Structural behavior of lapped cold-formed steel Z sections with generic bolted configurations

In order to improve the buildability of cold-formed steel structures, a series of research and development projects have been undertaken by the authors to study the structural behavior of bolted moment connections between cold-formed steel sections. As one of the major applications of cold-formed steel sections in building construction is modern roof structures with multi-span purlin systems, an extensive experimental and theoretical investigation on the structural behavior of lapped moment connections between cold-formed steel Z sections was carried out. Both generic configurations with high structural efficiency were adopted, namely, Config. W4 and W6, in which only the webs of the sections were bolted together for easy installation. Moreover, an analysis and design method was proposed to assess both the moment resistance and the effective flexural rigidity of the generic lapped connections. The research work aims to provide understanding on the structural behavior of cold-formed steel Z sections with lapped connections, and hence, to develop a set of rational design rules for multi-span purlin systems with overlaps.In order to verify the applicability of the proposed analysis and design method for other commonly adopted connection configurations, namely, Config. W2F2 and W4F2 where bolts were installed to both section webs and flanges within the lapped connections, an experimental and theoretical investigation on a total of 12 one point load tests on lapped cold-formed steel Z sections were carried out. Among all tests, section failure under combined bending and shear at the ends of lap was found to be critical while twisting of the lapped Z sections was apparent throughout the entire deformation ranges. In general, the structural behavior of lapped connections with Config. W2F2 and W4F2 was found to be similar to those with Config. W4 and W6.Moreover, the formulation of the proposed analysis and design method was modified to accommodate the presence of the flange bolts, and back analysis of the lapped Z sections against combined bending and shear using the proposed method was performed. After careful calibration against test data, the method was shown to be structurally adequate and efficient for lapped connections with Config. W2F2 and W4F2. Moreover, simple design expressions were also proposed for the evaluation of effective flexural rigidities of the lapped connections. Comparison on the structural behavior between the two sets of connection configurations was also presented.     

Analysis of cold-formed purlins with slotted sleeve connections

In multi-span purlin design, it is customary to assume beam continuity, avoiding joint analysis. This analysis is safe when employing full strength and stiffness connection. There are two usual joint typologies: overlapping and sleeve connections. The main advantage of sleeve joints is reduction of cost and time in the assembly process. Overlapping joints can be correctly designed selecting the overlap length to depth ratio, for a wide range of Z-shaped profiles. In this paper, a partial strength and stiffness Z profile sleeve joint is experimentally and numerically analyzed, studying their structural behavior and characterizing their rotational stiffness and strength.     

Buckling behavior of cold-formed zed-purlins partially restrained by steel sheeting

This paper presents a study on the buckling behaviour of purlin-sheeting systems under wind uplift loading. The restraint provided by the sheeting to the purlin is modeled by using two springs representing the translational and rotational restraints. The analysis is performed using finite strip methods in which the pre-buckling stress is calculated based on the same model used for the buckling analysis rather than taken as the ‘pure bending’ stress. The results obtained from this study show that, for both local and distortional buckling, the restraints have significant influence on the critical loads through their influence on the pre-buckling stress rather than directly on the buckling modes. While for lateral-torsional buckling, the influence of the restraints on the critical loads is mainly due to their influence on the buckling modes rather than the pre-buckling stress.     

Moment resistance and flexural rigidity of lapped connections in multi-span cold-formed Z purlin systems

This paper reports an experimental investigation on the structural behavior of lapped connections over the internal supports in multi-span cold-formed steel Z purlin systems. The moment resistance and effective flexural rigidity of two typical connection configurations, i.e. web bolts plus self-drilling screws at both flanges or at the top flange only, are considered. In the moment resistance test, a total of 30 specimens with different connection configurations and connection lengths are tested, from which, after careful calibration, the assessment of moment resistance of such lapped connections as well as several useful design suggestions are proposed.In the effective flexural rigidity test, full-scale two-spanned purlins, with two external pin ends and one lapped connection over the internal support, are adopted. Lapped connections with different configurations and two practical connection lengths of 10% and 20% of span are concerned. Based on test results, the effective flexural rigidities of lapped connections studied in this paper are proposed, respectively, for the stress analysis and deflection analysis.     

Flexural performance of symmetrical cold-formed thin-walled members under monotonic and cyclic loading

This paper focuses on the flexural behavior of cold-formed steel members with symmetrical axes. The cold-formed section is clamped at a flange location to form a closed loop. Forty-one specimens with various lengths and sectional dimensions were tested under monotonic and cyclic loading to study their static and dynamic responses. It was found that members with higher flexural rigidities exhibited stiffer pre-buckling behavior, however, higher strength loss was encountered during the post-buckling stages than in members with lower flexural rigidities. A modified analytical model based on test result calibration for the calculation of sectional properties is proposed for design reference.     

Effective Width Method for determining distortional buckling strength of cold-formed steel flexural C and Z sections

This paper proposes a design method, based on the Effective Width Method, for determining the nominal distortional buckling strength of typical cold-formed steel C and Z sections subjected to bending. The method can be integrated into the classic effective width design provisions specified in AISI S100, and it allows the conventional design approach to cover more comprehensive limit states. The proposed method is calibrated by the flexural distortional buckling strength predicted by the Direct Strength Method. Comparison with experimental results indicates that the proposed method yields reasonable predictions for the flexural distortional buckling strength of industrial standard C and Z sections. The method offers the same level of accuracy and reliability as the Direct Strength Method.     

