Research Highlights of the Department Earthquake Engineering a) Seismic Hazard Estimation and Microzonation of Indian Cities A spate of earthquakes in the recent past, causing extensive damage has heightened the sensitivity of engineers and planners to the looming seismic risk in densely populated Indian cities. Although considered to be part of the stable continental interior, many spots in the Indian shield are known to have generated moderate magnitude (M 6) earthquakes in the recent and historic past. Microzonation of Bangalore city is a project that puts together a wide range of data to evaluate the future seismic hazard of Bangalore. Studies recommend the upgradation of the southern part of Karnataka as zone III in the seismic zoning map of India. A microzonation map of Bangalore is generated using data from 950 boreholes drilled in various parts of the city, a much required data for planning the developmental activities (Figure 1). Based on the available information on distribution and dimensions of the suspected faults, ground motions have been simulated and the expected Peak Ground Accelerations (PGA) at rock depths are calculated within a distance of 220 square km area of Bangalore. For the Bangalore city, the maximum PGA at rock depth is estimated to be around 0.15g. Maps showing variations in PGA at ground level considering site response, sediment thickness, bedrock configuration as well as amplification maps are developed in GIS format.

Figure 1: Microzonation map for Bangalore city

Delhi city is in a seismically active region and hence it is important to understand the existing earthquake hazard to the city on a scientific basis. This study, after discussing the seismo-tectonic status of the city and its environs, presents probabilistic seismic hazard analysis of an area of 30 km by 40 km with its centre at India Gate, Delhi city. The quantified hazard in terms of the rock level peak ground acceleration value is mapped on a grid size of 1 km by 1 km, for a return period of 2500 years. In engineering parlance, this corresponds to a probability of exceedance of 2% in a design life of 50 years. This map can be used as the basis for further site-specific studies at soft soil deposits. An engineering approach to estimate the existing seismic hazard for Mumbai city has been developed. After assembling a catalogue of past earthquakes and analyzing the database statistically, the recurrence relation for the control region is found out. This is used to compute the probability of ground motion that can be induced by each of the twenty-three

known faults that exist around the city. Final results are presented in the form of peak ground acceleration and response spectra, for two different return periods. These results can be directly used by engineers as basic inputs in earthquake-resistant design of structures in and around Mumbai. b) Engineering Modeling of Earthquake Source

In the absence of strong motion records, ground motion during the earthquake on 26th January, 2001 in Kutch, India earthquake has been estimated by analytical methods. A contour map of peak ground acceleration (PGA) values in the near source region is provided. These results are validated by comparing them with spectral response recorder data and field observations. It is found that very near the epicenter, PGA would have exceeded 0.6 g. A set of three aftershock records have been used as empirical Green’s functions to simulate ground acceleration time history and 5% damped response spectrum at Bhuj City. It is found that at Bhuj, PGA would have been 0.31 g–0.37 g. It is demonstrated that source mechanism models can be effectively used to understand spatial variability of large-scale ground movements near urban areas due to the rupture of active faults. Strong motion array records are analyzed in this study to identify and map the source zone of four past earthquakes. The source is represented as a sequence of double couples evolving as ramp functions, triggering at different instants, distributed in a region yet to be mapped. The known surface level ground motion time histories are treated as responses to the unknown double couples on the fault surface. The location, orientation, magnitude, and rise time of the double couples are found by minimizing the mean square error between analytical solution and instrumental data. Numerical results are presented for Chi-Chi, Imperial Valley, San Fernando, and Uttarakashi earthquakes. Results obtained are in good agreement with field investigations and those obtained from conventional finite fault source inversions. c) Seismic Site Characterization A piezovibrocone has been developed for site characterization (Figure 2a). The system consists of four anchor rods, one hand controlled loading unit, one power pack mounted on two wheeler trolley. It has a cylindrical friction sleeve of 150 cm2 surface area, capped with a 600 apex angle conical tip of 15 cm2 cross sectional area. A load cell is kept at the tip to measure the cone resistance (qc). Another load cell is also kept in friction sleeve to measure the sleeve friction (fs), which is the local friction between the soil and shaft of the probe. Several cone penetration tests have been performed using the piezovibrocone for geotechnical site characterization. The Standard Penetration Test (SPT) and MASW tests have been also performed at these locations (Figure 2b) and the results are compared. Relationships have been developed between the in-situ shear wave velocity Vs, SPT value N70 and cone penetration resistance qc for a specific site.

