The Impact of Bearing Conditions on the Behavior of Cold-Formed Steel Stud Assemblies

The objective of this study is to explore the structural response of cold-formed steel stud assemblies (i.e., stud and track) with partial bearing conditions. It is hypothesized that studs bearing under partial bearing conditions (i.e., not fully bearing on a concrete slab) may result in reduced axial capacities. Currently, the behavior of these systems on concrete slabs due to member instabilities is not well-understood, and cold-formed steel design specifications provide no guidance. This study provides an integral experimental and numerical investigation of the stability response of the studs under partial bearing conditions in order to quantify the reduction of their axial capacities. A variety of partial bearing conditions are considered in this study by parametrically varying edge (i.e., where the steel stud assembly is close to the concrete slab edge) and overhang (i.e., steel stud assembly is outside the edge) distances. The non-uniform bearing stress underneath the stud caused by concrete cracking, crushing, or a combination thereof is measured to relate with the reduction of the axial capacity of the stud. The results of this study will be used to develop design guidelines for stud wall assembly under non-uniform bearing conditions

Experimental Seismic Behavior of the CFS-NEES Building: System-Level Performance of a Full-Scale Two-Story Light Steel Framed Building

In the summer of 2013, testing of two full-scale cold-formed steel (CFS) framed buildings under seismic excitations took place at the Structural Engineering and Earthquake Simulation Lab (SEESL) at the University at Buffalo. Utilizing the twin shake tables, the two-story building specimens were subjected to ground motions from the 1994 Northridge earthquake. These experiments were conducted as a part of the CFS-NEES experimental effort in an attempt to advance cold-formed steel earthquake engineering and design. Two buildings were tested: the first, a specimen constructed with only structural components (CFS-framed gravity walls, shear walls, floor and roof diaphragms, with OSB sheathing on shear walls and diaphragms); the second began with an exact replica of the first building, but saw the addition of various non-structural systems such as gravity wall sheathing, full diaphragm sheathing, interior partition walls, and exterior weatherproofing. Prior to these experiments, little experimental data existed on full building system behavior for CFS framing. This paper presents results on full-system behavior, specifically examining: drifts, acceleration amplification, shear wall behavior, base shear, diaphragm flexibility, damping, and period of vibration. Comparison to the North American specification for CFS, and design recommendations are also provided

Cyclic performance of fiber-cement-board cold-formed steel connections with varying edge distance

Seismic design of cold-formed steel shear walls is constantly evolving due to ever-growing interest in the CFS systems in high seismic zones. Cementitious panels used as sheathing for CFS shear walls and diaphragms can achieve superior strength capacity and fire resistance and are thus increasingly popular in light-framed construction. This study aims to assess the impact of three various edge distances on the performance of CFS studs sheathed with fiber cement boards subjected to cyclic loads. In this paper, the results of the 36 specimens are investigated. The experimental specimens are designed to represent a slice of a shear wall and contain two studs sheathed on both sides with two panels. Eight fasteners connect the panels to the studs. Steel thickness, fastener type and fastener edge distance are varied to determine the performance of the connections. In total, 36 specimens are tested via the FEMA 461 cyclic protocol, represented a large suite of experimental data for these connection types. Population statistics are performed on the data to quantify inherent variability in these screws fastened connections. Results will inform national design specifications and provide underlying data to enable design of CFS shear walls and diaphragms sheathed with fiber cement boards.The authors would like to thank the United States Gypsum Corporation (USG) for funding the work. Additionally USG and ClarkDietrich provided in-kind support through the donation of specimen materials. The assistance and guidance provided by the lab technician at UMass Amherst, Mr. Mark Gauthier, is highly appreciated

Incorporating cold-formed steel member and system design into the undergraduate curriculum

Cold-formed steel design, in an ideal scenario, deserves an entire advanced undergraduate or graduate level course. However, this is not practical in many institutions, where a program of study can only include a few courses in hot-rolled steel design due to teaching capacity and ever-expanding program requirements. Thus, instructors with expertise in cold-formed steel and repetitively-framed systems are forced to infuse it into other curricula, or simply not teach it at all. The pervasiveness of repetitively-framed structural systems worldwide motivates not only teaching the fundamentals of member behavior, but also system behavior, to prepare undergraduates for their careers as practicing engineers. This paper highlights efforts at the University of Massachusetts Amherst to do this in two courses: a second course in steel design (CFS members), and a course on structural systems (repetitive and light framed systems). Modularized lesson plans are presented, along with in-class active learning activities, examples of student work, and feedback from students in each of courses. This paper aims to enable effective modular cold-formed steel instruction, leading to significant learning in thin-walled member behavior and repetitively-framed system behavior.The authors gratefully acknowledge our students, who, with their enthusiasm, made teaching this material a joyful experience

Numerical investigation of the impact of bearing condition on the axial behavior of variable-height cold-formed steel stud wall assemblies

This paper is devoted to identifying and numerically characterizing the strength of cold-formed steel (CFS) wall assemblies of various height with non-uniform bearing conditions. The results are for a means of evaluating existing design guidelines presented in the North American Specification of the American Iron and Steel Institute (AISI S100-16). In this standard, the bearing condition of the members is not included in equations for predicting axial strength. However, based on the recent experiments done by the authors, non-uniform stress distributions at the ends of CFS studs, caused by different bearing conditions, can reduce the axial capacity of the assemblies. The sources of nonuniformity considered were finite flexibility of the concrete slabs, uneven bearing surfaces provided by the slabs, distance of the wall assemblies to the slab edge, or overhang conditions caused by construction error. In the experiments done by the authors, the height of lipped-channels was fixed to 12 inches to enable comparison across specimens. However, in typical construction, wall assemblies installed on concrete slabs are generally full-height (8 ft or higher) and thus globally-dominant. In this paper, various heights are considered for the studs. They are determined based on the local, distortional, and global buckling half-wavelengths. The impact of bearing conditions on the strength is further elucidated via high-fidelity 3D finite element analyses (FEA). The results of FEAs clarify how the non-uniform stress distribution at the ends of the studs or partial bearing conditions can impact their strengths when they buckle locally, distortionally, and globally. The finite element models are calibrated with existing experimental results. Comparison to available experimental results and to the governing design codes are provided.The authors would like to gratefully thank American Iron and Steel Institute (AISI) for their financial support and Mark Gauthier, the Universit

