Application of a system-level approach to the design of steel structures

Abstract

Design of steel structures according to Eurocode is currently performed by a two-step approach, where structural analysis is followed by design checks on each individual structural element. In this approach, the resistance of a structural system is determined by a single member that first reaches its capacity. However, the redundancy of a structure allows redistribution of forces after a member fails, which means that as a system, the structure has additional load-bearing capacity. This capacity is exploited in the recently introduced direct design method (DDM), which determines the resistance of the entire structural system through advanced structural analysis. While the framework of the DDM seemingly enables more efficient use of material, it is not clear how the method compares with the governing member-based design approach of the Eurocode.This study quantifies the efficiency of the DDM in terms of material consumption compared to the conventional two-step design approach. Trussed plane frames with hollow section members are designed for minimum weight employing the DDM and EN 1993 separately. These structures are widely used in the Nordic countries as main frames of hall-like buildings, and they possess redundancy to enable redistribution of internal forces. Therefore, they provide an interesting application for demonstrating the practical use of the DDM, with analysis of the benefits and challenges of the method.Results of numerical case study show that the DDM can achieve at least a material saving of 6.6 % compared to the conventional two-step design approach. This leads to a more economical use of steel, as well as a reduced environmental impact. The material savings can be further increased by considering the rotational stiffness of the joints, which is also illustrated in the case study.