Industrial portal frames near to other buildings must keep their vertical walls standing in fire in order to prevent fire spread.A recently developed analysis, implemented in the program Vulcan, using a combination of static and dynamic Mens Bags solvers, has shown that the strong base connections recommended by the current design method may not always lead to conservative design.A second-phase failure mechanism observed in numerical modelling, and the critical temperature at which final run-away collapse occurs, may be higher than the temperature at which the roof frame initially loses its stability, because a re-stabilisation often happens.A new method for estimating critical temperatures of portal frames in fire, using these two failure mechanisms, is presented.Numerical tests on typical industrial frames Wine Tumbler are used to calibrate this new method.