Analysis of roof live loads in industrial buildings

In design, structural engineers must have a clear understanding of live loads, both qualitatively and statistically. For decades, multiple studies have been published that relate live loads for floor loads in various occupancies such as offices and residences. However, survey data or probabilistic live load models for industrial building roofs are difficult to find. There are recommendations in major standards used in the modern world that give design live load values for roofs based on the accessibility of the rooftops. On the other hand, engineers may not understand the origin of these values. Comparison is made between current U.S standards for roof live loads and standards used in other parts of the world. To ensure that the most accurate live load assessment is implemented in the design, our understanding of live loads should be updated on a regular basis. Furthermore, in the United States, the current roof live load design value is 0.96 kN/m2 (20 psf), which is much greater than the values recommended by European, Australian, and Chinese standards. As a result, determining the source of live load on industrial building roofs is essential. To cover the gap in the literature, this article gives survey methodology and probabilistic studies related to design live load value on roofs. The sensitivity of existing probabilistic models to mean, variance, and time duration was also investigated.This work is part of the research project Roof Live Load Models for Metal Buildings which is sponsored by the Metal Building Manufacturers Association (MBMA) and the Steel Deck Institute (SDI). The authors would like to thank Dr. Zhanjie Li, Associate Professor at Suny Polytechnic, for his help translating the Chinese standards

Analysis and Design of Various Medieval Vaulting Technologies

Finite element modeling methods were used to design models of three medieval\ud vaulting technologies: an ideal quadripartite vault, ideal sexpartite vault, and vault from Beauvais Cathedral. These models were loaded with their own self-weight and wind loads on their horizontal projection, corresponding to realistic scenarios at Beauvais, France. The outward thrusts that would be transmitted to the flying buttress system were as anticipated for all models. Von Mises stresses were analyzed to qualitatively examine the stress distributions and structural efficiency of each model

Experiments on the Stability of Sheathed Cold-Formed Steel Studs Under Axial Load and Bending

This report provides test and analysis results for a typical cold-formed steel (CFS) stud as employed in light steel framing for a building. Specifically, tests are conducted on a 362S162-68 (50 ksi) stud connected to 362T162-68 (50ksi) track with varying combinations of sheathing connected to the two flanges. Loading consists of both axial load and a directly applied horizontal load to induce major-axis bending of the stud (and torsion due to the shear center of the stud). The sheathing configurations studied are intended to capture various stages of construction and final form and include: no sheathing; one-sided sheathing with Oriented Strand Board (OSB); and two-sided sheathing with OSB and gypsum board, or only OSB on the two sides, or only gypsum board on the two sides. The combinations of axial load (P) and bending load (M) studied are intended to capture nearly the complete P-M space.American Iron and Steel Institut

Experiments on the Stability of Sheathed Cold-Formed Steel Studs Under Axial Load and Bending

This report provides test and analysis results for a typical cold-formed steel (CFS) stud as employed in light steel framing for a building. Specifically, tests are conducted on a 362S162-68 (50 ksi) stud connected to 362T162-68 (50ksi) track with varying combinations of sheathing connected to the two flanges. Loading consists of both axial load and a directly applied horizontal load to induce major-axis bending of the stud (and torsion due to the shear center of the stud). The sheathing configurations studied are intended to capture various stages of construction and final form and include: no sheathing; one-sided sheathing with Oriented Strand Board (OSB); and two-sided sheathing with OSB and gypsum board, or only OSB on the two sides, or only gypsum board on the two sides. The combinations of axial load (P) and bending load (M) studied are intended to capture nearly the complete P-M space.American Iron and Steel Institut

Characterizing Wall-to-Diaphragm Moment-Rotation Response in Cold-Formed Steel Systems via Fastener Limit States

Ledger framing is currently the dominant framing system in cold-formed steel buildings and is a popular choice for its flexibility in floor joist spacing with respect to wall stud spacing. In ledger framing, the floor, which is sheathed with oriented strand board (OSB), is framed into the side of the wall via a rim track (ledger) and clip angle connection. Experimental studies on ledger-to-wall connections have demonstrated complex behavior, involving fastener pull-out through multiple plies, ledger flange buckling, and stud web crippling. This is in stark contrast to current design recommendations, which assumes the connection is a simple shear connection with no moment-rotation capacity; fastener shear capacity is the governing limit state. To improve design recommendations and understanding of load transfer mechanisms, a robust finite element model (FEM) on ledger-to-wall connections was modelled in ABAQUS. Experimentally-derived fastener behavior was adopted in the modelling approach proposed herein, and was found to accurately capture the flow of forces in the wall-diaphragm connection. The model is validated with experimental tests and is expanded to simulate the influence of metal deck on the connection stiffness and strength. In addition, effect of metal deck thickness and fastener spacing is explored. Ultimately, this work at the connection level will lead to more robust modeling and prediction capabilities for CFS diaphragms with metal deck sheathing.The authors would like to thank the CFS-NHERI Team for their contributions to the work. This material is based on work supported by the American Iron and Steel Institute, the Steel Framing Industry Alliance, the Steel Deck Institute, and the University of Massachusetts Amherst