Steel Buildings in Europe

Part 6: Fire Engineering 6 - 31 To meet the requirements of fire safety engineering, the designer must ensure that one of the following conditions is met:  At t fi,requ the temperature of the member is lower than the critical temperature: θ cr ≥ θ a,t or  At t fi,requ the resistance of the members is greater than the design effects: R fi,d,t ≥ E fi,d . 4.4 Simple structural fire design methods 4.4.1 Introduction The simple design methods follow the principle of ultimate limit design as for normal temperature. Fire safety requirements are satisfied by providing a design which will meet the above conditions. Simple calculation models take account of the reduction of the action effects for fire design and the variation of material properties at elevated temperature. The simple calculation models have broader application than the prescriptive approach, and can be applied to:  Unprotected steel members, including tension members, steel beams and steel columns.  Unprotected composite members, including composite slabs and beams, concrete encased beams and columns, concrete filled hollow columns.  Protected steel and composite members. Design effect of actions, E fi,d The fire action is designated as an accidental action in the fire parts of Eurocodes. Due to the low probability of both a major fire and full external actions occurring simultaneously, the effect of actions for fire design can be determined by reducing the design effects of external actions for normal design using a reduction factor  fi : E fi,d =  fi E d where: E d is the design effect of actions for normal design using EN 1990  fi is the reduction factor of the effect E d for fire design, calculated as  fi = Q,1 k,1 G k fi k,1 k G Q G Q      for combination 6.10 from EN 1990 G k is the characteristic value of the permanent action Q k,1 is the characteristic value of the leading variable action  fi is the combination factor given by  1,1 or  2,1 , see EN 1991-1-2  G is the partial factor for permanent actions  Q,1 is the partial factor for the leading variable action. For the calculation of the reduction factor for use with combination 6.10a and 6.10b, the designer should refer to EN 1993-1-2 §2.4.2.