Clearance Between Single-Story Steel Frames and Firewalls

Abstract

A simple formula is developed to estimate the separation distance required between single-story steel frames and firewalls so that during an uncontrolled fire the frame can expand without contacting and damaging the wall. Detailed analysis shows that the frames reach their maximum lateral displacement at the birth of plastic hinges in the girders. The hinges allow the girders to sag excessively, pulling inward on the columns and causing the frame to collapse away from firewalls. The simple method presented uses the frame geometry, member cross sections, loads, and the temperature-dependent steel properties to predict the steel temperature at which the plastic hinges form. The formula uses this temperature and the equilibrium of the deformed frame to calculate the minimum acceptable clearance. The fire is assumed stationary and fully developed. The exposed portions of the structure are assumed totally enveloped by the hot gas. The method includes the column resistance to thermal expansion and the plastic deformations that originate from the second-order effects due to the axial loads in columns acting on the deformed frame. The approach lends itself to spreadsheet calculations for fast design decisions and is accurate, which is demonstrated by good agreement with experimentally validated finite element results.