Estimation of Seismic Force in Shear Walled Building Using Site Specific Response Spectrum
Site specific ground response analysis is required to determine the response of a soil deposit to the motion of the bedrock immediate below the soil and also to determine the effect of local soil conditions on amplification of seismic waves. It is also required for estimation of the ground response spectra for future design purposes (Kramer, 1996).
The estimation of strong motion characteristic in terms of peak ground acceleration and spectral ordinates is important for any engineering design. Loading condition is the main input in structural design, which must satisfy certain condition regarding their level and frequency of occurrence during the lifetime of a structure. Generally, various important structures and buildings are designed for seismic forces specified in IS 1893 (Part I): 2002. The response spectra of codal provisions are applicable for general soil conditions but not reflecting local sub soil characteristics. Sub soil condition plays an important role in response of ground surface under specified bed rock motion and thus in development of response spectra for a specific site. For the important structures, response spectrum obtained from site specific ground response analysis is recommended rather than that of code specified response spectrum.
In present paper site specific ground response analysis is carried out for Ahmedabad soil sites. For development of site specific response spectrum ProSHAKE software is used while for dynamic analysis of multistoried wall frame buildings ETABS software is used. ProSHAKE is based on one dimensional geotechnical site response model and ETABS is widely used software for three dimensional structural analysis of buildings.
In highrise structures shear wall is widely used to resist earthquake forces. Earthquake forces produce large displacement, vibration and large stresses in building which leads to building an unsafe and causing discomfort to the occupants. The concrete shear walls are quite stiff in their own plane. Therefore, shear wall frame building of varying no. of storey are considered to understand effect of site specific response spectrum in this paper. Seismic forces in shear wall building is estimated considering site specific response spectrum and are compared with that obtained considering IS 1893 response spectrum.
Site Specific Response Analysis
- Characterization of Site
- Selection of input bedrock motions
- Ground response analysis for specific soil site
- Site specific response spectra for structural analysis
Characterization of SiteThe local characteristics of soil such as shear wave velocity, shear modulus of the soil deposits, unit weight of soil, thickness of soil layers are obtained from the geotechnical and geophysical investigations.
Selection of input bedrock motionsAppropriate rock motions i.e. natural acceleration time histories or synthetic acceleration time histories are selected to represent the design rock motion for the site. For the present study the strong motion recorded at Ahmedabad during 26thJanuary 2001 Bhuj earthquake is selected as input bedrock motions.
Ground response analysis for specific soil site
Site specific response spectra for structural analysisResponse spectra of calculated ground surface motion are statically analyzed to develop the design spectrum for the site. The response spectrum describes the maximum response of a single degree of freedom (SDOF) system to a particular input motion as a function of the natural period and damping ratio of the SDOF system. The response is expressed in terms of acceleration, velocity or displacement is referred as the spectral acceleration (Sa), spectral velocity (Sv), and spectral displacement.
Methods Used for Site Specific Ground Response AnalysisBased on dimensionality Site Specific Ground Response Analysis can be classified as:
- One-dimensional ground resp– onse analysis
- Two-dimensional ground resp–onse analysis
- Three-dimensional ground resp–onse analysis
One-dimensional ground response analysis
A complete ground response analysis should consider various factors such as rupture mechanism at the origin of earthquake, propagation of seismic waves through the crust to the top of the bedrock and are difficult to quantify thus, complete ground response analysis becomes highly complicated. Therefore, one dimensional ground response analysis is preferred over other analysis methods due to simplicity and also it is believed to provide conservative result.
Based on number of techniques available for ground response analysis, one dimensional ground response analysis is carried out using following method
- Linear analysis
- Equivalent linear analysis
- Non–linear analysis
Ground Response Analysis Using ProSHAKE SoftwareThe effect of local soil conditions on ground response during earthquake is evaluated using computer software ProSHAKE, software based on one-dimensional, equivalent linear seismic ground response analysis. ProSHAKE provides the results of acceleration time history, ground response spectra and depth plots of various sites (Edu Pro.2003). Site specific ground response analysis of following sites is performed:
- Indian Institute of Management (I.I.M.) Site
- Maninagar-Sukhipara Site
- Passport Office Site
- Nirma Institute of Technology (N.I.T) Site
- Bodakdev Site
- Sola Site
- Chandkheda Site
The data of soil profile of the sites corresponding to various parameters such as number of layers of soil, thickness of each layer, unit weight, shear modulus are obtained from borehole data and geophysical testing. The soil profiles for I.I.M. Maninagar, Passport Office, NIT, Bodakdev, Sola and Chandkheda Site are shown in Tables 1, 2, 3, 4, 5, 6 and 7.
