Structural Seismic Engineering

Overview

Structural seismic engineering deals with studying the behavior of structures subjected to seismic loading. Primarily, the focus of my research has been the behavior of non-seismically designed reinforced concrete frame structures. Such non-seismically designed structures form a major portion of the existing structures worldwide and are particularly vulnerable against seismic excitation. Some of the major issues making these structures vulnerable against earthquakes include insufficient joint shear capacity, poor detailing with small embedment lengths and inadequate lap splices, insufficient confinement, undesirable hierarchy of strength (e.g. strong beam – weak column) etc.

Seismic performance of beam-column joints

Beam-column joints play a crucial role in the seismic behavior of non-seismically designed (NSD) reinforced concrete (RC) frame structures. These structures are associated with typical non-seismic design features such as bottom beam bars not-sufficiently anchored in the joints, no shear reinforcement in the joint core, insufficient lap splices in critical regions (beam and column ends), insufficient transverse reinforcement in the lap region. Due to these detailing aspects, in particular, the beam-column joints become vulnerable against seismic loads undergoing failure due to joint shear, reinforcement pullout or insufficient confinement.

The objectives of this test program, which constituted an important part of my PhD (link to PhD Thesis), were to provide information on the seismic behavior of beam-column joints of NSD structures with an emphasis on the influence of longitudinal beam bar anchorage into the joint. The tests, conducted on full-scale joints, highlighted that when the beam bars are bent into the joint, the concrete strut remains stable even after first cracking and can therefore carry significantly higher load compared to the cases where the beam bars are bent out. The case where the longitudinal beam reinforcement is embedded straight for a short embedment length (typically 125 mm) into the joint, are susceptible to undergo bond failure prior to the attainment of joint shear strength. The test results provided the basis for the development of a multi-spring joint model and served as the benchmark for joints retrofitted with fully fastened haunch retrofit solution.

The tests were performed at the component testing facility at Bhabha Atomic Research Centre, Mumbai. This was the first time that such beam-column joint tests were not outsourced to other testing laboratories but were carried out within BARC. This demanded a thorough coordination with various divisions of BARC.

Seismic pushover tests on RC structures

A reinforced concrete (RC) frame structure when applied with gradually increasing lateral loads, undergoes several stages from first cracking until collapse. These stages (or limit states) are referred to as “performance levels”, which play a key role in the modern seismic design philosophy. In the linear analysis procedures, the response reduction factors and the importance factors are indirectly related to these performance levels. However, a non-linear analysis procedure such as “pushover analysis”, which provides the base-shear vs. monitored displacement curve (known as capacity curve), can provide a direct estimate of the performance level expected for a structure in the event of an earthquake. The reliability of the performance estimate is strongly dependent on the accuracy of the capacity curve obtained.

As a non-seismically designed RC frame structure can display various failure mechanisms, in order to generate a reliable capacity curve, good analytical models that can consider all the possible failure modes as well as their interactions are necessary. Currently, there are very few tests available that provide information on the non-linear behavior of non-seismically designed RC frame structures under gradually increasing lateral loads. In order to fill this gap of information, pushover tests on non-seismically designed RC frame structures were conducted.

A pushover test was conducted on a full-scale 4-storey structure (Total height 17m, bay width 5m), which was the first of its kind test to be conducted in India. The experiment was carried out as a round robin exercise in which several scientists, academicians and researchers participated from various institutes across India. The experiment was carried out at the tower testing facility of Central Power Research Institute, Bangalore in collaboration with Dr. Ramesh Babu and Mr. D. Revanna. The load applied on the structure was maintained at a ratio of P: 2P: 3P: 4P corresponding to 1^st floor: 2^nd floor: 3^rd floor: 4^th floor, respectively, resulting in an inverted triangular load pattern. A special loading arrangement was designed to apply the loads. Rock anchors were used in the foundation to prevent the uplift/rotation of the foundation. The structure was well-instrumented using strain gauges, LVDTs, load cells, dial gauges etc.

