Study of the Behaviour of Precast Connections under Progressive Collapse Scenario

Abstract

Progressive collapse is a situation where local failure of a primary structural component leads to the collapse of adjoining members, which in turn leads to spread of collapse. Progressive collapse of building structures is initiated when one or more vertical load carrying members are seriously damaged or collapsed due to extreme loading imposed by earthquake, flood, explosion, vehicle impact etc. As a result, a substantial part of the structure may collapse, causing greater damage to the structure than the initial impact. Progressive collapse of any structure causes catastrophic failure of structure and substantial loss of human lives as well as natural resources. In order to reduce risk of progressive collapse, it is necessary to design and detail the structure to develop alternate load path in the event of extreme loading. In precast concrete construction, the components of structures are produced in controlled environment, transported and individual precast elements are connected appropriately at site. Because of better quality control and faster rate of construction, the precast concrete construction is being adopted world-wide including India. However, in precast concrete construction, connections are the critical elements of the structure, because in past, major collapse of precast building took place because of connection failure. Therefore, it is very important to study the performance of precast beam column assembly under a progressive collapse scenario. Behaviour of precast beam column assemblies under progressive collapse condition is not reported in greater detail in literature. National Institute of Standard and Technology (NIST), U. S. Department of Commerce has presented report on experimental and computational study of two precast concrete moment-frame assemblies under a column removal scenario, in September 2015. During last couple of years, few more investigators have reported results of experimental and numerical studies conducted on different precast beam column connections under progressive collapse scenario. In the present research work, behaviour of different precast connections are evaluated under progressive collapse scenario by conducting experiments and performing numerical analysis. Experimental studies are conducted on sixteen reduced (1/3rd) scale precast test specimens, which are constructed by providing wet and dry connections. Each test specimen represents beam column assembly, consists of two span beam and three columns with removed middle column, which in turn represent column removal scenario. Connections are provided at beam olumn junction for eight precast test specimen, while for remaining eight precast test specimens, connections are provided away from the beam column junction i.e. within beam length, to avoid any discontinuity at beam column junction. Test specimens are extracted from 6-storey symmetrical precast building having overall plan dimensions of 16 meter × 12 meter. As, perimeter frames of any building are exposed to higher risk of occurrence of any undesired event due to ease of accessibility, the prototype of test specimens is assumed to be located at the middle of the perimeter frame in longer direction subjected to column loss at bottom storey. Two monolithic test specimens are also prepared to compare behaviour of precast connections with that of monolithic connection under column removal scenario. Monotonic vertical load is applied at the top of the removed middle column with the help of hydraulic jack till the complete failure of specimen takes place. Response of test specimens are evaluated in terms of ultimate load carrying capacity, load versus vertical deflection at the location of removed middle column, deflection profile of the specimen measured along the beam length, crack formation and failure propagation of test specimens. For some of the test specimens, strain measurement is also carried out at selected locations on concrete surface as well as on steel reinforcement bars. Numerical analysis using finite elements for monolithic and precast specimens is carried out using ABAQUS software for validation of results obtained from experimental studies. Finite Element (FE) models are developed for precast connections under column removal scenario and analysis is carried out by considering material and geometrical nonlinearities. Concrete damage plasticity model is used for nonlinear FE analysis. Concrete part of specimen is modelled as 8-node linear brick elements (C3D8R) with reduced integration. 2-node three dimensional truss elements (T3D2) are used for modelling of steel reinforcement bars. The numerical analysis results in terms of ultimate load carrying capacity, deflection profile of specimen at specified load values and failure modes are compared with that observed during experimental studies. From the experimental results, it is concluded that precast connections provided away from the beam column junction, specifically wet connections, where individual elements are connected by overlapping or welding of projecting reinforcement bars, can be used as an alternative of monolithic connection. Behaviour of such precast connections under progressive collapse scenario will be similar to that of monolithic connection. Further, from the experimental results, it is observed that, precast wet connections performs superior as compared to precast dry connections under progressive collapse scenario. Results of experimental studies also indicates that, behaviour of precast wet connections is further enhanced though inclusion of polypropylene fibres with cast-in-place micro concrete and by providing additional lateral confinement reinforcement within connection region. Experimental results on precast dry connections considered for the study, suggests that precast dry connections, constructed by welding steel plates, performs superior as compared to those constructed using bolting. From the results of numerical analysis, it is found that, Finite Element (FE) models developed for precast connections under progressive collapse scenario, using concrete damage plasticity model are about to capture the behaviour of specimen during initial phase up to flexural action and Compressive Arch Action (CAA). Stress contours and scalar stiffness degradation contours obtained from numerical analysis closely matches with the failure pattern of specimen observed during experimental studies, which indicates close agreement of experimental and numerical analysis results. Present study can be useful in developing guidelines for better performance of precast beam column assemblies during progressive collapse scenario.