A NEW DESIGN METHOD FOR INDUSTRIAL PORTAL FRAMES IN FIRE

For single-storey steel portal frames in fire, especially when they are situated close to a site perimeter, it is imperative that the boundary walls stay close to vertical, so that fires which occur are not allowed to spread to adjacent properties. A current UK fire design guide requires either that the whole frame be protected as a single element, or that the rafter may be left unprotected if column bases and foundations are designed to resist the forces and moments generated by rafter collapse, in order to ensure the lateral stability of the boundary walls. This can lead to very uneconomical foundation design and base-plate detailing. In previous studies carried out at the University of Sheffield it was found that a fundamental aspect of the collapse of a portal frame rafter is that it usually loses stability in a “snap-through ” mechanism, but is capable of re-stabilising at high deflections, when the roof has inverted but the columns remain close to vertical. Numerical tests performed using the new model show that the strong base connections recommended by the current design method do not always lead to a conservative design. It is also found that initial collapse of the rafter is always caused by a plastic hinge mechanism based on the frame’s initial configuration. If th