Indian Seismic Code Research Articles

Deterministic seismic hazard analysis considering different seismicity levels for the state of Goa, India

Deterministic seismic hazard analysis (DSHA) is carried out for the state of Goa, which is a small but one of the most popular touristic places on the west coast of peninsular India. Goa state is located about 250 km from Koyna region which experienced a 6.3 magnitude earthquake in 1967 and about 350 km from Latur which witnessed a 6.3 magnitude earthquake in 1993. Hence, it is important to carry out the seismic hazard analysis for Goa, such studies being scarce in literature till date. In this paper, DSHA for Goa is carried out by considering a 350 km distance from the state boundary taking into account the historic seismicity and identifying active faults and lineaments. Seven attenuation relationships developed for intraplate regions and shallow crustal earthquakes including one developed specifically for peninsular India are used. Three earthquake scenarios representing three levels of seismic activity are considered using the logic tree approach. The maximum magnitude estimated by empirical relations and the same from past earthquake events are considered in two scenarios for carrying out the DSHA, and the third scenario is derived from the previous two scenarios using equal weightage. The peak ground acceleration (PGA) values are obtained by considering a grid of 0.1° × 0.1° (i.e., 11.1 km × 11.1 km) covering the entire state of Goa. The deterministic seismic hazard map showing contours of PGA are developed for the state of Goa for the three scenarios. For the mildest scenario 2, highest hazard with PGA of 0.15 g is found at localized areas in the southwest of Goa. The hazard maps for the severe scenarios indicate high hazard in the central band of the state. The deterministic spectra are developed for five major cities of Goa and compared with the spectra given in Indian seismic code IS 1893-Part 1 (Criteria for Earthquake resistant design of structures. Bureau of Indian standards, New Delhi, 2002). PGA map and the spectra will be very useful for seismic design of important structures in Goa.

Seismic Liquefaction Hazard and Site Response for Design of Piles in Mumbai City

Mumbai city is the financial capital of India with the highest population density and formed by reclamation of land over time from original seven different islands. As per Indian seismic design code IS 1893-Part 1, Mumbai city is located in Zone III, hence may experience moderate intensity earthquake which may lead to liquefaction of some typical soil sites of Mumbai city. In this paper, using available recent procedures for liquefaction analysis, seismic liquefaction hazard maps for Mumbai city are prepared. Also the typical coastal soft soil strata of Mumbai may be prone to soil amplification for different bed rock earthquake motions and the present study shows that typical ranges of soil amplification factor for bed rock acceleration are 1.2–3.5. Hence, construction of pile foundation, which is mostly used for the effective use of the most precious land of Mumbai city for construction of high rise buildings, need special attention in design when such possibly liquefied soil strata with soil amplification during moderate earthquake intensity is considered. Present study shows the response of pile foundation in both non-liquefied and liquefied soil by considering both kinematic and inertial responses in terms of displacement and bending moments of piles, which are necessary to consider for seismic design of pile foundation in typical Mumbai soil.

Dynamic Site Characterization and Correlation of Shear Wave Velocity with Standard Penetration Test ‘N’ Values for the City of Agartala, Tripura State, India

