22.5Limiting Steel Temperature

The limiting steel temperature $(T_{1})$, in $^\circ \mathrm{C}$, in the case of ordinary steels, shall be calculated as follows:

where

$r_{f}$ = ratio of the design action on the member under fire to the design capacity of the member ( $R_{d} = R_{u}/\gamma_{m}$ ) at room temperature;

$R_{d}, R_{u} = \text{design strength and ultimate strength of the member at room temperature, respectively; and}$

$\gamma_{\mathrm{m}}$ = partial safety factor for strength.

The design action under fire shall consider the following:

a) Reduced bond likely under fire, and

b) Effects of restraint to expansion of the elements during fire.

Limiting steel temperature for special steels may be appropriately calculated using the thermal characteristics of the material obtained from the supplier of the steel.