A 10-story cold-formed steel (CFS)-framed building, referred to as CFS10, was constructed and tested under multi-directional seismic excitation at the 6-DOF Large High-Performance Outdoor Shake Table (LHPOST6, NHERI@UC San Diego). The test building was outfitted with a wide range of nonstructural components and systems (NCSs) designed and installed at different levels to evaluate their seismic performance as integrated in a full-scale building. These included suspended ceilings, windows, swing and roll-up doors, a resilient stair system, pressurized fire sprinkler system, pressurized gas piping system, and roof-mounted mechanical equipment. The building was subjected to a series of eighteen increasing intensity multi-directional earthquake motions, including two risk-targeted maximum considered earthquakes (MCER). This paper compares the measured acceleration response and horizontal response amplification with the component resonance ductility factor (CAR) prescribed in Chapter 13 of ASCE 7-22 for nonstructural components. A companion paper Hutchinson et al. [1] presents an overview of the overall test program, including the design and construction of the specimen, and results from the earthquake and fire test phases of the experimental program. Throughout the seismic tests, the building and installed nonstructural components exceeded expected performance including at MCER level demands for a risk category II building, exhibiting seismic robustness with continued functionality

Bolted shear connectors are being increasingly used in demountable solutions, especially in innovative coldformed steel (CFS)-concrete composite beams. The design of steel-concrete composite beams is closely related to the performance of the shear connectors that transfer shear forces between the steel beam and the concrete slab. However, while existing design codes provide formulas for predicting the shear resistance of welded shear studs, their applicability to bolted shear connectors remains uncertain, especially when dealing with thin-walled steel sections. Moreover, the available design procedures (especially in EN 1994-1-1) are specified for concrete with density more than 1750 kg/m3, imposing limitations for lightweight concrete (LWC), specific connector steel grades, and a limited ratio of connector diameter to steel beam flange thickness. Furthermore, the transferred shear force causes compression stresses in the CFS flange, and its effect has yet to be studied; this is, therefore, the subject of this study. This study first presents a numerical modeling approach for bolted shear connectors used in CFS-lightweight concrete systems and validates it against the available tests. Then, a parametric study is conducted to study the behavior of the bolted shear connector by varying the CFS thickness, CFS material properties, connector diameter, connector grade, and LWC strength. The numerical results were then compared with design shear resistance predictions following available design standards. Finally, formulations were proposed to predict the shear resistance of a bolted shear connector as a function of slippage between the CFS beam and concrete slab. The need for a new formulation arose from findings in this paper, which revealed that a high ratio of bolt diameter to beam flange thickness failed due to the curling of the CFS flange at the connection point. Consequently, a new equation was proposed to account for bearing forces combined with local bending of the CFS beam at the connection, as well as potential failure modes of the bolts and concrete.

The objective of this paper is to summarize ongoing and recently completed changes to steel structural engineering design specifications in America, emphasizing changes potentially relevant and of interest to the larger stability research community. The author participates and/or leads committees that span both carbon and stainless steel as well as cold-formed and hot-rolled steel, with applications in residential, commercial, and industrial buildings, storage racks, and energy infrastructure. Notable changes and interesting decisions related to stability concerns in recent editions of the stainless steel standards: ASCE 8-22, AISC 370-26, and carbon steel standards AISI S100-24, and AISC 370-26 will be highlighted. Looking forward, the ongoing efforts to develop new ASCE 8 and AISC 370 standards will be summarized. Notably, the AISC 360-33 edition is planned to be a merger of the cold-formed AISI S100 and hot-rolled AISC 360 steel provisions. An update on this process will be provided – along with thoughts on some of the critical stability relevant issues such as the extension of the Direct Strength Method to carbon structural steel. The handling of local-global interaction in carbon structural steel, the continued expansion of design by analysis methods, etc. The talk is intended to bring the audience up to speed on these changes in American provisions and open a dialog on how such changes relate to the many excellent new provisions also coming to fruition in Europe, Australia, China, and other parts of the world with an eye towards maintaining some form of global coherence in our collective understanding of steel stability and its design specifications.

In this paper, an extension of the Finite Tube Method for the Linear Buckling Analysis of tubular members, such as the supporting towers of wind turbines, is presented. The method utilizes the circular cross-section shape by employing Fourier-series approximation for the displacements, and accounts for the effect of the curved surface directly in the strain-displacement relationships; making the method computationally more efficient compared to general (shell) finite element calculation. Recently the method has been extended to tubes with conical shape, which provides a more accurate analysis in the case of tapered geometry which frequently occurs in certain parts of real wind turbine towers. In this paper, the extension of the method is briefly summarized, then the method is applied on a wind tower segment to calculate the critical load values and buckling modes. The results demonstrate the efficiency and accuracy of the Finite Tube Method.