Study on distortional buckling performance of cold-formed thin-walled steel flexural Members with stiffeners in the flange

This paper presents a finite strip program CUFSM used to calculate and analyze the elastic distortional buckling of cold-formed thin-walled steel flexural members with stiffeners in the flange, which has different sectional geometric parameters. According to the classical buckling stress formula, the distortional buckling coefficient of the flange can be calculated so as to analyze the influence of changed sectional geometric parameters on it. On this basis, this study provides a simplified formula of distortional buckling stress to calculate 40 members with different sections which are selected from the Technical Code of Cold-Formed Thin-Wall Steel Structures of China but not contained in this paper. Compared with the analysis results of CUFSM, it shows that the two simplified formulas have quite high accuracy and wide applicability for general members provided by the specification. So it is suggested that they can be used for engineering design and standard revision.     

Buckling analysis of thin-walled cold-formed steel structural members using complex finite strip method

In this paper, a generalised complex finite strip method is proposed for buckling analysis of thin-walled cold-formed steel structures. The main advantage of this method over the ordinary finite strip method is that it can handle the shear effects due to the use of complex functions. In addition, distortional buckling as well as all other buckling modes of cold-formed steel sections like local and global modes can be investigated by the suggested complex finite strip method. A combination of general loading including bending, compression, shear and transverse compression forces is considered in the analytical model. For validation purposes, the results are compared with those obtained by the Generalized Beam Theory analysis. In order to illustrate the capabilities of complex finite strip method in modelling the buckling behavior of cold-formed steel structures, a number of case studies with different applications are presented. The studies are on both stiffened and unstiffened cold-formed steel members.     

Analysis and design reliability of axially compressed members with high-strength cold-formed thin-walled steel

High-strength cold-formed thin-walled steel has been widely used in developed countries in the last several years. However, the application and popularization of the new materials in China is still very limited, and there are no related provisions in current design codes for reference in engineering practice. In this paper, considering the effects of the variations from material strength in structural members, geometrical parameters of sections, analysis methods for limit load-carrying capacity and external loadings, the second-order moment probability method was used for conducting the design reliability analysis of 550 MPa high-strength cold-formed thin-walled steel structures, from which the reasonable target reliability index, the corresponding resistance partial coefficient and the design strength values were discussed and proposed. Existing experimental data related have been collected and used to demonstrate the suitability and reasonability of the proposed results, which shows that, with reasonable determination of the design strength value of 550 MPa high-strength cold-formed thin-walled steel sections, the effective width-to-thickness ratio method considering coupling stability of plates recommended by “Technical Code of Cold-formed Thin-Walled Steel Structures (GB50018-2002)” can be efficiently used to estimate the load-carrying capacities of the axially compressed elements of high-strength cold-formed thin-walled steel structures, and well satisfy the target reliability index in structural design.     

Review: The Direct Strength Method of cold-formed steel member design

The objective of this paper is to provide a review of the development and current progress in the Direct Strength Method for cold-formed steel member design. A brief comparison of the Direct Strength Method with the Effective Width Method is provided. The advantage of methods that integrate computational stability analysis into the design process, such as the Direct Strength Method, is highlighted. The development of the Direct Strength Method for beams and columns, including the reliability of the method is provided. Current and ongoing research to extend the Direct Strength Method is reviewed and complete references provided. The Direct Strength Method was formally adopted in North American cold-formed steel design specifications in 2004 as an alternative to the traditional Effective Width Method. The appendices of this paper provide the Direct Strength Method equations for the design of columns and beams as developed by the author and adopted in the North American Specification.     

Analysis and design of lapped connections between cold-formed steel Z sections

This paper presents an analysis and design method for lapped connections between cold-formed steel Z sections after careful calibration against test data obtained from a total of 26 one point-load tests on lapped connections. Based on the experimental observations on the lapped connection tests where combined bending and shear is always critical in the cross-sections at the end of laps, an analysis method is proposed to evaluate all the internal forces within the lapped connections. Once the co-existing moments and shear forces in the lapped connections are evaluated, the critical sections are readily checked against combined bending and shear using codified design rules. Moreover, design expressions are also proposed for the evaluation of effective flexural rigidities of lapped connections with various bolt configurations against practical lap length to section depth ratios. Consequently, the structural behaviour of lapped sections between cold-formed steel Z sections in terms of strength and stiffness is quantified rationally for general design. The research project aims to provide understanding to the structural performance of lapped connections between cold-formed steel Z sections, and hence, to develop a set of rational design rules for multi-span purlin systems with overlaps in modern roof construction.     

冷弯薄壁槽形檩条的弯扭性能对比研究

通过对冷弯薄壁槽形檩条弯扭作用下截面各点应力的分析，并把理论计算结果与规范公式计算结果进行了对比，得到规范中正应力计算公式与理论计算结果基本一致，但剪应力计算结果却偏差较大。     

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.     

