Design Optimization of Long-Span Cold-Formed Steel Portal Frames Accounting for Effect of Knee Brace Joint Configuration

Abstract

Abstract:The application of cold-formed steel channel sections for portal frames becomes more popular for industrial and residential purposes. Experimental tests showed that such structures with long-span up to 20 m can be achieved when knee brace joints are included. In this paper, the influence of knee brace configuration on the optimum design of long-span cold-formed steel portal frames is investigated. The cold-formed steel portal frames are designed using Eurocode 3 under ultimate limit states. A novel method in handling design constraints integrated with genetic algorithm is proposed for searching the optimum design of cold-formed steel portal frames. The result showed that the proposed routine for design optimization effectively searched the near global optimum solution with the computational time is approximate 50% faster than methods being popularly used in literature. The optimum configuration for knee brace joint can reduce the section size of rafter and so the lighter frame could be obtained especially for long-span portal frame. The minimum weight of main frame obtained from optimization process is approximate 19.72% lighter than a Benchmark Frame used in the full-scale experimental test.

Coupled element and structural level optimisation framework for cold-formed steel frames

“…Column-to-rafter connections and rafter-to-rafter connections were assembled using eaves and apex joints, respectively, formed through plane brackets bolted between the webs of the channel-sections ( knee-to-column and knee-to-rafter connections can be reasonably considered as pinned joints [29,30], while moment connections with nine bolts used for apex joints were capable to carry bending moments [32]. It was also reported that rigid apex joints could be efficiently provided in the CFS portal frame by using bolt group length larger than 0.4 m [29,33]. In this study, it is assumed that the bolt group length is larger than triple of the beam depth, hence the effect of bi-moment caused by the presence of the bolts can be practically neglected [34][35][36].…”

“…To consider the design constraints for single objective optimisation in this study, an effective penalty approach [41,42] is applied, in which the penalised value for each violated design constraint is gradually decreased as the generation progresses [33,43]:…”

Determination of load transfer characteristics to connections between cold-formed steel truss and supporting system

“…One possible situation is the combination of CFS roof truss with supporting structural system made of HRS section or reinforced concrete. The reasonable location of knee brace joint on the supporting structural system should be one-third of the eaves height from the top and knee angle is approximate 42° (Phan et al , 2017). By optimising the frame geometry and knee brace, configuration can noticeably improve the structural performance and reduce the required structural weight (Phan et al , 2020).…”