Seismic Analysis of Soft Storey Buildings Considering Structural and Geometrical Parameters

Soft storeys in a high rise building play an important role on its seismic performance. At the soft storey level, there is a discontinuity in the rigidity of the structure due to lack of infill walls or due to variation in floor height. It is this continuity which is the cause of structural failure of multi stored buildings under earthquake loads. In this study, seismic analysis of soft storey building frames have been carried out considering 3 building plans, 15 soft storeys cases and 20 load combinations. Soft storeys have been created by varying the floor heights and effect of infill is ignored. In this way, total 45 frames are analysed. STAAD.pro software has been used for analysis purpose. Results are collected in terms of max. moment, max. storey displacements, max. shear force, max. axial force and max. drift, which are critically analysed to quantify the effects of various parameters .


introduCtion
Buildings are classified as having a "soft storey", if that level is less than 70% as stiff as the floor immediately above it, or less than 80% as stiff as the average stiffness of the three floors above it.Often, open-ground-storey buildings are called soft-storey buildings, even though their ground storey may be soft and weak.Generally, the soft or weak storey usually exists at the ground storey level, but it could be at any other storey level.Soft storey buildings, having first storeys much less rigid than the storeys above are particularly susceptible to earthquake damage because of large, unreinforced openings on their ground floors.Behaviour of soft storey building to seismic forces has to be critically examined considering various geometrical and seismic parameters.Some of the prominent literature on the topic are as follows: Ari Wibowo, et. al (2010) concluded that precast soft storey system have sufficient displacement capacity for lower seismic regions, but the performance was considered marginal for higher seismic regions.Plumier, et. al (2005) worked towards promoting safety without too much changing the constructional practice of reinforced concrete structures.He observed that most frequent failure mode of reinforced concrete (R.C.) moment-frame buildings was the so called soft storey mechanism.Mo and Chang (1995) described a practical system combining a flexible first storey with sliding frictional interfaces.The system utilized Teflon sliders at the top of the first storey reinforced concrete framed shear walls to carry a portion of the superstructure.Chen and Constantinou (1990) observed that the practical system deliberately introduces flexibility to the first storey of structures, The system utilised Teflon sliders to carry a portion of the superstructure.Energy dissipation was provided by the first storey ductile columns and by the Teflon sliders.Sivakumaran and Balendra (1994) presented a method of seismic analysis of three-dimensional asymmetric multistorey buildings founded on flexible foundations.The building-foundation system considered in this study was a linear elastic N-storey asymmetric building with a rigid footing resting on the surface of a linear elastic soil half-space.The method of analysis also included the P-Δ effects, in which the additional overturning moment and torsional moment at each storey due to P-Δ effects had been replaced by fictitious lateral forces and torques.Zekai Sen (2010) concluded that earthquake hazard assessment of existing buildings was among the most important issues for pre-and post-earthquake warning, preparation, vulnerability, and mitigation works.In any potential earthquake prone area, it was necessary to classify the existing building stoke into different categories according to rapid, simple, reliable, logical and expert view based models and software.Kirac Nevzat, et. al (2011) observed that the negative effects of this weak storey irregularity could be reduced by some precautions during the construction stage.Also, some recommendations were presented for the existing buildings with weakstorey irregularity.Manabu Yoshimura, (1995) conducted nonlinear dynamic response analysis, where strength deterioration was considered in representing member nonlinearity, The analysis was found to reproduce the observed damages well, such as residual displacement, mechanism and damages to members.It was also revealed that if first storey mechanism might occur, the collapse could be unavoidable even for buildings with a base shear strength of as much as 60% of the total weight.Sivakumaran (1990) proposed a method of analysis for the earthquake response of multi-storey mono-symmetric buildings founded on flexible foundations.The analysis also included the sway (P-Δ) effects.Vipul Prakash (2004)  Based Engineering (PBE) in India.He listed the pre-requisites that made the emergence of PBE possible in California, compared the situation in India and discussed the tasks and difficulties for implementing PBE in India.In India, the criteria for earthquake resistant design of structures are given in IS 1893, published by the Bureau of Indian Standards (BIS).IS 1893-2002 reduced the number of seismic zones to four by merging zone I with zone II and adopted a modified CIS-64 scale for seismic zoning and dropped references to the MMI scale.
In this paper, seismic analysis of soft storey buildings considering structural and geometrical parameters have been carried out using STAAD.PRO software.Soft storeys have been created by increasing the floor heights.Effect of infill has been ignored.Results, in terms of moment, displacement, shear force, axial force and drift are critically examined and salient conclusions are drawn.

StruCturAl modellinG And AnAlYSiS (a) modelling of building frames
All the building Frames considered are 16m x 16m in plan area and 10 storey (G+9).
Height of the buildings are 29m.1.    Poisson ratio : 0.17 The column size is 500mm x 300mm and the beam size is 450mm x 250mm.

(d) loading conditions
Following loading are conducted for analysis:-

(e) Structural Analysis
Structural analysis of the building frames are carried out using STAAD.Pro software (Ref.10).All the columns are rigidly supported at ground and 20 load combinations, given in Table 2, are considered for the analysis purposes.

reSultS And diSCuSSion
Results of structural analysis can be described under following heads -

a. moments in columns and beams
Comparison of max.bending moment are given in Table 3. Plot of max.moment vs soft storey for all types of buildings is given in Fig. 5

e. drift
Comparison of max.drift is shown in Table 7. Plot of max.drift vs soft storey for all types of buildings is given in Fig. 9.For type A building, max.drift is in case 11 at second floor.For type B building, max.drift is at third floor for case Figure 1: Plan of building Figure 2: Soft storey building TYPE-1
storey cases The following 15 cases have been framed for analysis purpose-CASE-1 : Without soft storey CASE-2 : Soft storey at first floor CASE-3 : Soft storey at second floor CASE-4 : Soft storey at third floor CASE-5 : Soft storey at fourth floor CASE-6: Soft storey at fifth floor CASE-7 : Soft storey at sixth floor CASE-8 : Soft storey at seventh floor CASE-9 : Soft storey at eighth floor Soft storey at ninth floor CASE-11: Soft storey at first and second floor CASE-12 : Soft storey at third and fourth floor CASE-13: Soft storey at fifth and sixth floor CASE-14: Soft storey at seventh and eighth floor CASE-15: Soft storey at eighth and ninth floor (c) material and geometrical properties Following material properties have been considered in modelling :-Density of RCC: 25 kN/m 3 Density of Masonry: 20 kN/m 3

Figure 7 :
Figure 7 : Max.Shear force in buildings

Figure 9 :
Figure 9 : Max.Drift in buildings described the prospects for Performance

table 1 :
No. of beams and columns.

table 2 :
Details of load cases.Comparison of max.storey displacement are given in Table 4. Plot of max.storey displacement vs soft storey buildings for all types of buildings is given C building is at first floor for case 3. Max.moment for all types of buildings are at first floor.b.Storey displacement

table 5 :
Comparison of Max.Shear force(kN)Comparison of max.axial force is given in Table6.Plot of max.axial force vs soft storey for all types of buildings is given in Fig.8.It is observed that max.axial force in Type A is in case 15 for all types of floor.It is also observed that max.axial force is at ground floor.Max.axial force for Type B building is at ground floor for case 10.Case 10 has max.axial force for all type of floor in
Figure 8 : Max.Axial force in buildings