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Mechanical properties of steel and concrete under the sequential action of cyclic loading and elevated temperatures
thesisposted on 2017-03-02, 04:04 authored by Sinaie, Sina
Current design guidelines allow structures to dissipate seismic energy through plastic deformations. As a result, a fi re following an earthquake would find a structure in a weaker state than what had been originally assumed during its fire-resistant design. This thesis aims to investigate the potential changes that the mechanical properties of construction materials experience under earthquake-then-fire loading sequence. Earthquake and fire are both extremely random events, so before any attempt to investigate their effects in the material scale, they have to be projected into a more quantifiable form. For this purpose, in the following work, a post-earthquake fire event is replaced by a multi-phase loading history that includes cyclic loading at ambient temperature followed by exposure to high temperatures. In order to make the findings of this research widely applicable to current and future structures, this thesis focuses on two of the most common construction materials: mild steel and normal strength concrete. The results of this research indicate that a prior history of cyclic loading can significantly affect the ductility and strength of steel and concrete at elevated temperatures. These variations in material properties have great implications when it comes to post-earthquake fire analyses, suggesting that any history of cyclic loading should be included in the post-earthquake fire-resistant design of a structure. Therefore, it is desirable to establish a relationship for each mechanical property, not only as a function of temperature but also the level of damage induced by cyclic loading. Developing predictive models is a main objective of this thesis. For this purpose, a generic model has been proposed for steel and concrete. This model takes advantage of a special class of functions known as Bezier curves. The proposed model proves to be highly versatile, in the sense that it can successfully take the effect of temperature and pre-induced damage into account. In addition to Bezier curves, more advanced models have also been incorporated in this research. This includes the theory of plasticity for steel and the discrete element method for concrete. The knowledge gained from this study is not only useful when it comes to designing new structures, but is also suitable for assessing the fire-resistant capacity of structures that have already sustained an earthquake. Moreover, the results of this research can be used to develop fire-resistant guidelines for structures located in earthquake-prone areas.