Seismic site characterization is the basic requirement for seismic microzonation and site response studies of an area. Site characterization helps to gauge the average dynamic properties of soil deposits and thus helps to evaluate the surface level response. This paper presents a seismic site characterization of Agartala city, the capital of Tripura state, in the northeast of India. Seismically, Agartala city is situated in the Bengal Basin zone which is classified as a highly active seismic zone, assigned by Indian seismic code BIS-1893, Indian Standard Criteria for Earthquake Resistant Design of Structures, Part-1 General Provisions and Buildings. According to the Bureau of Indian Standards, New Delhi (2002), it is the highest seismic level (zone-V) in the country. The city is very close to the Sylhet fault (Bangladesh) where two major earthquakes (M w > 7) have occurred in the past and affected severely this city and the whole of northeast India. In order to perform site response evaluation, a series of geophysical tests at 27 locations were conducted using the multichannel analysis of surface waves (MASW) technique, which is an advanced method for obtaining shear wave velocity (V s) profiles from in situ measurements. Similarly, standard penetration test (SPT-N) bore log data sets have been obtained from the Urban Development Department, Govt. of Tripura. In the collected data sets, out of 50 bore logs, 27 were selected which are close to the MASW test locations and used for further study. Both the data sets (V s profiles with depth and SPT-N bore log profiles) have been used to calculate the average shear wave velocity (V s30) and average SPT-N values for the upper 30 m depth of the subsurface soil profiles. These were used for site classification of the study area recommended by the National Earthquake Hazard Reduction Program (NEHRP) manual. The average V s30 and SPT-N classified the study area as seismic site class D and E categories, indicating that the city is susceptible to site effects and liquefaction. Further, the different data set combinations between V s and SPT-N (corrected and uncorrected) values have been used to develop site-specific correlation equations by statistical regression, as ‘V s’ is a function of SPT-N value (corrected and uncorrected), considered with or without depth. However, after considering the data set pairs, a probabilistic approach has also been presented to develop a correlation using a quantile–quantile (Q–Q) plot. A comparison has also been made with the well known published correlations (for all soils) available in the literature. The present correlations closely agree with the other equations, but, comparatively, the correlation of shear wave velocity with the variation of depth and uncorrected SPT-N values provides a more suitable predicting model. Also the Q–Q plot agrees with all the other equations. In the absence of in situ measurements, the present correlations could be used to measure V s profiles of the study area for site response studies.

Spatial variation of probabilistic seismic hazard for Mumbai and surrounding region

Mumbai city, the economical capital of India, is located on the west coast of stable intra-plate continental region of Peninsular India which has an experience of significant historical earthquakes in the past. The city stood as the fourth most populous city in the world. Recent seismo-tectonic studies of this city highlighted the presence of active West coast fault and Chiplun fault beneath the Deccan basalt. In the present study, spatial variability of probabilistic seismic hazard for Mumbai region (latitudes of 18.85–19.35°N and longitudes of 72.80–73.15°E at a grid spacing of 0.05°) which includes Mumbai city, Suburban, part of Thane district and Navi Mumbai, in terms of ground motion parameters; peak horizontal acceleration and spectral acceleration at 1.0-s period for 2 and 10 % probability of exceedance in 50 years are generated. The epistemic uncertainty in hazard estimation is accounted by employing seven different ground motion prediction equations developed for worldwide shallow crustal intra-plate environments. Further, the seismic hazard results are deaggregated for Mumbai (latitude 18.94°N, longitude 72.84°E) to understand the relative contributions of earthquake sources in terms of magnitude and distance. The generated hazard maps are compared with the zoning specified by Indian seismic code (IS1893: Part 1 in Indian standard criteria for earthquake-resistant design of structures, Part 1—General provisions and buildings. Bureau of Indian Standards, New Delhi, India, 2002) for rocky site. Present results show an underestimation of potential seismic hazard in the entire study region by non-probabilistic zoning prescribed by IS1893: Part 1 with significantly higher seismic hazard values in the southern part of Navi Mumbai.

Estimation of local site effects using microtremor testing in Vijayawada city, India

The microtremor method is very useful in urban areas to estimate local site effects for microzonation purposes. As seismic waves propagate from bedrock to the ground, their characteristics are site specific due to the heterogeneity of substrata. Source, path and site effects are prime parameters that affect ground response. Many destructive earthquakes that occurred in the past (e.g. Bhuj 2001, Bingol 2003, Kashmir 2005, Haiti 2010, Tohoku 2011, Van 2011 and Yunnan 2012) clearly illustrate the effects of local soils on damage severity and pattern. The Indian sub-continent has more than 400 major faults that influence seismic activity and India has experienced several devastating earthquakes (Assam 1897 (M = 8·7), Kangra 1905 (M = 8·6), Bihar-Nepal 1934 (M = 8·4), Assam-Tibet 1950 (M = 8·7), Latur 1993 (M = 6·4), Chamoli 1999 (M = 6·8), Bhuj 2001 (M = 7·6) and Sikkim 2011 (M = 6·9)). The state of Andhra Pradesh is located in the central part of peninsular India (zones II and III according to Indian seismic code) and has a record of earthquakes along the coast. This study considered Vijayawada (zone III), the third largest city in the state. Microtremor surveys were carried out at 75 different locations in the Vijayawada urban area and analysis was carried out using the Nakamura technique. Dynamic characterisation was carried out by considering the shape of the response curve, horizontal to vertical (H/V) amplitude, predominant frequency and the characteristic soil profile at all the test sites. Based on this detailed analysis, a classification is proposed and a predominant frequency map of the study area was developed. It was found that areas in the north eastern and south eastern parts of the city, with silty and clayey sand formations, have comparatively high predominant frequencies (≥ 4 Hz). The northern and western parts of the city, with high silty clay and silty sand, are characterised by moderate frequency values (2–4 Hz). Low (< 2 Hz) values of predominant frequencies were observed at a few locations. The H/V amplitudes are high (2–3) in the eastern and western regions.

