SECOND-ORDER ANALYSIS FOR LONG SPAN STEEL STRUCTURE PROTECTING A HERITAGE BUILDING

摘要

Second-order analysis has been widely referred in codes and claimed by structural analysis software. There are actually many types of second-order analysis with the simplest one that considers only the P-Δ effect and other more advanced ones that include the P - Δ - δ effect with careful consideration of initial imperfections as well as plastic hinges. In Hong Kong, which has one of the most highest world's density in highrise steel composite buildings with height greater than 150 m, this method of design is widely used and the success is partly due to the use of one-element-per-member model which saves time and efforts in design and improves safety as well. This paper is concerned with the-protection of a heritage building. A long span steel building of clear span 30 m and clear height 14 m is designed to house the heritage building using the second-order analysis to Eurocode-3~([1]) without the assumption of effective (or buckling) length. The computer method employs the reliable curved element with curvature set as member initial imperfection so that the member P-δ effect could be directly considered in the analysis~([2,3]). It further utilizes the critical mode as initial imperfection mode for the global frame P - Δ imperfection in its checking of sway stability. The advantages of the proposed design method based on second-order analysis include its ignorance of the uncertain assumption of effective length for the vertical columns which are subject to heavy axial forces from the loads of a restaurant at the top of the steel building. As the design method uses only the basic material properties of the material as well as their imperfections, it could be applied to the analysis of structures of different ages allowing for deterioration of material properties under various scenarios like seismic~([4]) and materials by simply adopting the properties of materials adopted for the structures. Consequently, the proposed new method could become a unified design method for design for structures under complex loads and scenarios and this new codified method is replacing the old effective length method.