Figure 2a: Piezovibrocone Figure 2b: Site classification based on SPT -N values

By using the bender and extender elements tests in resonant column apparatus (Figure 3a), with the measurements of the travel times of the shear (S) and primary (P) waves, the variation of Poisson ratio (ν) was determined for dry sands with respect to changes in relative densities and effective confining pressures (σ3). The tests were performed for three different ranges of the particle sizes. The magnitude of the Poisson ratio was observed to decrease invariably with an increase in both the relative density and the effective confining pressure (Figure 3b).

0.15

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30 40 50 60 70 80 90Relative Density, %

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Figure 3a: Resonant Column Figure 3b: Variation of Poisson’s Test Set-up ratio with relative density d) Pre and Post Liquefaction Behaviour of Soils Cyclic triaxial tests (Figure 4a) are carried out to evaluate the dynamic properties as well as the liquefaction behavior of soil samples collected exclusively from the sites close to epicenter of recent Bhuj earth-quake, sites close to Sabarmati river belt in Ahmedabad and Meizoseismal region of Shillong Plateau, Assam. Stress controlled and strain controlled cyclic triaxial tests are conducted to study the potential for liquefaction of the sandy soils

(Figure 4b). The normalized values of pore water pressure and loading cycles showed an establishment of a unique relationship between pore pressure ratio and cycle ratio independent of the number of cycles of loading, relative density of sand, confining pressure, amplitude of shear strain and percentage of non plastic fines. Also it has been observed that stress path has an insignificant influence on the dynamic properties of granular materials. Large diameter cyclic triaxial testing equipment, which can accommodate a sample of dimensions up to 300 mm diameter and 450 mm height is being procured by the Department, to facilitate static and cyclic triaxial testing of soils, soft rocks, ballast, concrete etc. to extend the studies on evaluation of dynamic properties of these materials and liquefaction behaviour of soils.

Figure 4a: Cyclic Triaxial Test Set-up Figure 4b: Pore pressure response of original

Ahmedabad sand specimens e) Earthquake Load Modeling Using Critical Excitation Method Method of critical excitations enables modeling of the worst case scenarios in problems of earthquake load specification. These excitations are tailor-made to produce the highest response in a specified structure while satisfying constraints on reliably known features of ground motion. Specific contributions made in this topic of research include: (a) development of critical power spectral density function models for spatially varying earthquake ground motions and their applications for seismic response analysis of piping structures in nuclear power plant structures, (b) development of reliability based critical excitation models for linear and nonlinear engineering structures, and (c) development of an inverse reliability based method for structural design under critical excitations. f) Structural Vibration Control

In recent years, significant progress has been made in the area of structural control as a viable technology for enhancing structural functionality and safety against natural hazards such as earthquakes and wind gusts. The control system may be passive, active, semi-active or hybrid in nature and be deployed as an absorber or as a base isolation system. While passive systems suffer from a limited capability, active systems have a high power requirement from an

external source. A hybrid system alleviates the limitations that exist from either a passive or active system acting alone. Findings emerging from these studies are:

• Fuzzy logic control (FLC) algorithms incorporate an inherent robustness and uncertainty handling capability that results in better control, compared to classical control algorithms. FLC based algorithms account for structural nonlinearities effectively. The major advantage of the controller is its simplicity and the use of a limited number of measured structural responses. Sensor noise, quantization and saturation errors are considered in the developed model to offer a realistic representation of the control implementation. An artificial neuro-fuzzy model with supervisor module has been shown to detect changes in structural system parameters and adjust controller parameters online to offer effective control.

• Acceleration in addition to velocity feedback results in improved control performance.

• A multi-objective optimal design approach that offers a set of Pareto optimal designs for making an appropriate selection has been developed.

• Experiments on hybrid base isolated building model using MR dampers have shown the efficacy of genetic algorithm based fuzzy logic control in mitigating the structural responses under near and far field excitations (Figures 5 and 6).

• Model based control algorithms (two-stage optimal dynamic inversion and integrator back stepping) developed for MR damper based control are efficient and offer improvements in performance over FLC based control.

• Mitigation of vibration levels in long span cable supported bridges due to vehicular traffic, seismic or wind-induced excitations are often critical design criteria. Cable nonlinearity, uncertainty with reference to magnitude and frequency content of dynamic loads, possibility of sensor saturation, limitations in actuator capacity and saturation have been given due consideration in developing control algorithms for use with MR dampers.