Determination of Time Period of Vibration Effect on Seismic Performance of Building

Most seismic codes specify empirical formulas to obtain the fundamental period of buildings. The equations specified in present IS codes, are according to the available data on the time period of buildings measured from their recorded accelerograms. Shear-wall dominant reinforced concrete buildings, constructed, using codes specification are commonly built in different countries, facing a substantial seismic risk, in spite of their high resistance against ground motions. Current seismic code provisions including the Uniform Building Code (International Conference of Building Officials, Whittier, CA, 1997) and the Indian Seismic Code (Criteria for earthquake resistant design of buildings, fifth revision, 2002) are considered to evaluate the effect of time period on seismic behavior of building.In this study, time period obtained by code formulas are compared with those obtained by modal analysis in SAP2000. Also the top story displacement (as an adequate parameter of determination the seismic performance of building) correspond to the values of mentioned time period are estimated using uniform building code and software respectively. It is observed that current empirical equation for calculating the time period of RCC buildings is rather inaccurate. Also it is shown that the time period has very effective influence on seismic performance of building.

An Analytical Study on the Seismic Vulnerability of Masonry Buildings in India

Two analytical models for unreinforced masonry (URM) buildings are proposed with the aim to simulate their seismic response and to estimate corresponding vulnerability functions. The proposed models are implemented in SAP 2000 nonlinear software to obtain capacity curve parameters for representative Indian URM buildings, based on a field survey and statistical analysis. Vulnerability functions are estimated using the obtained capacity curves. Damage Probability Matrices (DPMs) are obtained using the approximate PGA-intensity correlation relationship as per Indian seismic building code and are compared with the commonly used intensity scales and empirical damage data observed after the 2001 Bhuj earthquake.

Influence of host structure characteristics on response of rooftop telecommunication towers

The increasing trend of mobile communications has seen exponential growth in the last three years. Increased competitions among mobile operators also have contributed to the installation of many towers to enhance both coverage area and network reliability. The tower locations as specified in terms of latitudes and longitudes with the height of mounted antenna dictated by functional requirements of the network. The availability of land which satisfies ideal installation conditions in urban areas is extremely limited giving no alternative but to adopt roof top towers (with marginal adjustment in position but not in height). In the present study, the seismic response of 4-leged telecommunication tower has been studied under the effect of design spectrum from the Indian seismic code of practice for Zone-IV. The analysis has been performed on ground tower and tower located on the roof of host structure by varying positions of tower with increasing the stiffness of host structure in the both direction (X and Y). Their dynamic analyses are performed by SAP2000 program. The axial forces of the tower members are considered as the main parameter. The comparison has been made between rooftop tower and ground tower members at the same height from the ground.

Limitations of Soil Amplification Provisions in the 2002 Indian Seismic Code

The design response spectra of the 2002 Indian seismic code are compared with analytical and empirical results, and design spectra of other major national codes. Amplification at 37 sites in India is studied analytically using 1-D wave propagation. Next, Generation Ground-Motion Attenuation (NGA) models are used to obtain the spectra for corresponding site classes using different magnitude-distance (M-R) pairs. Average amplification factors for the four selected NGA models are compared with the design codes and analytical results and it is observed that there are considerable differences among them. We consider the provisions for local soil amplification in the current Indian seismic code to be inadequate and in need of improvement.

