RESPONSE OF RC FRAMED STRUCTURES SUBJECT TO POST - EARTHQUAKE FIRE

Abstract

Fires can sometimes be regarded as collateral damage consequent to earthquakes in extremely inhabited areas and in general are an integral part of the emergency response strategies focused on life safety in most developed economies. As a mitigation strategy, the design guidelines proposed by di erent agencies in many countries across the globe considers the e ect of seismic and re loading on the structures as two separate entities. Adequate level of resistance to these hazards is speci ed considering one hazard at a time. Although they have a unique signi cance in an occurrence of such events individually, a sequential occurrence cannot be pretermitted. Fire following earthquakes, also termed as post-earthquake re events manifests a greater deal of credible threat to built environment when it behaves as a single damage causing agent. There are no regulations specifying the design for required resistance for these hazards occurring in a sequential manner. The compound e ect of two di erent hazards makes the structure more vulnerable to permanent structural damages either leading to stark failure or leaving it un t to be put to structural use upon retro tting. The risk of re consequent to earthquake is enhanced by mass urbanization. The havoc caused by the re following earthquakes in the past is widely known from the timeline of events in the retrospect for almost a century till now. Two such largest events that history has seen were the res followed by 1906 San Francisco earthquake and 1923 Tokyo earthquake. In both these events, re was the dominating factor that led to a major loss in life and property than the earthquake. Reports of multiple re events and a few large con agrations after earthquakes have been quite common in most of the major earthquakes: 1971 San Fernando, 1994 Northridge, 1995 Hanshin earthquakes. World most densely populated cities located on the fault lines; San Francisco and Tokyo are highly vulnerable to re following earthquake events. However, no outbreak of major re has been reported following earthquakes in developing economies like India and China. Nevertheless, rapid urbanization in major cities calls in for consideration of such events as a measure of earthquake preparedness. Response of buildings and structures to preceding extreme loading conditions is extremely useful in development of design guidelines. No research attempt has been made in the past for assessment of structures under re following earthquake. A critical literature review mandates consideration of such loading conditions under performance based design techniques. Since re following earthquake falls out of scope of practising professionals in earthquake and re service elds, it has stimulated the interest amongst various groups of researchers in the structural engineering community to carry out a rigorous investigation in this front. Substantial research e orts required in the investigation may be identi ed at two di erent levels: Structural and Material. Structural level represents the holistic behaviour of a framed structure consisting of beams, columns, slabs and joints, whereas, material level constitutes the studies on the behaviour of constituent materials such as steel and concrete. In light of the above discussion, the principal objective of the current research was to investigate the response of a full-scale framed structure under post earthquake re scenario, both experimentally and analytically and behaviour of pre-loaded / damaged steel / con ned concrete at elevated temperatures. Material level investigations at elevated temperature nd its applications in development of constitutive models that aid analytical studies. A pre-load or a damage induced in the materials simulates the loading conditions experienced by the materials during earthquakes. Material level tests helps in establishing type-speci c stress-strain relationships that can be utilized in the non-linear analysis of reinforced concrete structures using computer software application packages. This ensures a far more accurate analysis of structural members induced with earthquake and re damage. Firstly, tests were carried out on steel reinforcing bars under elevated temperatures as a part of material level test. Mechanical properties of steel reinforcing bars of varying diameter at elevated temperature with and without pre-load were investigated to work out their stress-strain relationships. Steel reinforcing bars of category TMT EQR (Earthquake resistant, soft-core enhanced ductility) having diameter 8 , 10 , 16 and 20 were considered in the investigation. Bars under no pre-load category were exposed to three target temperatures, 250 C, 500 C and 750 C and tested in tension under steady state. Bars under the pre-load category were initially stressed to a certain known limit (0:58fy) to simulate the prevailing conditions caused by an earthquake. The bars were then exposed to the target temperatures and tested in tension at elevated temperature. Stress-strain behaviour for rebars of di erent diameter were plotted and compared. Stress-strain curve of rebars under no pre-load category shows a degradation in peak stress and modulus of elasticity at high temperatures. Specimens in pre-load category showed further degradation in peak stress upto 500 C. Thereafter, the specimen failed to retain su cient strength and sti ness before attaining the next level of target temperature, 750 C. Secondly, under material level test category, con ned concrete specimens were examined at elevated temperatures. Undamaged and damaged con ned concrete specimens, with two di erent levels of con nement (42 mm and 68 mm) were investigated at elevated temperatures. The specimens under undamaged category were directly exposed to the target temperature and the target temperature was maintained constant to attain a steady state. Thereafter, the specimens were loaded to failure in heated state. The specimens under damaged category were initially stressed by applying a load corresponding to a desired level of strain (0:0033, 0:0056 and 0:008), representing three di erent levels of damage to simulate the condition imparted by an earthquake. The specimen was then unloaded and exposed to the target temperature (250 C, 500 C and 750 C) and upon attaining the steady state, it was loaded to failure. Mechanical properties like stress-strain relationship, peak stress, modulus of elasticity, strain at peak stress and ultimate strain were extracted from the data and the in uence of damage and con nement were studied rigorously. Reduction in peak stress and modulus of elasticity at elevated temperatures was evident from the results analysed. Following the material level tests, a full-scale test was conducted on a single storey reinforced concrete (RC) frame representing an intermediate portion of a G + 3 storey building. The main scope of conducting the test was to evaluate the response of the RC frame under simulated re following earthquake. The test frame sub-assemblage consisted of four oor beams, four columns, four roof beams and a slab. The dead load from the storeys above was superimposed evenly on the four columns by using hydraulic jacks. A rigorous instrumentation was devised on the frame to reckon its structural response. The test frame was imparted with quasi-static simulated earthquake loading i by two double-acting hydraulic jacks. A bi-directional loading was achieved in eleven displacement cycles with a combination of 10 mm and 20 mm displacement per cycle. A maximum displacement of 150 mm that corresponds to the \Collapse Prevention" level of structural safety as per FEMA 366 was achieved. Thereafter, the structure was enclosed in a 3m 3m 3m compartment and it was exposed to one hour pool re. Ultimately, the re damaged RC frame was pushed to reckon its residual capacity. Response of the frame was measured in terms of load-displacement hysteresis, strains at rebar level, temperature experienced in various structural entities and displacement during re. The reinforced concrete frame was inferred to be intact without collapse. However, the frame underwent a degradation in strength and sti ness following the simulated earthquake and re. With a rigorous experimental work under structural and material level, focus was nally laid on analytical work on reinforced concrete frame. A nite element model was developed using commercial software suite, ABAQUS version 6:9. Modelling was achieved using solid homogeneous element, C3D8 for concrete and truss element, T3D2 for steel reinforcement. Di erent structural elements such as beams, columns and slab were discretized using octahedral elements. A detailed heat transfer analysis was carried out on the frame to generate temperature pro les, which were then compared with the experimentally assessed temperature pro les. Thereafter, a cyclic analysis was carried out to simulate the earthquake damage on the frame model. The load-displacement hysteresis obtained was compared with the experimental results.