Using ProSHAKE software ground response analysis is performed and various results are obtained. Acceleration time history graphs of I.I.M Site, Maninagar-Sukhipara Site, Passport Office Site, N.I.T Site, Bodakdev Site Sola Site and Chandkheda Site are shown in Figure 1. Normalized response spectra at all the seven sites are obtained by dividing ordinates of response spectrum by peak ground acceleration. They are further compared with similar plot (Sa/g®Time period) given in IS 1893:2002 (Part-1) as shown in Figure 2.
From ground response analysis various parameters like peak ground acceleration, velocity and displacements are obtained for all the three components of the ground motion. But as the longitudinal direction is critical these parameters are compared for all the seven sites as shown in Table 8.
Response Spectrum Analysis of Shear Wall Frame BuildingThis study has been carried out to understand the effect of local soil conditions on response of shear walled building with 10, 15, 20, 25 and 30 storey. Shear walled frame building is chosen for study purpose because shear wall is an efficient way of stiffening the structure. Shear walls have been the most ommon structural systems used in building to resist horizontal forces caused by earthquakes. The shear wall frame buildings considered are three by three bays with fixed base resting on different sub soil condition. Plan of the shear wall frame building is shown in Figure 3. Story height of building is considered as 4m. The dimension of various structural elements is given in Table 9.
The response spectrum analysis of the multi–storey shear wall frame buildings is carried out using ETABS software. The response spectra obtained from ProSHAKE for all the seven sites are provided in ETABS for response spectrum analysis (Wilson et al. 2002). Response spectrum analysis is carried out considering the IS 1893:2002 Response Spectrum (medium soil) as well as for the Site Specific Response Spectrum. The response spectrum corresponding to 5% damping is considered which is reasonable for concrete structure.
The time period of all the buildings are obtained using dynamic analysis and are compared with that of empirical formula given in IS1893-2002. The same is shown in Figure 4.
From Response spectrum Analysis for all structures and various results are compared. The comparison of Spectral Acceleration Coefficient and Base Shear considering IS 1893:2002 response spectrum and site specific response spectrum are presented in Figure 5 and 6 respectively.
Results and DiscussionsSite specific response spectra for seven sites of Ahmedabad city are developed by performing Site specific ground response analysis. From the available data of sub-soil strata and input motion data of acceleration time history during Bhuj earthquake of 26thJanuary 2001, acceleration time histories on ground and response spectra for the sites are developed. With the help of these plots, peak ground acceleration (g), Spectral acceleration coefficient (Sa) are evaluated. The comparison of site specific response spectrum with that of IS 1893 (medium soil) indicates significant difference for lower time period range. This affects base shear of buildings. Subsequently, site specific response spectrum analysis and time history analysis of multistoried building shows difference in total seismic force on building. From Figure 5 it is observed that spectral acceleration coefficient is different for various sites compared to that specified as per IS 1893. From Figure 6 it is observed that total seismic force on building expressed as base shear varies for various sites in comparison of IS 1893. For buildings having 10, 15, and 30 storey site specific response spectra gives higher base shear.
- Local sub soil characteristics have considerable effect on acceleration time histories on ground and response spectra
- Time period obtained from Dynamic analysis for all the R.C.C. shear wall framed structures is higher in comparison with the time period obtained from IS1893-2002
- IS 1893-2002 gives lower value of Sa/g coefficient for 10, 15 storey building in comparison to site specific response spectrum. While IS 1893-2002 gives higher value of Sa/g coefficient for 20, 25 and 30 storey buildings in comparison to site specific response spectrum.
- Base Shear obtained considering Site Specific response spectra is governing for 10, 15 and 30 stories buildings while in case of 20 and 25 storey building base shear considering IS 1893-2002 response spectra is governing.
AcknowledgmentThe authors acknowledge the support provided by Prof. Sandip S. Trivedi, Associate Professor of Department of Civil Engineering, Nirma University and Research Scholar of I.I.T. Delhi for providing geotechnical and geophysical data of various sites of Ahmedabad.
- EduPro Civil Systems, Inc., “ProShake Ground Response Analysis Program, Version 1.1,” User Manual, Redmond, Washington, 2003.
- Govinda Raju, L., Ramana, G. V., Hanumantha Rao, C. and Sitharam, T. G., (2004), “Site-specific Ground Response Analysis,” Current Science, Volume. 87, No.10, pp.1354-1362.
- IS-1893, “Criteria For Earthquake Resistant Design of Structures” Part 1), Bureau of Indian Standards, New Delhi, 2002.
- Kramer, Steven L., (1996), “Geotechnical Earthquake Engineering,” Prentice Hall, Upper Saddle River, NJ.
- Wilson E. L., and Habibullah A., “Extended Three Dimensional Analysis of Building Systems ETABS Version 9.0.7,” Users manual, Computers and Structures, Inc., Berkeley, California, 2002.