A Novel Tapered Box Cold-formed Steel Portal Framing System: Full-Scale Testing, Simulation and Design Optimisation

Abstract

This thesis presents the development of a novel cold-formed steel portal framing system trademarked as VERTEBEAM® (VB) which is jointly patented by New Zealand Steel and Donovan Group. The portal frame uses tapered/parallel box members comprising two cold-formed nested channel sections intended for use for buildings with clear spans between 16m to 48m. The advantage of the system lies in the structural and non-structural benefits a boxed section presents, such as improved bird, dust and corrosion resistance, aesthetic improvement, and enhanced seismic performance. The novelty of the system also lies in the tapering ability of VB, which results in material and paint savings. Following validating the value propositions of the system in the first chapter, this research provides the following technical developments through chapters two to four : a) Full-scale experimental tests and finite element analysis verification Two portal frames of 18.2m span were built and tested to failure for two common load combinations. The first was lateral cyclic loading into the inelastic range and the associated vertically acting permanent load. The second was vertical loading to failure. The possibility of ductile plastic hinge formation in a severe earthquake was investigated by lateral cyclic testing. Non-linear elasto-plastic finite element models of the test frames were developed in the finite element program ABAQUS, which showed good agreement with the experimental test results in terms of both initial stiffness and failure loads. An elastic beam idealisation using non-prismatic beam elements was made in SAP2000, which showed good agreement with test results in terms of initial stiffness. Recommendations are made for beam modelling of the VB frames for engineering design purposes. b) Design methodology: The Direct Strength Method (DSM) is used for the design of VB members. The capacity of the governing sections by DSM has been verified against the full-scale test results. The available Finite Strip Analysis (FSA) tools developed for calculating required local and distortional buckling stresses required in DSM are not currently able to incorporate the effects of depth variation, moment variation, and contact between the section elements. This study shows the influence of these effects on VB sections by conducting a parametric study using Eigen Buckling Analysis in ABAQUS. Although these effects are minor (less than 5% difference with test result if not considered), reduction or increasing factors are suggested to be applied to the buckling stresses coming from FSM. Local and distortional buckling stresses for a wide range of possible VB sections have been determined using THIN-WALL software, and then they are formulated to be used in the optimisation software. c) Optimisation using Genetic Algorithm: Design of tapered portal frames is a relatively time-consuming repetitive process due to the number of input variables and constraints involved. This makes the problem well suited for optimisation. A user-friendly frame analysis and design program, entitled OPTIDES hereafter, has been developed by the author to implement the optimisation study for the VB frames. The software can be considered as the starting point for commercialisation of automated analysis, design, optimisation and drafting portal frame software in New Zealand to increase time efficiency and quality of the steel portal frame designs. The Genetic Algorithm is incorporated to minimise the weight and cost of the example frames. OPTIDES produces efficient designs in a considerably shorter time compared to those designed by conventional tools. In order to indicate the advantage of cost minimisation over the weight minimisation approach, two VB portal frame examples are studied. For indicating the economic viability of VB, cost and weight of two sample VB portal frames are compared with that of Tapered I-member option. The result supports the idea of cost minimisation as minimum weight may not always be reflecting the minimum cost. d) Moment end-plate (MEP) connection design for VB members using FEM: No straightforward procedure was found for designing Flush MEP connections for box members; hence a simplified calibrated Finite Element modelling in SAP2000 has been proposed in Appendix C.