Optimized design of steel car parks for fully spread fires

The design optimization of steel structure is usually carried out by choosing a profile that minimizes the amount of steel, under the boundaries of carrying mechanical loads and being compliant with maximum allowed displacements. The fire verifications are typically performed at a later design stage by applying the amount of insulation needed to keep the profile below its critical temperature during fire. This strategy keeps the steel costs to a minimum, but often results in relatively high costs for insulating the structure, which may nullify the scope of the original design optimization. This study investigates possible beneficial effects of including fire verifications at the early design stage of a structure. The methodology is presented with reference to a steel car park structure insulated with intumescent paint against a fully spread fire and is based on the minimization of the sum of the steel and insulation costs. The results show that the optimization leads to a reduction of 36% of the material costs.
The price is also compared to the case of an uninsulated structure designed against local fires. This design assumption allows avoiding fire insulation, but has been challenged by past cases of major fire spread, such as the recent car park in Liverpool. The paper shows that the conservative assumption of a full spread fire corresponds to the doubling of the material costs, when the structure is traditionally optimized by minimizing the amount of steel for the ordinary design conditions. However, the proposed optimization limits the cost increment to 19% of the material costs and just 6% of the total costs of the structure. This increment lies within the expected variation of costs during the realization of a structure and indicates that a higher safety level of steel car parks can be achieved, while maintaining a competitive market price against concrete structures.