Cost-optimization based target reliabilities for design of structures exposed to fire

Adequacy of structural fire design in uncommon structures is conceptually ensured through cost-benefit analysis where the future costs are balanced against the benefits of safety investment.Cost-benefit analyses,however,are complicated and computationally challenging,and hence impractical for application to individual projects.To address this issue,design guidance proposes target reliability indices for normal design conditions,but no target reliability indices are defined for structural fire design.We revisit the background of the cost-optimization based approach underlying normal design target reliability indices then we extend this approach for the case of fire design of structures.We also propose a modified objective function for cost-optimization which simplifies the evaluation of target reliability indices and reduces the number of assumptions.The optimum safety level is expressed as a function of a new dimensionless variable named“Damage-to-investment indicator”(DII).The cost optimization approach is validated for the target reliability indices for normal design condition.The method is then applied for evaluating DII and the associated optimum reliability indices for fire-exposed structures.Two case studies are presented:(i)a one-way loaded reinforced concrete slab and(ii)a steel column under axial loading.This study thus provides a framework for deriving optimum(target)reliability index for structural fire design which can support the development of rational provisions in codes and standards.

Thermal response of steel framing members in open car park fires

For open car park structures,adopting a performance-based structural fire design is often justified and allowed because the fire does not reach flashover.However,this design approach requires an accurate assessment of temperatures in structural members exposed to car fires.This paper describes a numerical study on the thermal exposure on steel framing members in open car park fires.Steel temperatures are computed by the coupling of computational fluid dynamics and finite element modeling,and by analytical models from the Eurocodes.In addition,the influence of galvanization on the steel temperature evolution is assessed.Results show that temperatures in unprotected beams and columns are influenced by the section geometry,car fire scenario,modeling approach,and use of galvanization.Galvanization slightly delays and reduces peak temperature.Regarding the different models,CFD-FEM(CFD:computational fluid dynamics,FEM:finite-element method)coupled models predict lower temperatures than the Hasemi model,because the latter conservatively assumes that the fire flame continuously touches the ceiling.Further,the Hasemi model cannot account for the effect of reduced emissivity from galvanization on the absorbed heat flux.Detailed temperature distributions obtained in the steel members can be used to complete efficient structural fire designs based on the member sections,structure layout,and use of galvanization.