Structural protection systems based on vibration control technology will be studied to evolve alternate repair / retrofit strategies that are adaptive in nature, and easy to deploy on bridges and buildings to mitigate seismic and wind induced vibrations.

Figure 5: Schematic view of the experimental set-up of hybrid base isolated building (MR damper + sliding bearing), for study on fuzzy logic control.

Figure 6: Comparison of controlled and uncontrolled response

g) Seismic Response of Reinforced Soil Retaining Walls Seismic response of reinforced soil retaining walls is studied through shaking table tests (Figure 7). Wrap faced and rigid faced reinforced soil retaining walls of size 750× 500 mm in plan and 600 mm height are built in rigid and flexible containers and are tested under controlled dynamic conditions using a uni-axial shaking table (Figure 8). The effects of frequency and acceleration of the base motion, surcharge pressure on the crest, number of reinforcing layers, effect of container boundary, wall structure and reinforcement layout on the seismic performance of the retaining walls is studied through systematic series of shaking table tests. Results are analyzed to understand the effect of each of the considered parameters on the face displacements, acceleration amplifications and soil pressures on facing at different elevations of the walls. A numerical model is developed in a parallel effort to simulate the shaking table tests on wrap faced reinforced soil walls using computer program FLAC (Fast Lagrangian Analysis of Continua). The experimental data is used to validate the numerical model and parametric studies are carried out on 6 m height full-scale wall using this model.

Figure 7: Laminar box on shake table Figure 8: Model of rigid faced

retaining wall h) Shock Table Studies for Earthquake Resistant Design of Masonry Buildings

Shock table test facility for evaluating seismic performance of buildings was designed and constructed (Figure 9). The table is of size 3.5m by 2.5m and is supported on 4 wheels with ability to move horizontally in one direction on rails. The table can be subjected to shocks through a swinging pendulum of 600kg mass with provision to increase the mass up to 1000kg. On the side of the table opposite to the pendulum, provision is made to generate a reverse shock through a reaction beam. The impulse force that can be given to the table can be varied by changing the swing angle of the pendulum, mass of the pendulum and the material to which the pendulum impacts. The reverse force to the table can also be varied by changing the gap between the table and the reaction beam before the start of the test. The photograph shows two brick masonry building models with different earthquake resistant features on the shock table.

Laminar box

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Pendulum (1.8m length & 600kg mass Max. swing 400)

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Figure 9: Shock table test facility with masonry building models

Behaviour of two masonry building models (1/4th scale) were studied (Figure 9). The models were of plan size 1.5m × 0.75m and height 0.75m and were without roof. One of the models (model 1) had all the earthquake resistant features recommended by IS: 4326-1993 like R C lintel and roof band and vertical corner reinforcement. The second model (model 2) had additional earthquake resistant features like R C sill band and containment reinforcement which is a new earthquake resistant feature incorporated to impart ductility to the otherwise brittle masonry (Figure 10). The building models were constructed with scaled soil-cement bricks of size 40mm X 40mm X 90mm with cement-lime sand mortar. The fundamental frequency of the models was extracted by conducting free vibration experiments and was found to be about 71.23Hz for model 1 and 73.55 Hz for model 2. The models were subjected to 14 shocks with different swing angles against wood. The building model 1 with Bureau of Indian standard code (IS 4326 – 1993) recommended earthquake resistant features collapsed after 12 shocks whereas the improved model even though suffered damage did not collapse even after 14 shocks (Figure 11). The test clearly demonstrated the need for additional features over and above the existing earthquake resistant features to prevent collapse of masonry buildings during earthquakes and thus save precious lives.

containment reinforcement with link

Corner containment reinforcement with triangular link

Figure 10: Construction sequence of buildings with containment reinforcement

Model 1 (ERF as per IS 4326:1993)

Model 2 (ERF as per IS 4326:1993 plus additional R C band at Sill level and Containment reinforcement

One fourth scale models

Figure 11: Behaviour of the building models after 12 shocks

i) Inelastic Seismic Analysis of R C Framed Structures Ductility demands have been obtained from inelastic analysis of multistory R C structures subjected to near-field ground motions. It is observed that the demands due to normal components are nearly 2 to 3 times more than those due to parallel components while the distribution of ductilities over the height of the structure is nearly the same. The pulse periods of near-field ground motions have significant influence on the responses. Many other features like influence of eccentricities of structures asymmetric in plan, Responses due to one-component as compared to those due to two components of the ground motion, feasibility of designing an inelastic energy absorber at the top or towards the upper storeys are more or less similar to those subjected to normal ground motions.