The structural response of industrial portal frame structures in fire
A number of recent fires in single-storey warehouses have drawn attention to a current lack of understanding about the structural response of industrial portal frame structures to elevated temperatures. This research project has investigated the subject by conducting fire tests on a scaled model and by computer modelling using the non-linear finite element program VULCAN. This program has been developed in-house by the University of Sheffield and is capable of modelling the behaviour of three-dimensional steel and composite frames at elevated temperatures. It has been validated throughout its development. An initial investigation was conducted to validate the program for analysing inclined members, which form part of a pitched- roof portal frame, but for which it was not initially developed. Additional features were implemented into the program where necessary. A series of indicative fire tests was conducted at the Health and Safety Laboratories, Buxton. A scaled portal frame model was designed and built, and three major fire tests were conducted in this structure. In the third of these tests the heated rafters experienced a snap-through failure mechanism, in which fire hinges could clearly be identified. The experimental results were then used for validating the numerical results produced by VULCAN analyses. The correlations were relatively close, both for predictions of displacements and failure temperatures. This gave increased confidence in using VULCAN to conduct a series of parametric studies. The parametric studies included two- and three-dimensional analyses, and a number of parameters were investigated, including the effects of vertical and horizontal load, frame geometry, heating profiles and base rotational stiffness. The influence of secondary members was investigated in the three-dimensional studies using different fire scenarios. A simplified calculation method has been developed for estimating the critical temperatures of portal frames in fire. The results compare well with predictions from VULCAN. The current guidance document for portal frames in boundary conditions has been reviewed, and the concept of performance-based design for portal frame structures has been discussed.