The initial cracks were observed on the tension face at the base of the columns and at the tension face on beam ends at first floor level (base shear = 300 kN). At a base shear equal to 500 kN, the first shear cracks started to appear at the beam-column joints at 1^st floor level. These cracks opened with the further increase in the load and more cracks at higher elevation (2^nd and 3^rd level) were also observed in the beams and beam-column joints. The beams transverse to the direction of loading underwent torsion failure.

The complete details about the experiment can be obtained from the following publications:

Paper 1

Paper 2

A similar test on a small scale structure was conbducted at Structural Engineering Research Centre, Chennai. The details of this test can be found in the following publication:

Paper 3

The tests provided valuable insight into the structural behavior under lateral loads and the interactions of different failure modes. The tests serve as the benchmark problems to validate the modeling procedures for RC frame structures under seismic loads

Dynamic (Shake table) testing of RC structures

Shake table tests come closest to providing the information on the behavior of structures under real life type seismic loads. The shake table is given the acceleration time history as the input to the base of the structures as in case of real earthquakes. Within the framework of my PhD thesis, shake table tests were performed on 2/3rd scale planar reinforced concrete frame structures in which the exterior joint corresponded to the beam-column joints sub-assemblies tested earlier. The tests provided an insight into the behavior of non-seismically designed RC frame structures under seismic loads and served as benchmark for the seismic behavior of structures retrofitted with fully fastened haunch retrofit solution. The tests were conducted at the Central Power Research Institute, Bangalore in collaboration with Dr. Ramesh Babu. The details of the tests can be obtained from the following publication:

Paper 1

In order to understand the interactions between the various components of a reinforced concrete frame structure under seismic loads, verify the influence of brick infill panels on structural frequencies as well as to investigate the influence of Tuned Liquid Dampers (TLD) on the seismic response of structures, shake table tests were performed on one-third scale 3-dimensional RC frame structure. The tests were carried out in Central Power Research Institute, Bangalore. The details of the tests can be obtained from the following publication:

Paper 2

Numerical modeling of seismic performance of beam-column joints

Since beam-column joints of non-seismically designed RC frame structures dominate the seismic behavior of these structures, modeling of the joints is essential to reliably generate the capacity curve of structures. 3D finite element modeling of joints are able to capture the realistic behavior of the joints, provided good modeling techniques and constitutive laws are used, they are not practical to model the complete structures and generate their capacity curve due to computational efforts and costs involved. Multi-spring models within the framework of lumped plasticity approach offer a good solution to perform nonlinear seismic analysis of RC frame structures considering nonlinear joint behavior. One such model was developed within the framework of my PhD Thesis which uses one rotational spring and two shear springs to simulate the contribution of the joint shear distortion to the global drift, in case of an exterior joint. The model uses critical principal tensile stress as the failure criterion to identify first joint shear cracking and ultimate failure of the joint, in case of joints with beam bars bent in. The model is reliable, fast, accurate and is based on the realistic deformation behavior of beam-column joints under seismic loads. The model is easily implementable in any software capable of performing nonlinear analysis within the framework of lumped plasticity approach. Further details on the joint model can be obtained from the following publication:

Paper Joint Model

Nonlinear static pushover analysis of RC structures

Developing numerical models and analysis procedures for performing nonlinear static pushover analysis of reinforced concrete frame structures considering different possible modes of failure. The modeling procedure has been validated against different experimental results.

Pivot hysteresis model for nonlinear seismic analysis of RC structures

Development of pivot hysteresis model parameters for nonlinear seismic analysis of reinforced concrete structures including beam-column joints. The model originally proposed by Dowell et al (1998) for circular columns is extended for its applicability on rectangular sections and beam-column joints.

Nonlinear dynamic analysis of RC structures

Developing numerical models and analysis procedures to carry out the detailed nonlinear dynamic analysis of reinforced concrete structures also considering the joint distortion. The analysis procedure has been validated against several test results and has been proven to capture the seismic behavior of the structures realistically.

Modeling seismic behavior of masonry infilled RC structures

Developing models for considering the seismic performance of reinforced concrete frame structures with masonry infill panels. The models are developed within the framework of lumped plasticity approach on the basis of validated 3D finite element analyses.