اثرات عمق لایه خاک روی کارایی ساختمان ها جهت زمین لرزه های ویژه محل

ABSTRACT Effect of depth of soil stratum on performance of buildings for site-specific earthquakes

New generation seismic design codes are shifting towards performance based design of buildings [1–3]. This necessitates development of methodologies for performance evaluation of designed buildings for earthquakes. In this context, provisions for inclusion of the effect of critical parameters viz., soil–structure interaction, type of foundation, nature of ground motion and soil are reported in literature [4–7]. Effect of soil amplification has been well recognised in existing seismic design codes. Indian seismic design code IS-1893-2002 Part I [8] has three distinct design spectra exclusively for soft, medium and hard soils. As an improvement over this, amplification factors based on empirical and theoretical data [9] have been introduced in International Building Code (IBC) [10] for site class A to F to take care of the behaviour in short as well as long period range. Elghazouli [11] reported that in Eurocode (EC 8 Part 5), two types of response spectra have been proposed for five different soil conditions (Type A to E). Response spectra are identified with different soils and the expected magnitude of ground motion. Classification of site soil A to E has been made on the basis of average shear wave velocity of top 30 m soil.

Sun et al. [12] have showed that the site coefficients specified in IBC [10] are not valid for Korean Peninsula due to the large difference in the depth of bedrock and the soil stiffness profile. On extending this observation, application of the soil amplification factors specified by Borcherdt [9] to several other regions can produce results of unacceptable error. Further, building design codes are highly simplified tools and do not adequately represent any single earthquake event from a probable source for the site under consideration. It has been recommended [13] that in addition to use of codal provisions, site-specific analysis which includes generation of strong ground motion at bedrock level and propagating it through soil layers [13–15] and arriving at the design ground motions and response spectra at surface should also be carried out. On the other hand, considering the loss due to collapse or cost of repair, substantial revisions are taking place in major seismic design codes to evolve next generation design procedures [16–18]. Important considerations for the revisions are to account for acceptable levels of damage to buildings in a seismic event of known characteristics and also to include the actual performance of a building at the proposed site conditions. Performance evaluation of buildings is a process which involves both the structure and soil characteristics which need proper treatment in analysis phase.

To the authors’ knowledge, no methodology or a comprehensive study is reported on seismic performance evaluation of buildings for site-specific earthquakes. Despite the understanding and observation of the role of local soil on seismic wave amplification, practical design of structures is being made based on the spectra suggested by seismic design codes. Within the framework of performance evaluation of structures, design spectra act as performance objective. Besides this, design spectra also acts as demand curve in a design using conventional procedures. It is well known that the response spectra at soil surface can be significantly different from that of the bedrock response spectra due to modification of ground motion characteristics, as the wave travels through soil layers overlying the bedrock. In this background, a simple methodology is proposed and applied in this paper to evaluate the performance of buildings for site-specific earthquakes including depth of soil stratum as a parameter.

Basically, the methodology involves comparison of demand spectrum corresponding to an earthquake under the given soil condition against capacity spectrum of a building. Well-established procedures are available to evaluate the capacity spectrum of a building to a reasonable accuracy while considerable uncertainty exists in the generation of demand spectrum. In the present study, demand spectra corresponding to site-specific earthquakes are generated by carrying out the following two steps: (i) evaluation of ground response spectra using strong ground motion generated at bedrock level for a scenario earthquake and conducting a one-dimensional equivalent linear wave propagation analysis and (ii) generation of Depth Dependent Demand Spectra corresponding to the individual site conditions.

More details about the methodology are described in the next section. The methodology is verified using particulars of Delhi city in India. The main reason for choosing this city is the availability of geotechnical details for successful application of the methodology. It is to the belief of authors that the proposed methodology is general and, therefore, applicable to any other region at which local soil is expected to contribute significantly to the seismic behaviour of the structure. Comparison among the computed numerical values provides valuable information about the inelastic behaviour of buildings due to the effect of local soil.