Experimental study of composite cold-formed steel C-section floor joists

Twelve large-scale slab specimens and twenty-two companion push-out specimens were tested to study the behavior and capacity of composite slab joists consisting of cold-formed steel C-sections and concrete. Four shear transfer mechanisms, including surface bond, pre-fabricated bent-up tabs, pre-drilled holes, and self-drilling screws, were employed on the surface of the flange embedded in the concrete to provide shear transfer capacity. Results indicated that specimens employed with shear transfer enhancements showed a marked increase in strength and reduced deflection compared with those relying only on a natural bond between steel and concrete to resist shear. Of the three shear transfer enhancements investigated, bent-up tabs provided the best performance at both the strength and serviceability limit states, followed by drilled holes in the embedded flanges. The use of self-drilling screws resulted in the lowest strength increase. The correlation of shear transfer capacity of push-out specimens with the ultimate capacity of large-scale specimens indicated that the average experimental flexural capacity of the slab specimens was approximately 1.16 times the average predicted value based on push-out test results.     

A new formulation for web crippling strength of cold-formed steel sheeting using genetic programming

This study presents Genetic programming (GP) as a new tool for the formulation of web crippling strength of cold-formed steel decks for various loading cases. There is no well established analytical solution of the problem due to complex plastic behaviour. The objective of this study is to provide an alternative robust formulation to related design codes and to verify the robustness of GP for the formulation of such structural engineering problems. The training and testing patterns of the proposed GP formulation are based on well established experimental results from the literature. The GP based formulation results are compared with experimental results and current design codes and found to be more accurate.     

Fire performance of cold-formed steel wall panels and prediction of their fire resistance rating

Recent research at the Queensland University of Technology has investigated the structural and thermal behaviour of load bearing Light gauge Steel Frame (LSF) wall systems made of 1.15 mm G500 steel studs and varying plasterboard and insulation configurations (cavity and external insulation) using full scale fire tests. Suitable finite element models of LSF walls were then developed and validated by comparing with test results. In this study, the validated finite element models of LSF wall panels subject to standard fire conditions were used in a detailed parametric study to investigate the effects of important parameters such as steel grade and thickness, plasterboard screw spacing, plasterboard lateral restraint, insulation materials and load ratio on their performance under standard fire conditions. Suitable equations were proposed to predict the time–temperature profiles of LSF wall studs with eight different plasterboard-insulation configurations, and used in the finite element analyses. Finite element parametric studies produced extensive fire performance data for the LSF wall panels in the form of load ratio versus time and critical hot flange (failure) temperature curves for eight wall configurations. This data demonstrated the superior fire performance of externally insulated LSF wall panels made of different steel grades and thicknesses. It also led to the development of a set of equations to predict the important relationship between the load ratio and the critical hot flange temperature of LSF wall studs. Finally this paper proposes a simplified method to predict the fire resistance rating of LSF walls based on the two proposed set of equations for the load ratio–hot flange temperature and the time–temperature relationships.     

Cyclic axial response and energy dissipation of cold-formed steel framing members

This paper summarizes results from an experimental program that investigated the cyclic axial behavior and energy dissipation of cold-formed steel C—sections structural framing members. Fully characterized cyclic axial load–deformation response of individual members is necessary to facilitate performance-based design of cold-formed steel building systems. Specimen cross-section dimensions and lengths were selected to isolate specific buckling modes (i.e., local, distortional or global buckling). The cyclic loading protocol was adapted from FEMA 461 with target displacements based on elastic buckling properties. Cyclic response showed large post-buckling deformations, pinching, strength and stiffness degradation. Damage accumulated within one half-wave after buckling. The total hysteretic energy dissipated within the damaged half-wave decreased with increasing cross-section slenderness. More energy dissipation comes at the cost of less cumulative axial deformation before tensile rupture.     

Structural performance of cold-formed lean duplex stainless steel columns

The structural performance of cold-formed lean duplex stainless steel columns was investigated. A wide range of finite element analysis on square and rectangular hollow sections and other available data, with a total number of 259 specimens, were considered. An accurate finite element model has been created to simulate the pin-ended cold-formed lean duplex stainless steel columns. Extensive parametric study was carried out using the validated finite element model. The column strengths predicted from the parametric study together with the available data are compared with the design strengths calculated from various existing design rules for cold-formed stainless steel structures. It is shown that the existing design rules, except for the ASCE Specification as well as the stub column and full area approach, are conservative. Modifications are proposed for the AS/NZS Standard, EC3 Code, and direct strength method. Reliability analysis was performed to assess the existing and modified design rules. It is also shown that the modified design rules are able to provide a more accurate and reliable predictions for lean duplex stainless steel columns. In this study, it is suggested that the modified design rules in the AS/NZS Standard and the modified direct strength method to be used in designing cold-formed lean duplex stainless steel columns.     