冷弯薄壁型钢屋架受力性能及杆件计算长度研究

为研究低层冷弯薄壁型钢结构屋架的受力性能,对两个不同构造形式的屋架足尺试件进行了受弯性能试验研究,考察了冷弯薄壁型钢结构屋架的工作原理和破坏模式。试验结果表明：钢桁架的破坏主要是支座处节点连接的破坏,而杆件没有发生屈曲,不属于强度破坏。采用非线性有限元分析方法对钢结构屋架进行了变参数分析,结果表明：钢材强度、截面形式以及钢材厚度对桁架承载力都有较大的影响。采用柱挠度曲线(CDC)法分析了节点连接半刚性对冷弯薄壁型钢桁架压杆计算长度的影响,研究表明,冷弯薄壁型钢桁架压杆半刚性连接的计算长度与节点转动刚度和构件自身刚度有关,建议受压弦杆、端斜腹杆以及端竖腹杆平面内计算长度系数取为1.0,受压腹杆平面内计算长度系数取为0.9。     

高强冷弯薄壁型钢卷边槽形截面轴压构件畸变屈曲性能研究

建立了考虑材料和几何双重非线性的550MPa高强冷弯薄壁型钢卷边槽形截面轴压构件畸变屈曲性能分析的有限元模型,并通过对两种厚度高强冷弯薄壁型钢轴压构件畸变屈曲试验已有结果的分析比较验证了其有效性;采用该模型进一步分析了厚度、长度、初始缺陷模式及幅值等参数对畸变屈曲轴压构件承载力的影响,并对轴压构件畸变屈曲发生机理进行了探讨。结果表明：厚度、长度和初始缺陷模式是影响畸变屈曲轴压构件承载力的主要因素,且卷边面内屈曲是槽形截面轴压构件发生畸变屈曲的主要原因。通过理论计算与试验结果的对比分析,表明可以采用建议方法计算此类复杂截面轴压构件的畸变屈曲承载力。     

Experimental and numerical analysis on the structural behaviour of cold-formed steel beams

A research study on the structural behaviour of cold formed steel beams with C-, I-, R- and 2R-shaped cross-sections at ambient temperature is presented, based on the results of a large programme of experimental tests and numerical simulations. Firstly, several four-point bending tests were carried out in order to assess mainly the failure loads and failure modes of the beams. Secondly, a suitable finite element model was developed to compare with the experimental results, and finally, a parametric study was undertaken in order to investigate the influence of the thickness, height and length of the beams on its structural behaviour.     

谈某冷弯薄壁型钢结构体系低层住宅施工技术

以某二层冷弯薄壁型钢结构体系住宅为依托,介绍其现场施工关键技术、工程创新点,通过解决现场施工过程中遇到的重点、难点问题,总结了基础施工、墙体施工、楼板施工及屋面施工等关键技术,为同类冷弯薄壁型钢结构体系施工提供了技术参考。     

某冷弯薄壁型钢结构住宅用钢量优化分析

结合某冷弯薄壁型钢结构住宅实例,采用大型通用有限元软件ANSYS建模,运用零阶法、一阶法对梁、柱构件截面进行优化分析。结果表明,与原设计相比,优化分析后工程的用钢量得到了大大的减少,这不仅有利于钢材的节约,带来可观的经济效益,而且有利于冷弯薄壁型钢结构住宅在我国的推广和应用。     

冷弯薄壁型钢组合楼盖和自攻螺钉连接的抗剪性能试验研究

对3个足尺冷弯薄壁型钢楼盖试件的面内受力性能进行了水平低周往复加载试验研究,得到其破坏特征、承载力指标以及自攻螺钉连接的破坏模式,试验结果表明C形边梁与压型钢板间的自攻螺钉连接破坏导致了楼盖试件破坏。基于楼盖试件面内破坏现象,对C形楼盖梁与U形边梁间、C形楼盖梁与压型钢板间两种自攻螺钉连接形式进行了抗剪性能试验研究,得到自攻螺钉连接的破坏形态和单颗自攻螺钉的抗剪承载力,分析了中国规范GB 50018-2002对于计算单颗自攻螺钉抗剪承载力的适用性。采用基于性能的抗震设计理论对冷弯薄壁型钢楼盖中自攻螺钉连接的性能水平、性能目标以及承载力指标进行了探讨,其结果可用于指导冷弯薄壁型钢结构体系的抗震设计。     

冷弯薄壁方钢管长柱轴压性能试验研究

进行了3组共9个试件冷弯薄壁方钢管受压长柱的试验研究与有限元计算分析,截面尺寸均为120 mm×120 mm×2 mm,考察长细比变化时,试件的破坏模式、荷载-位移曲线以及承载力等改变情况,并利用ANSYS软件进行了数值模拟计算,结果表明:1)对于轴心受压柱,在长细比λ≤36.4时,柱发生的是强度破坏,承载力较高;当长细比增大到λ≤65.5时,柱承载力降低近20%,破坏模式为整体破坏。2)有限元计算结果与试验结果较为接近,差值在12%以内,具有较高准确度。