This paper studies the elastic shear buckling behaviour of cold-formed steel plates with edge-stiffened central holes of three different shapes (circular, square, and diamond) and proposes simplified approximate formulae for design use. The analysis is conducted utilising the finite element analysis software ABAQUS through a comprehensive parametric study comprising 4200 FE simulation models of plates with both edge-stiffened and unstiffened holes over a wide range of plate slenderness ratios and hole sizes. The parametric study investigates the influence of hole geometry with and without edge-stiffeners with key parameters including hole size, plate width, plate length, stiffener size, and plate thickness. The findings reveal that the elastic shear buckling capacity of plates with circular or diamond holes can be substantially enhanced through the inclusion of edge-stiffeners. The edge-stiffeners are shown to recover the elastic shear buckling capacity lost due to perforation and, in many cases, even exceed the initial capacity of unperforated plate. However, for plates with square holes, the addition of edge-stiffeners resulted in only minor improvement in elastic shear buckling capacity. For design purposes, simplified approximate equations for the buckling stress (τcr) are proposed, reproducing the numerical database with COV below 3% for unstiffened perforations and below 10% for edge-stiffened perforations. The proposed formulae are compatible with the ASD/LRFD design formats of ANSI/SDI AISI S100 and AS/NZS 4600, providing a direct and practical means to account for edge-stiffened holes in routine design. A design example is also included in this paper to demonstrate the application of the proposed equations. The predicted buckling stress, or the corresponding elastic buckling load Vcr, provides the stability parameter required in ultimate-strength formulations for the nominal shear resistance Vn

The paper describes recent research on developing a methodology for designing 6–8 storey (mid-rise) buildings entirely in cold-formed steel. The focus is on the application of built-up sections for the flooring and vertical load bearing systems with emphasis on shear walls for the horizontal load transfer. The paper first summarises the main advantages of using built-up sections for floor beams and uprights. These include the significant enhancement in torsion rigidity (GJ) achievable by introducing closed loops in connecting multiple open profiles, thus creating beam sections with high lateral buckling strength, ideal for long-span applications including mid-rise applications, and upright sections with high flexural-torsional buckling capacity. The paper then outlines a recent test series on cold-formed steel shear walls featuring built-up uprights with a closed loop. Two levels of shear wall at a total height of 2 × 2.4 m = 4.8 m were tested quasi-statically, varying the screw size, screw fastening pattern and sheathing thickness and steel grade. An optimal combination of shear wall design parameters was identified from the tests. Next, a series of tests on the screw connection between upright and sheathing is described. The tests varied the angle of the shear force relative to the edge of the sheathing, as occurs round the perimeter of a shear wall assembly, as well as the screw size and the thickness of the sheathing. Lastly, the paper addresses the design of mid-rise cold-formed steel buildings. The load transfer is described, followed by the analysis and design of an archetype 8-storey building featuring shear panels with built-up sections for the primary members. The study demonstrates the feasibility of using built-up cold-formed steel for all primary and secondary load bearing components of the building.

Errata to Vol. 62, No. 1 paper Generalized Elastic Lateral-Torsional Buckling of Steel Beams

This study aims to establish a novel foundational framework for designing pultruded glass fiber-reinforced polymer (pGFRP) columns using the Direct Strength Method (DSM). The approach incorporates: (i) the influence of initial geometric imperfections (GI) and (ii) the interaction between material crushing and elastic buckling, for global buckling (GB) failure. In this paper, only global flexural (FB) and flexural-torsional (FTB) buckling failures are considered, including potential interactions with crushing. The proposed methodology builds upon the single-curve approach adopted in steel design codes and applied in the DSM framework. A rigorous selection process identified 124 experimental tests of axially compressed pGFRP columns from the literature. Using this data, a simplified design approach is developed through an optimization problem solved using an evolutionary algorithm. The proposed design methodology is compared with the ASCE 74–23 and CEN/TS 19101:2022 standards, offering insights and suggestions for improving the current design procedures. The findings from this study aim to drive the development of a unified DSM-based design approach capable of: (i) accommodating different material characterization standards for predicting crushing phenomena, (ii) addressing local-global-crushing interactions, (iii) incorporating GIs, and (iv) accounting for the post-buckling strength reserve associated with local buckling modes. Future research will focus on reliability-based design, computational simulations, and parametric studies to extend the applicability of the DSM proposal to structural scenarios beyond the scope of current experimental tests.

