Innovative cold-formed steel nested tapered box section portal frames with bolted-side plate joints

Abstract

Nested tapered box (NTB) portal frames made entirely of cold-formed steel (CFS) are widely used in New Zealand. Shahmohammadi (2019) recently described a full-scale test of such a system. The tested frame had a span of 18.16 meters and a height to the eaves of 4.27 meters. The joints of this NTB portal frame were rigid, constructed with bolted end plates. However, bolted end plate joints can be expensive due to the full penetration butt weld. This thesis proposes an alternative jointing system that uses bolted-side plates. The numerical work conducted in this study yields the following findings: • A shell finite element (FE) model was developed and validated using previously conducted full-scale portal frame tests. This validated model was then employed to compare the performance of bolted-side plate joints with bolted end-plate joints. The analysis revealed that a portal frame with 10 mm thick bolted side plates could sustain the same load as an NTB portal frame with bolted end-plate joints. Increasing the thickness of the bolted-side plates from 10 mm to 16 mm resulted in a 20% increase in load-carrying capacity. This improvement was due to the confinement effect provided by the side plates, which reduced the slenderness ratio near the eaves joint. However, the NTB portal frame with bolted-side plates exhibited greater apex displacement compared to the frame with bolted end-plate joints. • FE models were developed for both the eaves and apex joints of the NTB portal frame. A parametric study was then carried out involving 1,000 FE models. Using FE models, design equations are proposed for the moment capacities of the eaves and apex bolted side plates, accounting for both opening and closing moments under partial restraint. • A parametric study was conducted using the developed design method on eight models, considering a range of spans from 25m to 40m and eaves heights from 5.5m to 6.25m, with a consistent frame spacing of 7.5m. The results showed that portal frames with bolted-side plates can carry on average 7% more load compared to those with bolted end plate joints. • A methodology was presented to predict the load-carrying capacity of portal frames with back-to-back channel sections. It was shown that the predicted vertical loads from the proposed approach were, on average, within 1% (on an average) of the FEA results for portal frames. The same approach was applied to NTB portal frames through worked examples in Appendix B and C, demonstrating good accuracy.