Author: Mahbubur Rahman

Publisher:

Published: 2012

Total Pages: 0

ISBN-13:

Practicing engineers can confidently design hot-rolled steel portal frame structure if it is kept at ambient temperature. However, as they are not aware of the potential collapse behaviour of such frames in fire, they tend to use heavy foundations with expensive fire protection materials applied to all the columns, rafters and column bases to ensure the structural integrity, and prevent premature collapse. The research presented in this thesis aims to provide computational techniques and solutions for studying the possible behaviour of different hot-rolled steel portal frames in fire considering the partial strength of column bases with partial insulations applied to the columns. Before tackling the effect of partial strength of column bases, a comparative study between two different dynamic methods for solving such problems, the implicit dynamic method and the explicit dynamic method, has been undertaken considering fire, large deformations, complex geometry, boundary conditions and degradation of material stiffness. For such analyses, the cost of computation is important as well as the accuracy, robustness and stability of the analysis. It is found that obtaining similar results are possible by using both of the dynamic methods, however, the analyses time differ significantly. It has been established that if the applied artificial inertia forces in terms of residual forces can be magnified and if the automatic time incrementation scheme is activated in the implicit dynamic method then this method shows significant superiority over the explicit dynamic method both in terms of cost of computation and accuracy of results for analysing such structure. Once the proper dynamic method has been selected, all of the analyses of portal frame structure in fire have been conducted by using this particular dynamic method. The developed model using the implicit dynamic method has been used for studying the effect of partial strength of column bases. A non-linear elasto-plastic implicit dynamic finite element model of a single-span pitched roof steel portal frame building in fire is set-up and used to assess the adequacy of the design method provided by the Steel Construction Institute (the SCI design method). Both 2-D and 3-D models are used to analyze a building similar to the Exemplar frame described in the SCI design guide. Using the 2-D model, a series of parametric study on different frames is conducted. It is shown that the value of the overturning moment,MOTM, calculated in accordance with the SCI design method, is not sufficient to prevent collapse of the frame before 890°C. It is established that if MOTM is increased the eaves rotations are reduced significantly and reach close to 1° of the original shape, the limit specified by the SCI design method. The developed model has been extended for analyzing three other portalised frames, such as, multi-span portal frames, portalised truss frames and asymmetric portal frames. It is found that, apart from the multi-span frame, the model can be readily applied to the portalised truss frames and asymmetric portal frames without any computational overhead and loss of accuracy. However, for the multispan frame, the cost of computation is increased significantly. The computational cost is reduced by relaxing some tight tolerance parameters without losing any accuracy. For all of the frames, it has been observed that all the frames collapses when the column bases are perfectly pinned. However, when a partial strength is introduced at the column bases, the behaviour of the frames changed considerably. It is found that though the snap-through-buckling temperatures remain almost same, the collapse temperatures vary and the eaves rotations differ significantly. Similar to the single-span pitched roof portal frame it has been found that when MOTM is increased the eaves rotations are reduced significantly and reached close to 1° of the original shape. Based on the studies on different frames it is suggested that the MOTM given by the SCI method should be increased and considered within the region of 1.5MSCI to 2.0MSCI. Key words: steel portal frames, stability, snap-through-buckling, quasi-static and dynamic analysis, partial strength, semi-rigidity.