Seismic hazard and site-specific ground motion for typical ports of Gujarat

Economic importance of major ports is well known, and if ports are located in seismically active regions, then site-specific seismic hazard studies are essential to mitigate the seismic risk of the ports. Seismic design of port sites and related structures can be accomplished in three steps that include assessment of regional seismicity, geotechnical hazards, and soil structure interaction analysis. In the present study, site-specific probabilistic seismic hazard analysis is performed to identify the seismic hazard associated with four typical port sites of Gujarat state (bounded by 20°–25.5°N and 68°–75°E) of India viz. Kandla, Mundra, Hazira, and Dahej ports. The primary aim of the study is to develop consistent seismic ground motion for the structures within the four port sites for different three levels of ground shaking, i.e., operating level earthquake (72 years return period), contingency level earthquake (CLE) (475 year return period), and maximum considered earthquake (2,475 year return period). The geotechnical characterization for each port site is carried out using available geotechnical data. Shear wave velocities of the soil profile are estimated from SPT blow counts using various empirical formulae. Seismicity of the Gujarat region is modeled through delineating the 40 fault sources based on the seismotectonic setting. The Gujarat state is divided into three regions, i.e., Kachchh, Saurashtra, and Mainland Gujarat, and regional recurrence relations are assigned in the form of Gutenberg-Richter parameters in order to calculate seismic hazard associated with each port site. The horizontal component of ground acceleration for three levels of ground shaking is estimated by using different ground motion attenuation relations (GMAR) including one country-specific GMAR for Peninsular India. Uncertainty in seismic hazard computations is handled by using logic tree approach to develop uniform hazard spectra for 5% damping which are consistent with the specified three levels of ground shaking. Using recorded acceleration time history of Bhuj 2001 earthquake as the input time motion, synthetic time histories are generated to match the developed designed response spectra to study site-specific responses of port sites during different levels of ground shaking. It is observed that the Mundra and Kandla port sites are most vulnerable sites for seismic hazard as estimated CLE ground motion is in order of 0.79 and 0.48 g for Mundra and Kandla port sites, respectively. Hazira and Dahej port sites have comparatively less hazard with estimated CLE ground motion of 0.17 and 0.11 g, respectively. The ground amplification factor is observed at all sites which ranges from 1.3 to 2.0 for the frequency range of 1.0–2.7 Hz. The obtained spectral accelerations for the three levels of ground motions and obtained transfer functions for each port sites are compared with provisions made in Indian seismic code IS:1893-Part 1 (2002). The outcome of present study is recommended for further performance-based design to evaluate the seismic response of the port structures with respect to various performance levels.

Probabilistic Seismic Hazard Macrozonation of Tamil Nadu in Southern India

Abstract The south Indian state of Tamil Nadu in the peninsular shield is a zone of low to moderate seismic activity with a sparse historical record of significant earthquakes. The current intensity-based zoning adopted by the Indian seismic code stipulates an effective peak ground acceleration (PGA) of either 0.10 or 0.16 g for different parts of the state, for the maximum considered earthquake (MCE), and the service life of a structure. In the current study, probabilistic seismic hazard contour maps for Tamil Nadu and the union territory of Pondicherry, in terms of the ground-motion parameters, PGA and spectral accelerations, at 0.1, 0.5, and 1.0 sec for 2%, 5%, and 10% probabilities of exceedance in a 50 yr period, have been produced. Hazard computations have been performed over a grid of sites covering the territory at an interval of 0.2°. A comprehensive earthquake catalog has been compiled for the region extending between 2 and 20.7° N latitude and 68 and 88° E longitude and spanning ∼950 yrs. The hazard maps are produced by suitably accounting for epistemic uncertainty in the hazard computations within a logic-tree framework incorporating parameters such as different probabilistic hazard analysis methods (classical Cornell–McGuire and zone-free approaches), catalog completeness estimation methods, maximum cutoff magnitude, and ground-motion predictive equations for shallow crustal intraplate environments. The hazard maps are compared to the zoning prescribed by the seismic code. The current estimations show that the potential seismic hazard in considerable parts of the state is underestimated by the broad zoning adopted by the Indian Standards.