The objective of this paper is to evaluate the seismic design parameters for modular metal building systems in high seismic zones in the United States using the FEMA P695 (ATC, 2009) methodology. Modular metal buildings, commonly used for large open spaces such as warehouses and data centers, combine traditional built-up tapered steel frames with intermediate gravity-only columns. A suite of archetype buildings with varying spans, heights, and number of modules was developed in collaboration with industry and analyzed using nonlinear static and dynamic procedures. High-fidelity shell finite element models, validated against component and shake table tests, capture key failure modes, including local and global buckling. Results from pushover and quasi-static cyclic analyses were used to calibrate nonlinear single-degree-of-freedom models for subsequent incremental dynamic analyses against the FEMA P695 earthquake suite. The study demonstrates that modular metal buildings designed with a response modification factor R = 3.5 meet the FEMA P695 collapse performance criteria, provided that lateral bracing systems are designed to meet both strength and stiffness requirements of AISC 360. Buildings using traditional strength-only bracing exhibited limited ductility due to premature column buckling, whereas AISC-compliant bracing achieved stable post-peak response and improved collapse margins. A collapse drift limit of 6% is proposed based on system flexibility and observed behavior. The findings confirm the adequacy and applicability of current ASCE 7 ordinary moment frame provisions for modular metal building systems, with important implications for design practice in high-seismic regions.

The main objective of this study is to share a novel experimental setup that was used to supply and support an open cold-formed steel shape under torsion and bending. The studied shape is a lipped channel-like profile with multiple intermediate stiffeners and compound return lips formed from 690MPa sheet steel. Under torsion and bending the cross-section is likely to distort—to address this a unique set of collars were machined that support the entire cross-section at the load points and supports but still leave the section free to warp. Tests were conducted with an eccentric load applied to the collars—resulting in combined torsion and major-axis bending. The eccentricity can be large enough that torsion fully dominates the response or aligned with the shear center such that one can test a single section in pure bending. Initial imperfections are measured through laser scanning, and deformations in the sections under load are recorded at a variety of points. The combined bending and warping stresses along with the highly stiffened nature of the section provide an unusual stress state and cross-section for assessing local, distortional, and global stability modes. The tests provide an intriguing benchmark for both design approaches and shell finite element collapse simulations; both of which are discussed in reference to the testing.

The objective of this study is to investigate the flexural strength of cold-formed steel (CFS) members fabricated from Advanced High Strength Steel (AHSS). An experimental program of 54 four-point bending tests on AHSS lipped channels was conducted. The testing program includes 6 AHSS grades with nominal yield strengths ranging from 340 MPa to 1200 MPa, and two lipped channel cross-section types - C and Sigma. The experiments involving sections formed from the higher yield stress materials (nominally 700 MPa and higher) were observed to experience local-distortional (L-D) buckling interactions prior to reaching peak moment. Strength predictions by the Direct Strength Method (DSM) implemented in AISI-S100 and AS/NZS 4600 are compared with the experiment's ultimate loads. The current DSM is found to be unconservative for the studied sections. Alternative DSM strength predictions are investigated which consider the use of a reduced yield stress for lower ductility steels and an L-D buckling interaction limit state. The study finds that using reduced yield stresses and introducing L-D buckling interaction can lead to reliable predictions. Overall, this study advances design against buckling limit states for CFS flexural members in the context of high yield strength steel grades.

This paper presents a summary of experimental findings from axial compression tests on columns featuring a cold-formed lipped channel section with intermediate stiffeners and return lips, roll-formed from high-strength low-allow steel with a nominal yield strength of 690 MPa (100 ksi). Additionally, the paper provides an analysis of the elastic stability of the studied section, a complete description of the initial geometric imperfections of the tested columns, results of tensile coupon tests, and comparison of the observed strengths of the columns with design predictions. The results provide important additional benchmarks for the wider adoption of high-strength cold-formed steel sections and indicate conditions where existing design methods may be reliably extended.

In this paper, a new set of mechanical criteria is employed to define the fundamental mode classes of thin-walled members: global, distortional and local, for use in modal analysis methods such as the Generalized Beam Theory (GBT) and constrained Finite Strip Method (cFSM). The objective of this work is to employ force characteristics, orthogonality, and completeness for defining the fundamental mode classes of thin-walled members, instead of using kinematic constraints, as in current modal decomposition methods. In this new framework the three basic mode classes span the entire deformation space of thin-walled members (i.e., they are inclusive of shear, transverse extension, and all other modes that conventional kinematic-based constraint methods separate out) and the mode classes are orthogonal to each other. Based on these criteria, this new method is implemented in the context of the Finite Strip Method (FSM) and termed fcFSM, i.e., force-based constrained finite strip method. The paper demonstrates buckling mode decomposition and identification results with fcFSM as well as the methods applicability for open/closed and polygonal/curved thin-walled cross-sections. In addition, the differences between the newly proposed fcFSM and the current kinematic-based constrained finite strip method (kcFSM) and GBT are highlighted and discussed through several specially selected examples.