Shape optimization of cold-formed steel columns with fabrication and geometric end-use constraints

The objective of this paper is to present constrained optimization results for a cold-formed steel (CFS) cross-section shape with maximum axial capacity. In the authors׳ previous work unconstrained shape optimization was performed via stochastic search and gradient-based algorithms. Unconstrained shape optimizations produced a significant capacity increase, more than 140%, above standard CFS cross-sections, but many of the solutions are highly unconventional and have potential limitations both with respect to end use (e.g. attaching boards for walls and floors) and cost of manufacturing. Column capacity is determined using the Direct Strength Method (DSM) which requires inputs for the local, distortional and global critical buckling loads. These critical loads are obtained using the finite strip method, as implemented in the open source software CUFSM, which allows essentially any potential cross-section to be evaluated. To advance the applicability of the optimized results, end-use constraints and manufacturing constraints on the number of rollers employed in forming were both successfully incorporated in the shape optimization presented in this paper, resulting in optimized cross-sections that are more practical and economical with only marginally decreased capacity (usually less than 10%) from the earlier unconstrained optimized solutions. The constraints are implemented within a simulated annealing (SA) algorithm for the optimization. Optimized sections from multiple runs show uniformity, partially indicating the robustness of the final optimized shapes. The implemented constrained shape optimization provides a thorough search with high computational efficiency. The optimized cross-sections from this research provide promising potential shapes for the development of new commercial product families, and the member-level optimization methodology can also be integrated into building optimization in the future.     

自身形状优化原则:薄壁型材截面承载力的优化

为实现经济效益,已对大批量生产的钢结构产品的优化进行了广泛研究。旨在不削弱结构构件的强度和实用性的条件下,尽量减少材料的用量。当前研究主要集中于传统截面形状主要尺寸的优化,而并未考虑采用新的最佳截面。介绍了一种新的优化方法,利用遗传算法(GA)的进化和自适应特点,使截面的自身形状达到最佳效果。采用该方法对薄壁型材的截面承载力进行优化,验证了该方法的可行性和精确性。特别地,通过简单参数对截面的优化方法中,这种分析方法是众所周知的,即双轴对称闭口截面的优化。结果表明,经过几代演化,截面自身形状产生出精确的最优解。给出了影响收敛的因素。该方法可推广至冷弯型钢开口截面柱的优化。     

运用复合刚度方法对Z型构件支撑屋面系统的侧向约束力分析

复合刚度方法是一种可以预测Z型截面构件支撑的屋面系统侧向受力的方法。该方法将一系列Z型截面作为单自由度系统分析,并采用刚度公式计算系统中不同的构件对于抵抗侧向位移的贡献。从力学理论中推导出屋面系统所需的抗力。     

An experimental investigation on the seismic behavior of cold-formed steel walls sheathed by thin steel plates

The use of cold-formed steel (CFS) frames has grown extensively in recent years, particularly in the earthquake-prone regions. However, the behavior of lateral resisting systems in CFS structures under seismic loads has not been scrutinized in detail. Towards this, an experimental investigation has been conducted on cold formed steel frames sheathed by thin galvanized steel plates, the results of which are presented here. The experiments involve 24 full-scale steel plated walls tested under cyclic loading with different configurations of studs and screws. Of particular interest were the specimens׳ maximum lateral load capacity and the load-deformation behavior as well as a rational estimation of the seismic response modification factor, R. The study also evaluates the failure modes of the systems. The main factors contributing to the ductile response of these shear walls are also discussed in order to suggest improvements so that the walls respond plastically with a significant drift and without any risk of brittle failure.     

Flexural behaviour and strength of cold-formed steel L-headers

Cold-formed steel headers are structural components used over wall openings in cold-formed steel residential and light commercial construction. Recently, there has been an increased interest in cold-formed L-headers among homebuilders primarily due to their ease of installation and low material cost. The findings from an extensive laboratory testing program, of full-scale single and double cold-formed steel L-headers are presented in this paper. The objective of the research was to investigate the flexural behaviour and strength of L-headers under both gravity and uplift loads. Based on the results improved ultimate strength design expressions and new deflection expressions for a wide range of L-header assemblies have been proposed.