Analysis and design of semi-rigid modular structures

Abstract

Steel bolted joint is a feasible option for the connection of modular constructions, and the engineering challenge is to ensure that the joints are well designed with sufficient strength and stiffness, so that there is no risk of collapse or excessive deformation. An integrated Curved-Quartic Function (CQF) beam-column element to include large displacement, plastic hinges and geometric imperfections has been developed to assess the structural performance of a steel frame with semi-rigid joints in terms of collapse loads and side-sways. The ultimate load is found to be reduced by only about 10% relative to a fully rigid joint for a fixity factor of 0.7. A series of laboratory tests on semi-rigid joints have been conducted to determine their strength and stiffness, and the results are verified rigorously with the large deformation elastoplastic finite element analysis. The stiffness in terms of the fixity factor of a typical bolt joint is about 0.5, which could be increased to 0.7 or more by the “Unicorn” stiffening scheme. The strength of the bolt joints could also be greatly increased by proper stiffening. By nature of the different response in the two opposite directions, the asymmetric joints ought to be analysed following the concept of semi-rigid joints such that the response depends on the direction of the moment. If the asymmetric joint is analysed as a symmetric joint, the joint may fail due to moment reversal on the weaker direction, and the force distribution of the other structural members may not be correct. The collapse load could be increased by 35% if the proper asymmetrical approach is adopted in the analysis compared to a nominally rigid joint design. The characteristics of a typical modular construction of a scaffold system are the rapid erection, scalability, repeated components and ease of dismantle for reuse. Two major factors likely to cause structural failure are the ground settlement and the global stability. Two approaches, which are the CQF beam-column element capable of capturing the global buckling load and the Effective Length Method (ELM) for local buckling of individual members, have been applied to analyse the collapse load of a scaffold system. It is found that for lateral restrained scaffold systems, in employing the ELM, a value of 2 for effective length of members should be allowed to ensure structural safety of the scaffolding. Whereas for scaffold systems without adequate lateral restraints, global stability analysis ought to be carried out as the load bearing capacity could be reduced to as low as 1/5, which cannot be covered by any factor of safety.