Effect of load

It is known from full scale fire tests that a simply supported beam carrying a concrete floor slab and 60% of its cold load bearing capacity will become plastic at about 620ºC. It is also known that if it carries a lower load then plasticity will occur at a higher temperature.  Thus, at low loads, fire resistance is increased.

In BS5950 Part 8, (see Code of Practice for Fire Resistant Design),  load is expressed in terms of the ‘Load Ratio’ where

Load Ratio =  the load at the fire limit state
                         the load capacity  at 20˚C

The load at the fire limit state is calculated using load factors given in BS5950 Part 8. A fully loaded beam in bending would normally have a load ratio of about 0.50 - 0.6.  It is known from the research data that, with a load ratio of 0.25, for example, failure in simply supported beams carrying concrete slabs will not occur until the steel reaches 750ºC, an increase of 130ºC on the limiting temperature in the fully loaded case, Figure 32 shown below.  Also please see Limiting temperature method.