Analysis of the structural response of tall buildings under multifloor and travelling fires
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The last decades have seen a surge in the construction of tall buildings all over the world. Due to their, often, innovative and complex layouts, tall buildings can pose unique challenges to architects and engineers. Previous tall building failures raised significant concerns on the applicability of prescriptive fire design for these structures. The use of structural fire engineering can enhance the safety of a tall building under fire by strengthening any vulnerable areas in the structure and at the same time reduce the costs of fire protection by removing it when unnecessary. Commercial finite element and specialist structural fire engineering software have their advantages and disadvantages. In this thesis, the object-oriented and open-source finite element software OpenSees is presented along with its development with structural fire capabilities by the author and other researchers at the University of Edinburgh. Specifically, new pattern, element, section and material classes have been introduced. All the developed code follows the object-oriented paradigm and is consistent with the ethos of the existing framework. Verification and validation studies of the developed code are also presented. Several procedures including that for dynamic analysis of structures in fire for the collapse assessment of structures are discussed. The development of OpenSees with structural fire capabilities allows the collaboration of engineers across geographical boundaries and disciplines using a community tool. In this work, the behaviour of tall buildings under different fire scenarios has been modelled using the developed OpenSees code. Firstly, the collapse mechanisms of generic tall buildings are investigated, namely the strong and weak floor mechanisms are demonstrated, and criteria are established on when each of these mechanisms occurs. The parametric study performed demonstrated that the weak floor collapse is less probable for generic composite buildings however this type of failure can become easier to appear as the number of floors in fire increase. The effect of vertically travelling fires on these mechanisms is also examined. The results of the study show that slower travelling rates delay or avoid the global failure of a tall building compared to quicker travelling rates allowing for the time required for steel members to regain their strength during cooling to ambient temperature. However, it was seen that higher tensile membrane forces were observed in the floors as the travelling rates increased which could result in possible connection failure. Most of the research and design codes, such as Eurocode, typically assume a uniform thermal environment across the floor area of a structure when defining the design fire. However, in reality fires are more likely to travel in large enclosures, hence there is a need to understand how tall buildings behave under more realistic fire conditions such as travelling fires. A methodology for defining the thermal environment of large enclosures using travelling fires has been recently developed at the University of Edinburgh. Taking into account OpenSees' programmable architecture and its recent inclusion with heat transfer capabilities by other researchers, there was a collaborative effort in order to understand the thermal and structural response of a generic composite tall building under horizontally travelling fires. The findings of the study showed that larger travelling fire sizes produce quicker heating to the steel beams while smaller fire sizes produce higher peak temperatures in the concrete slab. The structural analysis also demonstrated that travelling fires produced higher midspan deflections in comparison to Eurocode parametric fires and higher plastic deformations which is an indication of higher damage. Further work focused on looking at the behaviour of tall buildings under the combined scenario of horizontally and vertically travelling fires. The results of the study showed that the travelling fires produce lower maximum compressive and tensile membrane forces in the composite floor compared to the Eurocode parametric fires for the building examined and thus in a multi-floor scenario the columns are pulling-in less after large deflections develop in the floor. More specifically, the short-hot fire produced the most demanding response. This suggests that in long floors where uniform heating is really impossible, the time of failure predicted by parametric fires in a multi-floor scenario can be more onerous. The outcomes of this work can aid designers when considering the structural fire response of tall buildings in a performance based design context. It was demonstrated that multi-floor fires could be a threat for tall buildings, and thus this possibility should be considered in design. The use of more realistic fire definition for large enclosures, such as travelling fires, should also be considered. The travelling fire methodology can provide an enhanced level of confidence for the safety of a building since it can represent a range of similar fires to those that may occur in a real fire scenario.