Research

Thesis Title

Evaluation and Improvisation of the Seismic Collapse Performance of Special Concentrically Braced Frames

Thesis Supervisor: Prof. Dipti Ranjan Sahoo and Prof. A.K. Jain (IIT Delhi)

Special Concentrically Braced Frames (SCBFs) are popular lateral load resisting frames because of their structural efficiency, high stiffness, ease of construction, and low cost. In India, where around 58.6% of the landmass is affected directly by moderate to high-intensity earthquakes, the SCBF system has emerged as the most favorable mode of structure for construction. The increase in the trend is also observed in industrial building and warehouse construction. However, past earthquakes highlighted the premature failure of the braced frame system due to the low-cyclic fatigue fracture of braces, failure at the end-protected zones and beam-column connection, occurrence of soft-story mechanism, and inefficient limit states design criteria. An increase in the downtime of the structures after a seismic event is also observed to have one major concern. Hence, further research is demanded to improve the seismic performance and resiliency of the SCBFs structures.

The present study is focused on the performance evaluation and improvement of the braced frames at the local (component/ sub-assemblage) and global (system) levels. The proposed study is primarily focused on three issues: to quantify the inelastic capacity of the brace components and brace-beam-column connections, to propose the limit state equation to prevent premature failure, and to evaluate and improvise the performance of the global braced frame systems. The present study's main aim is to quantify, improvise, and increase the resiliency of the SCBFs structures.

To achieve the desire objectives, the present work is broadly divided into three types, i.e., experimental, analytical, and numerical investigations. Four sets of large-scale experiments will be conducted in the Heavy Structural Laboratory of IIT Delhi. The experimental studies will highlight, a) the influence of the end protected zone on the inelastic deformation capacity of the brace members, (b) to quantify and compare the brace member's fracture ductility consists of closed section (torsional rigid) and open section, (c) to study a new replaceable fuse connection to enhance the resiliency of the SCBF system (expected to file a patent), and (d) to study hybrid brace members for higher buckling strength and inelastic deformation capacity. The sub-assemblage system will be tested under reserved cyclic quasi-static loading up to the failure of the specimen using the hydraulic actuators of a combined capacity of 200 tons (2000 kg). Large scale sub-assemblage components of the braced frame system are tested to quantify the proposed design guidelines that is derived from the validated numerical investigation and theoretical studies. The experimental studies conducted is also provided an idea about the reliability of the fracture initiation methodology that will be used in the numerical investigation to quantify the performance of the braced frame system.

As large-scale experiments are very costly, and it is very difficult to study all the parameters that will influence its behavior, so an advanced finite element numerical investigation will be carried out using the micromechanical cyclic void growth fracture model to predict the initiation of the fracture under ultra-low cyclic fatigue loading. A recently developed stress-weight damage model will be used in the initial phase to simulate the fracture initiation of the brace specimen. Simultaneously, research will be conducted to incorporate the influence of the local buckling on the fracture initiation to enhance its performance for better fracture prediction. Large-scaleexperimental studies will be taken as the yardstick for the quantification of the effectiveness of the numerical simulation.

Performance-Based Earthquake Engineering necessitates the development of simulation models that can predict the nonlinear behavior of structural components as part of a building subjected to seismic loading. For reliable seismic assessment of buildings, fiber-type FE model using OpenSees framework will be developed. The model attempts to combine the realism of FE approach and the computational efficiency of the physical theory model. The proposed brace frame model will be able to simulate the inelastic deformation of the brace components, end-protected zones, beam, column, brace fracture, and the post-buckling behavior of the brace member. A series of nonlinear static and dynamic analyses of different story-height SCBF buildings will be conducted on the proposed model with three different steel shapes, i.e., hollow square section, hollow circular section, and wide-flange section. The influence of the different brace members and end connections on the seismic collapse performance of the SCBF building were quantified, and design recommendations were provided. The impact of the R values on the different brace members of the SCBFs system is also quantified, and an interrelationship is established.

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