Cold-formed steel sheathing connections at elevated temperature

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Highlights

  • Sheathing bracing is critical to the performance of cold-formed steel studs employed in buildings.

  • Performance of OSB and gypsum sheathing connections degrade precipitously with elevated temperature causing a loss of stud bracing.

  • Fire performance-based design of CFS structures is enabled through retention factors for cold-formed steel-to-sheathing connections.

Abstract

The objective of this paper is to provide experimental results related to the elevated temperature performance of connections between cold-formed steel members and sheathing. Cold-formed steel building structures rely on sheathing for their mechanical benefits including bracing against member twist, global flexural and flexural-torsional buckling, and cross-section distortional buckling, as well as to supply lateral strength and energy dissipation in shear walls and diaphragms. Sheathing is also relied upon for non-structural benefits, including: fire, acoustic, and thermal performance. Predicting the degradation of the connection performance between cold-formed steel members and sheathing at elevated temperature is critical for any attempt to predict the structural performance of cold-formed steel buildings under fire demands. Steady-state connection tests were conducted under in-plane shear and pull-through at temperatures up to 400 °C for cold-formed steel members attached to gypsum board and oriented strand board. By combining the conducted tests with others in the literature retention factors for initial stiffness and ultimate strength of the connections are proposed.

Introduction

Buildings framed from cold-formed steel (CFS) rely on open singly-symmetric thin-walled members – typically lipped channels for wall studs and floor joists, and unlipped channels for tracks to frame in ends of the studs and joists as shown for a common wall in Fig. 1. The thin open CFS sections that frame the wall or floor are weak in torsion, further since they are singly-symmetric the shear center and centroid do not align so common out-of-plane loading creates first-order torsion demands. Secondary steel bracing including bridging (shown in Fig. 1a), strapping, and blocking are commonly used to resist this torsion, but are costly. To address fire, acoustic, and thermal demands essentially all cold-formed steel members in a building are sheathed on one, or more commonly both sides. In addition to providing non-structural benefits the sheathing also provides mechanical stabilization to wall studs and floor joists. This additional bracing is critical not only for resisting torsion, but since the sections are thin-walled, is useful for inhibiting cross-section buckling modes as illustrated in Fig. 2 such as distortional, and global (minor-axis flexure, or combined major-axis flexure and torsion) that must be addressed in design.

Connection between a CFS steel member, such as a stud, to the sheathing is commonly provided by a self-drilling screw as illustrated in Fig. 3a. In CFS design this connection is idealized by at least two restraining springs as shown in Fig. 3b: one for in-plane shear resistance, kx, and a second for rotational restraint, kφ. If sheathed on both sides kx provides resistance against minor-axis flexure and torsion as well as provides the primary resistance against lateral racking of the wall (i.e., as a shear wall) or a floor (i.e., as a diaphragm). If sheathed on one side kx still provides resistance against the translation component of distortional buckling and if stiff enough can transform global flexural-torsional buckling to one more consistent with restrained-axis torsional buckling (Fig. 2e). The rotational spring, kφ, is the primary restraint against distortional buckling – and obviously restrains global torsion as well. The mechanical origin of the rotational restraint derives from a combination of bearing and pull-through resistance as illustrated in Fig. 3c.

The stiffness and strength of these connections are critical to successful performance of CFS systems and, after significant study at ambient temperatures, are now embedded in current CFS design. However, the performance of the CFS steel member-to-sheathing connections has seen limited study at elevated temperature. For CFS to move from prescriptive fire design to performance-based design the strength prediction methods must be enabled at elevated temperatures. This paper addresses the performance of typical steel CFS member-to-sheathing connections at elevated temperature for oriented strand board (OSB) and gypsum sheathing. Fifty-four connection tests at elevated temperature under in-plane shear and pull-through are summarized. Tested temperatures of the connections range from 20 °C to 400 °C. It is noted that in CFS construction rapid mechanical degradation with temperature is observed and therefore the base layer sheathing may have additional (fire-exposed) gypsum sheathing layer(s) that delay the time in the fire at which these temperature levels are reached in the stud-to-sheathing connection. The test temperature of 400 °C at the connection is thus selected as an upper value given the very large reduction of mechanical properties already experienced at this temperature level as will be shown from the test results herein. Based on tests results, recommendations for retention factors for these types of connections are then provided.

Section snippets

Background

The importance of connection in-plane shear stiffness and rotational restraint (derived from pull-through resistance) of CFS member-to-sheathing connections has been well established at ambient temperature. Connection in-plane shear stiffness may be determined by a variety of test methods including a small-scale member-sheathing assembly using eight fasteners first developed by Winter [1], a simple lap-shear test with a single fastener as detailed in standards [2], or hybrids that aim to

In-plane shear performance at elevated temperature

A total of 26 tests were completed to experimentally obtain the in-plane shear stiffness of CFS-to-sheathing connections, with the primary objective of determining the degradation of the connection stiffness with increasing temperature.

Pull-through performance of stud-to-sheathing connection at elevated temperature

Aiming to characterize the degradation of pull-through fastener stiffness of CFS member-to-sheathing connections, a total of 28 tests were completed using different sheathing materials at temperatures ranging from ambient (20 °C approximately) to 300 °C. It is worth noting that even in axially loaded members with similar sheathing on both sides, due to cross-section buckling and/or thermal bowing, pull-through is a common failure mode at elevated temperature. An example of an intermediate

Recommended retention factors

Retention factors for in-plane shear and pull-through/rotational connection stiffness and strengths were developed based on the tests reported herein (i.e. from the first author's dissertation [13]). However, subsequent to Ref. [13] additional testing as reported in Ref. [15] provided more data for single layer gypsum sheathed specimens. In addition, previous work did not provide retention factors for connection strength, only stiffness. Finally, through committee work with the American Iron

Conclusions

Connections between cold-formed steel members and sheathing are critical to the successful performance of cold-formed steel buildings. The mechanical stiffness and strength of cold-formed steel-to-sheathing connections degrade significantly with temperature for both gypsum board and oriented strand board sheathing. Steady-state connection tests were conducted at elevated temperatures for cold-formed steel members connected to gypsum board and oriented strand board under in-plane shear and

Author statement

Jean C. Batista Abreu: Investigation, Visualization, Writing - Original Draft Luiz C. M. Vieira Jr.: Funding acquisition, Methodology, Supervision Thomas Gernay: Writing - Review & Editing Benjamin W. Schafer: Formal analysis, Visualization, Writing - Review & Editing, Conceptualization, Supervision.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

The authors received materials for testing from Simpson Strong Tie, Constalica-Soufer, Fundo de Apoio ao Ensino, à Pesquisa e à Extensão (FAEPEX) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) the authors also acknowledge past research funding, though not directly for this work, from the American Iron and Steel Institute, the Steel Framing

Acknowledgments

The authors would like to acknowledge the support of the laboratory staff at the University of Campinas. The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), grant no 2014/26217–9, for partially funding the research.

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