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

Towards formulating guidelines for performance evaluation of buildings to site-specific earthquakes, studies are reported in literature on the effect of various critical parameters. No study is, however, reported on the effect of depth of soil stratum. In this paper, a methodology is proposed and applied for performance evaluation of buildings for site-specific earthquakes including depth of soil stratum as a parameter. The methodology integrates independent procedures meant for performance evaluation of buildings and site-specific seismic analysis. Application of the proposed methodology enables to determine performance point of a building in terms of inelastic displacement and base shear. Numerical application of the methodology is demonstrated using the particulars of Delhi region. Two typical RC buildings (B1 and B2) with significantly different inelastic behaviour, assumed to be located on soil depths ranging from 10 to 200 m are chosen for the application study. Capacity spectra of the buildings are generated from nonlinear static analysis. Studies indicate that for building B1, with elasto-plastic behaviour, the depth of soil stratum strongly influences demand on inelastic displacement compared to that on inelastic base shear. For building B2, with continuously varying inelastic behaviour, the depth of soil stratum is observed to have significant influence on both the inelastic base shear as well as inelastic displacement. Responses of the buildings are compared with that obtained based on design spectrum of Indian seismic code. For both the cases, inelastic displacements as well as inelastic base shears are underestimated by Indian seismic code for certain depths of soil stratum. Proposed methodology enables the calculation of realistic values of inelastic base shear and corresponding displacement of a building for site-specific earthquakes by considering the actual characteristics of soil stratum.

Evaluation of seismic response of soft-storey infilled frames

In this study two single-bay, three-storey space frames, one with brick masonry infill in the second and third floors representing a soft-storey frame and the other without infill were designed and their 1:3 scale models were constructed according to non-seismic detailing and the similitude law. The models were excited with an intensity of earthquake motion as specified in the form of response spectrum in Indian seismic code IS 1893-2002 using a shake table. The seismic responses of the soft-storey frame such as fundamental frequency, mode shape, base shear and stiffness were compared with that of the bare frame. It was observed that the presence of open ground floor in the soft-storey infilled frame reduced the natural frequency by 30%. The shear demand in the soft-storey frame was found to be more than two and a half times greater than that in the bare frame. From the mode shape it was found that, the bare frame vibrated in the flexure mode whereas the soft-storey frame vibrated in the shear mode. The frames were tested to failure and the damaged soft-storey frame was retrofitted with concrete jacketing and, subjected to same earthquake motions as the original frames. Pushover analysis was carried out using the software package SAP 2000 to validate the test results. The performance point was obtained for all the frames under study, therefore the frames were found to be adequate for gravity loads and moderate earthquakes. It was concluded that the global nonlinear seismic response of reinforced concrete frames with masonry infill can be adequately simulated using static nonlinear pushover analysis.

Simulation of the damage to a simple RC structure in the Bhuj, India, earthquake to estimate the level of input-motion

Analysis of a simple reinforced concrete (RC) structure damaged by the Bhuj, India, earthquake was carried out to estimate the level of shaking in the epicentral region. For this, an attempt was made to estimate the level of input motion to cause inelastic behavior to the extent observed during the field visit. To consider the inelastic effects, both yielding of steel bars as well as crushing of the concrete cover has been investigated employing the hysteretic model known as the Fiber model. The only available record at Ahmedabad of the Bhuj earthquake and four additional earthquake records from Japan and California were used in the analysis. Considering simple scaling of input motion, the level of input motion to cause crushing and spall-off of the concrete cover as observed in the field was estimated to be of the order of 6 times the original instrumental record obtained at 240 km away from the epicenter. The methodology proposed was promising in providing a useful quantitative indication of the level of shaking when instrumental records are not available. It was also noted that the design response spectrum specified in Indian seismic code IS1893: 1984 appears inadequate compared to the extent of shaking estimated. Copyright © 2005 John Wiley & Sons, Ltd.

Seismic Retrofitting of R/C Shaft Support of Elevated Tanks

The circular, reinforced concrete (R/C) shaft-type support for elevated tanks lacks redundancy, damping and additional strength typically present in building framing systems and, therefore, should be designed for larger seismic resistance. However, the Indian seismic code IS:1893-1984 prescribes the same basic seismic force as that for the most ductile building framing system for which the design force is the least. Furthermore, the code-specified one-mass idealization of elevated water tanks is not appropriate for large (large width to depth ratio) and partially filled tanks. The low design forces lead to a weak and slender support—a very unfavorable feature in high seismic areas, as evidenced in the failure of two water tanks in the 1997 Jabalpur earthquake and a great many in the 2001 Bhuj earthquake. It is rather difficult to enhance the ductility and energy dissipation capacity of thin-walled, R/C shaft supports. Concrete jacketing is used as a retrofit measure to enhance the lateral strength and ductility by changing the failure mode of concrete crushing to a more ductile tension yielding. This scheme requires substantial strengthening of the existing foundation.