 2 ratio and seismic zone are given in Tables 19  through 22 for both infill and no infill.     From Tables 19 through 22, the following observations are made:  The maximum storey drift in x-direction increases monotonically with the severity of the zone.  Maximum value of storey drift in x-direction in any zone occurs when L 1 /L 2 ratio is unity.  The storey drift in x-direction in any case is smaller when infill is considered in the analysis.  The response spectrum method predicts lower maximum storey drift in x-direction compared to the equivalent static lateral force method in all cases.

Maximum Storey Drifts in Y-Direction
The maximum values of storey drift in y-direction for various values of L1/L2 ratio and seismic zone are given in Tables 23  through 26 for both infill and no infill.     From Tables 23 through 26, the following observations are  made:  The maximum storey drift in y-direction increases monotonically with the severity of the zone.  Maximum value of storey drift in y-direction in any zone occurs when L 1 /L 2 ratio is 0.25 as per ESLFM and 0.5 as per RSM.  The storey drift in y-direction in any case is smaller when infill is considered in the analysis.  The response spectrum method predicts higher maximum storey drift in y-direction compared to the equivalent static lateral force method in all cases.

Design Storey Drifts
 The absolute maximum design storey drift in x-or ydirection occurs in Zone V.  The maximum design storey drift in x-or y-direction for any zone and any value of L 1 /L 2 ratio is smaller when infill wall is considered in the analysis. Thus the effect of infill walls is to reduce the storey drifts.
(i) No Infill  As L 1 /L 2 ratio increases the maximum design storey drift in x-direction also increases in all zones. The maximum design storey drift in x-direction increases monotonically with the seismic severity of the zone.  In zones II and III, the maximum design storey drift in x-direction occurs either at floor no.4 or 18. In zones IV and V, the maximum design storey drift in x-direction occurs at floor no.18.  As L 1 /L 2 ratio increases the maximum design storey drift in y-direction decreases monotonically with increase in L 1 /L 2 ratio in all the zones.  As seismic severity of the zone increases, the maximum design storey drift in y-direction varies and is maximum for zone V.  In all the zones, the maximum design storey drift in y-direction occurs either at floor no.5 or 6  The maximum design storey drift in x-direction, for any given zone, increases with L 1 /L 2 ratio.  The maximum design storey drift in y-direction, for any given zone, decreases with L 1 /L 2 ratio.  The absolute maximum (maximum of maximums) design storey drift in x-or y-direction occurs in zone V.  The maximum design storey drift in y-direction is greater than that in x-direction for the same zone and L 1 /L 2 ratio in majority of cases.
(ii) With Infill  In zone II, the maximum design storey drift in xdirection occurs at floor no.4. In zones III, IV and V, the maximum drift in x-direction occurs at either floor no.4 or 18.  As L 1 /L 2 ratio increases the maximum design storey drift in x-direction also increases in all zones.
 As L 1 /L 2 ratio increases the maximum design storey drift in y-direction decreases monotonically with increase in L 1 /L 2 ratio in all the zones.  In zones II, III and IV, the maximum design storey drift in y-direction occurs either at floor no.5 or 6. In zone V the maximum design storey drift in ydirection occurs at floor no.5.  As seismic severity of the zone increases, the maximum design storey drift in y-direction varies and is maximum for zone V.
 The maximum design storey drift in x-direction, for any given zone except zone V, increases with L 1 /L 2 ratio. In the case of zone V, the maximum design storey drift increases initially, becomes maximum at L 1 /L 2 ratio = 0.75 and later decreases.  The maximum design storey drift in y-direction, for any given zone except zone III, decreases with L 1 /L 2 ratio. In the case of zone III, the maximum design storey drift decreases initially, becomes minimum at L 1 /L 2 ratio = 0.75 and later increases.  The absolute maximum (maximum of maximums) design storey drift in x-or y-direction occurs in zone III at L 1 /L 2 ratio = 1.0.  The maximum design storey drift in y-direction is greater than that in x-direction for the same zone and L 1 /L 2 ratio.

Design Ultimate Moments in Transfer Girders and Main Beams
 The variation with L 1 /L 2 ratio and severity of seismic zone is not significant.  The influence of infill wall on the moments is significant. Both the design ultimate positive and negative moments decrease in magnitude when the effect of infill wall is considered in the analysis.

Equivalent Static Lateral Force Method Versus Response Spectrum Method (L-Shaped Buildings)
 The response spectrum method predicts lower maximum storey drift in x-direction compared to the equivalent static lateral force method in all the cases. The response spectrum method predicts higher maximum storey drift in y-direction compared to the equivalent static lateral force method in all the cases.