System-level Design Method for High-strength Steel Hollow Section Structures : Eurocode-compliant approach

Abstract

In advanced design methods, an entire structural system can be directly designed based on the response of a nonlinear finite element analysis by monitoring the so- called load proportionality factor. This factor expresses the ratio of the system resistance to the design loads. Sufficient resistance of the system is ensured simply by comparing the load proportionality factor to the safety factor of resistance. This design approach differs remarkably from the conventional method, where the design of a structural system is verified by checking each member separately after structural analysis.

In recent literature, a sophisticated advanced design method called as the “Direct Design Method” (DDM) has been developed for framed steel structures for American, Australian and New Zealand standards. The heart of DDM is the partial factor of resistance, denoted as the system safety factor, <i>γADM</i>, which is determined based on extensive system-level reliability analyses. The <i>γADM</i> ensures that sufficient system reliability is achieved in design by considering all the relevant uncertainties regarding the resistance of the structural system. Consequently, the <i>γADM</i> is a structural family-specific safety factor which is tailored for certain family of systems.

In this research, a “Eurocode-compliant Direct Design Method” is developed for Warren truss steel portal frames. The developed DDM adopts the Eurocode framework for load combinations and yields the same system reliability as systems designed by the conventional Eurocode 3 method. The same reliability level with the conventional method is achieved by determining system-level reliability index of conventionally designed systems, which is further used as a target reliability level for the developed DDM.

Investigated trussed frames are built from members with cold-formed rectangular hollow sections made of the high-strength steel grade S700. Accurate modelling of these sections in nonlinear analyses requires tools for considering the effects of residual stresses and strain-hardened material. This research develops a probabilistic residual stress model and the so-called Effective Material Model (EMM) through which these effects are incorporated into reliability studies and finite element analyses. A wide variety of system-level reliability analyses are carried out for various trussed systems and load combinations to derive the target reliability index and the corresponding <i>γADM</i>.

Finally, the determined <i>γADM</i> is applied to a practical comparison in which trussed systems are designed both by the conventional design method and by the DDM developed in this dissertation. This comparison reveals that the DDM has a significant potential for reducing material consumption in trussed portal frames compared to the member-based conventional design method. This reduction is achieved by the capability of DDM to harness the full computational power of modern computers for the design use through nonlinear finite element analysis. By the sophisticated nonlinear analysis, DDM accurately captures the buckling behavior of continuous members and considers redistribution of forces and material plasticity in an entire structural system. These powerful capabilities, combined with the tailored system safety factor, provide ingredients for the next- generation computer-aided design tool developed in this research.