Steel Buildings in Europe

Part 7: Fire Engineering 7 - 26 Table 5.3 Values of reduction factor  fi as function of non dimensional slenderness at normal temperature  and the steel grade Steel grade Steel grade  S235 S275 S355  S235 S275 S355 0,2 0,8480 0,8577 0,8725 1,7 0,1520 0,1549 0,1594 0,3 0,7767 0,7897 0,8096 1,8 0,1381 0,1406 0,1445 0,4 0,7054 0,7204 0,7439 1,9 0,1260 0,1282 0,1315 0,5 0,6341 0,6500 0,6752 2 0,1153 0,1172 0,1202 0,6 0,5643 0,5800 0,6050 2,1 0,1060 0,1076 0,1102 0,7 0,4983 0,5127 0,5361 2,2 0,0977 0,0991 0,1014 0,8 0,4378 0,4506 0,4713 2,3 0,0903 0,0916 0,0936 0,9 0,3841 0,3951 0,4128 2,4 0,0837 0,0849 0,0866 1 0,3373 0,3466 0,3614 2,5 0,0778 0,0788 0,0804 1,1 0,2970 0,3048 0,3172 2,6 0,0725 0,0734 0,0749 1,2 0,2626 0,2691 0,2794 2,7 0,0677 0,0686 0,0699 1,3 0,2332 0,2387 0,2473 2,8 0,0634 0,0642 0,0653 1,4 0,2081 0,2127 0,2200 2,9 0,0595 0,0602 0,0612 1,5 0,1865 0,1905 0,1966 3 0,0559 0,0565 0,0575 1,6 0,1680 0,1714 0,1766 Steel beams The design moment resistance for the fire design situation of a laterally unrestrained beam with a Class 1, 2 or 3 cross-section, at a uniform temperature  a is given by:   Rd θ y, M,fi M0 LT,f fi,t,Rd k M M i       (21) where: θ y, k is the reduction factor for the yield strength of steel at the steel temperature θ reached at time t Rd M is the moment resistant of the gross cross-section (plastic moment resistant pl,Rd M or elastic plastic moment resistant el,Rd M for the normal temperature design calculated using EN 1993-1-1 LT,fi  is the reduction factor for lateral-torsional buckling in the fire design situation. It may be calculated in the same way as the reduction factor for flexural buckling but using the appropriate non- dimensional slenderness For laterally restrained beams, the same design method can be used, adopting 1 LT,fi   . Often structural members will not have a uniform temperature. An adaptation factor κ 1 can be introduced to take account a non-uniform temperature distribution over the height of the steel section. A further adaptation factor κ 2 can be also introduced to account for variations in member temperature along the length of the structural member when the beam is statically indeterminate.