In this paper an efficient numerical method for the static analysis of cylindrical tubes is introduced. The method is designed for the linear buckling analysis of wind turbine support towers which are, most typically, built up from conical and/or cylindrical cans. Accordingly, the developed method uses cylindrical tube segments as elementary building blocks, along with specialized shape functions, and is named the Finite Tube Method. Within a tube segment the displacements are approximated by two-dimensional Fourier series. The curved nature of the surface is directly considered in the kinematic equations. The segments are joined and/or supported to the ground by constraint equations or by elastic links. In the current implementation internal stresses are determined in a simplified way: the circumferential stress distributions are calculated from the internal forces/moment by classic strength of material formulae, while the longitudinal distribution within each segment is quadratic. The considered internal forces/moments are: normal force, shear force, bending moment, and torsional moment. The internal forces can be arbitrarily combined. In the paper the underlying derivations are briefly summarized, then the method is demonstrated and validated by numerical examples, comparing the results to analytical and alternative numerical solutions. The authors are actively developing the method and will provide future work on utilization of the method for buckling mode identification and decomposition, as well as practical advancements to make the method a useful tool in the engineering design and analysis of wind turbine support towers.

The objective of the study is to develop a suite of closed-form local buckling stress equations for cold-formed steel (CFS) C sections with centered rectangular web holes. Finite strip analyses are performed over a representative collection of over 1000 CFS C-sections under four loading conditions to generate data. Closed-form equations of buckling coefficients are developed in the simple forms of ratios of polynomials with modification factors used to consider the influence of holes. The suite of equations is named as the section method. The proposed section method has excellent accuracy over the C-sections considered in this study (mean is an element of [1.00, 1.01], COV is an element of [0.02, 0.06] for holed sections) and far exceeds the performance of the existing closed-form equations in the AISI S100 known as element method (mean = [1.55, 3.64], COV = [0.32, 0.51] for holed sections). The section method provides an alternative method for practicing engineers not using finite strip analyses and will lead to a more streamlined CFS design workflow.

Cold-formed steel (CFS) products are highly adaptable and suitable for several structural applications. Recent experiments conducted by the authors have demonstrated that incorporating composite action within systems comprising built-up CFS beams and lightweight concrete (LWC) is viable and can significantly enhance structural performance. However, current design methodologies lack specific guidelines for innovative demountable CFSLWC composite beams, which are necessary to leverage the benefits of composite action under a sagging bending moment. The available composite design procedures are for steel beams and concrete slabs connected by welded shear studs, while in CFS-LWC composite beam systems, bolts are used as shear connectors, and the ratio of bolt diameter to the CFS top flange thickness is higher than 2.5 (limit provided by EN1994-1-1). This research aims to assess the possibility of extending the available design specifications to predict the bending resistance of CFSLWC composite beams. First, the numerical modeling procedure was provided for CFS-LWC composite beams, and its results were validated against the available experimental data. A large parametric study was undertaken by considering different beam spans, CFS and LWC geometries, and material properties. The numerical results were then compared with design predictions per EN 1994-1-1 and AISC-360. The findings indicate that the design specifications tend to overestimate the bending resistance of CFS-LWC composite beams due to the absence of suitable expressions to determine the shear resistance of bolted shear connectors and thin steel plates, where the local buckling of the CFS section is not considered. Therefore, the influence of the localized buckling was explicitly taken into account by conducting pushout simulations. A good agreement between numerical results and design predictions following EN 1994-1-1 and AISC-360 was observed when the shear resistance of the bolted shear connector was obtained by numerical pushout simulation.

Modern wind turbines are often supported by tubular steel towers made from globally conical, locally cylindrical shells with relatively large diameter-to-thickness ratios-approximately between 100 and 300-which enables the tower material to be used as efficiently as possible. Wind turbine towers face complex loading resulting from both environmental and operational load cases and are sensitive to geometrical imperfections that inevitably arise during the fabrication process. Whereas bending often controls at the base of turbine towers, the upper sections are controlled by combined bending and torsion. Although extensive studies have been conducted on the stability and design of cylinders subjected to isolated actions, investigations into the structural response of thin-walled cylinders under combined actions, such as bending and torsion, remain limited. To address this knowledge gap, an experimental program was carried out to study the structural behavior of thin-walled steel cylinders under combined bending and torsion. A total of 48 cylinders were tested with varying diameter-to-thickness ratios and torsion-to-moment ratios found in wind turbine towers. To gain insights into the imperfection sensitivity of these tests, a laser scanner was used to measure the geometric imperfections of each specimen before testing. The test setup, instrumentation, loading procedures and structural response of the cylinders, including ultimate resistances, load-deformation characteristics, and failure modes, are reported. The primary objective of this study is to provide benchmark test data for the validation of numerical models and the development of advanced design methodologies, such as reference resistance design (RRD), for cylindrical shells under combined bending and torsion. Future work will involve formulating guidelines for using laser-scanned data to evaluate geometric imperfections, developing laboratory- and full-scale wind turbine tower finite element models, and ultimately providing improved design guidance on combined bending and torsion.