Steel Buildings in Europe

Part 7: Fire Engineering 7 - 29 composite column provide that an appropriate buckling curve is used. Checking the column consists of proving that the axial compression (for the combination of actions considered in fire situation according to EN 1991-1-2) is less than the buckling resistance of the column. For a given temperature distribution across the cross-section, the design resistance of a composite column N fi,Rd can be determined from the appropriate buckling column curve relating the load capacity N fi,Rd to the plastic load N fi,pl,Rd and the elastic critical load N fi,cr as follows:   fi,pl,Rd θ fi,Rd . N N    (22)  is the reduction factor for flexural buckling depending on the slenderness in fire situation θ  .For composite columns, θ  may be defined as: fi,cr fi,pl,R θ / N N   (23) where: fi,cr N is the Euler buckling load fi,pl,R N is the value of N fi,pl,Rd according to (24) when the partial security factors  M,fi,a ,  M,fi,s , and  M,fi,c ,of the materials are taken as 1.0 The reduction factor   is determined as for normal temperature design but using an appropriate buckling curve defined as function of column type (partially encased steel section, filled hollow steel section). The ultimate plastic load, N fi,pl,Rd of the cross-section is determined by summing the strengths of every part of the cross-section (yield stress for steel parts, compressive strength for concrete parts) multiplied by the corresponding areas, taking into account the effect of temperature on these elements, without considering their interaction (due to differential thermal stresses), i.e.:       m c k s j f A f A f A N ) ( ) ( ) ( . M,fi,c θ c, M,fi,s θ s, M,fi,a θ ay, a fi,pl,Rd    (24) N fi,cr is the Euler buckling load calculated as a function of the effective flexural stiffness of the cross-section fi,eff ( ) EI and the buckling length   of the column in fire situation, i.e.: 2 θ fi,eff 2 fi,cr ( ) π  EI N  (25) The effective rigidity ( EI ) fi,eff is determined from:       m k j I E E I E I EI ) ( ) ( ) ( ( ) θ c, θ sec, c, θ c, θ s, θ s, θ s, θ a, θ a, θ a, fi,eff    (26) where: θ , i E is the characteristic modulus of material i at the temperature  . For steel, it is the modulus of elasticity. For concrete: / 2 3 c,sec c, E E  