Structural Fire Performance of Steel Portal Frame Buildings (2005)
AuthorsBong, Ming Weishow all
This report investigates the behaviour of steel portal frame buildings at elevated temperatures using the finite element programme SAFIR. The finite element analysis carried out in this report is three dimensional and considers several different locations and severities of fires within the building, different support conditions at the column bases, the presence of axial restraints provided by the end walls, different levels of out-of-plane restraint to the columns and the effect of concrete encasement to the columns. This report also provides recommendations for the design of steel portal frame structures for fire resistance. Steel portal frame buildings are a very common and popular form of construction used in New Zealand, usually with reinforced concrete precast wall panels attached to the steel frames. In the past, concrete boundary wall panels were required to remain standing after a fire, but it is now considered acceptable for the panels to collapse inwards provided that they remain connected to each other. However there remains concern that under fire conditions, the concrete panels may collapse outwards, creating a danger to fire-fighters and to adjacent property. The analysis of this report was conducted using SAFIR, a non-linear finite element program developed at the University of Liege, Belgium. The analytical models consisted of a 410UB54 section forming the steel portal frames, with steel purlins and brace channels forming the roof structure and supported on the rafters. The concrete panels themselves were not included in the models, but they were represented by appropriate boundary conditions for the steel members. The ISO 834 Standard Fire and the Eurocode External Fire with and without a decay phase were used in this report. From a large number of analyses, it has been shown that the bases of the steel portal frames at the foundations must be designed and constructed with some level of base fixity to ensure that the structure will deform in an acceptable way during fire, with no outwards collapse of the walls. The analyses also show that it is not necessary for steel portal frame columns to be fire-protected unless the designer wishes to ensure that the columns and the wall panels remain standing, during and after the fire.