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Behaviour of steel I-beam to box column connections under post-earthquake fire
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posted on 21.02.2017by Song, Qian-Yi
Various kinds of structural damage, even collapse, can be caused to buildings by extreme loads such as earthquake, fire and impact. Among these extreme loads, not only is earthquake a common extreme natural disaster that can cause widespread damage and loss, but also it can induce other secondary disasters including fire, tsunami and debris flow. It is certainly a more destructive event when structures survive through an earthquake with various damage but are attacked by extreme secondary impacts. For the safety of life and property, evaluation of post-earthquake structural behaviour is necessary. Historical records show that, among all the events caused by earthquake, fire is a frequent event. In this study, the post-earthquake fire (PEF) scenario is the focus in which structural behaviour is investigated.
PEF events are recognised as a major threat to building structures. Yet understanding of the behaviour of structural components subjected to PEF is still limited, especially understanding based on experimental investigation. In regions of low to medium seismic risk, structural systems usually survive through the heavy ground motion but are left with partial damage. It is worthwhile, therefore, to investigate and evaluate the residual fire-resisting capacity of partially damaged structures. Since simple steel I-beam-to-tubular-column connections are widely used in low- to medium-rise steel structures, three types of steel connection were proposed for the experimental and numerical investigation in this study.
The purpose of this study is to investigate the effects of damage on fire-resisting behaviour of steel connections. In order to achieve the target, a series of experimental investigations were carried out, including tests under monotonic and cyclic loadings at ambient temperature, fire loading and PEF loading scenarios of interest consisting of a pre-damage seismic loading phase and a subsequent fire loading phase. Damage assessment was performed mainly on the basis of the test results under seismic loading at ambient temperature. The ISO-834 standard fire curve was adopted as the target fire scenario for the fire and PEF tests.
Using conventional material constitutive models, preliminary FE modelling analyses, including heat transfer analysis and mechanical behaviour analysis of steel tubular connections were also carried out and verified by experimental results under the loading scenarios at ambient temperature and fire. Although the modelling results were relatively good agreed with test data, the limit of the conventional modelling method was seen to model the damage phenomena, as well as the complex PEF process. Considering that fracture phenomena are complex and no well-accepted material model coupling with fracture particularly that under cyclic loading and fire was available, a simplified method was adopted to investigate the influence of pre-induced fracture damage on the fire-resisting performance of the connections. Pre-induced fracture was simulated by the element removal method. Besides the measured fire scenarios from the tests, a slow fire scenario was also adopted in the modelling investigation of the pre-damaged connections subjected to fire.
Besides the investigation of the steel connections under PEF, preliminary investigation of steel ductile fracture under monotonic loading at ambient temperature was carried out through material coupon tests and FE modelling. Two typical stress states were investigated including the uniaxial tensile loading case and the in-plane shear loading case. The influence of load rate over a small range was investigated within the material coupon tests. FE modellings were established and verified by the material coupon test results. The established fracture mode was finally applied in the modelling of connection CWH5S with improved modelling result.
Through the experimental investigation, fracture was found as one of the main pre-damage phenomena that significantly influence the fire-resisting capacity of steel connections subjected to PEF events. According to the investigations by experimental and numerical modelling methods, the fire-resisting capacity of the connections was found degraded significantly with the increase of the pre-induced damage level. The fire duration of the steel connections with heavy damage was shown decreased around 30%. As time is the most critical factor that determines evacuation of people, fire fighting and search-and-rescue, this study provides useful understanding of degradation of the fire-resisting capacity of steel connections pre-damaged by earthquake. For important buildings, it is also necessary to consider fire as secondary extreme event in design for safety reasons.