Structural analysis of tubular truss in fire

Tubular trusses with welded gap joints are internally indeterminate if they are modeled with beam elements using continuous chords and eccentricities at the joints. Trusses may be externally statically determinate or not, depending on their support conditions. The start point was a tubular truss, which was sizing and shape optimized. This means that the members and the joints can resist just the load in normal situation. Next the same truss was analyzed using material and geometrical non-linear theory with reduced load in fire allowing horizontal displacement at the other end and restraining it. The idea was to study: 1) is the normal tubular truss such that in externally determinate case the linear theory can be used in fire, 2) what happens in externally indeterminate case? The results were: 1) the linear analysis can be used in fire to determine the stress resultants and 2) the linear model is very conservative.