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بخشی از مقاله انگلیسیعنوان انگلیسی:Reliability of cold-formed steel framed shear walls as impacted by variability in fastener response~~en~~

Abstract

The objective of this paper is to examine the reliability of cold-formed steel framed shear walls with a particular emphasis on walls sheathed with wood structural panels. A sheathed cold-formed steel framed shear wall is a system consisting of studs, tracks, and sheathing often with bridging and/or blocking, connected with steel-to-steel and sheathing-to-steel fasteners. The shear walls may be integrally connected to foundations, floors, or other shear walls through a variety of means including hold downs, straps, diaphragm chords and collectors. Shear wall lateral resistance in cold-formed steel framed buildings varies because of the randomness in the components and connections that comprise the wall. The interaction between fasteners and sheathing is particularly important because (1) sheathing-to-steel fastener response is the main source of shear wall nonlinearity (2) there is high variability in this fastener response. Although the nominal strengths for different shear wall configurations are stated in current design specifications (e.g., AISI S400), variability of shear walls has not been explicitly considered. Existing resistance factors are extrapolations from steel diaphragm testing. To explore the impact of fastener response variability on shear wall reliability, Monte Carlo simulation of typical cold-formed steel framed wood sheathed shear walls with random fastener input was conducted. Variability in fasteners was determined based on existing physical fastener tests. Statistical properties of shear wall strength, demand capacity ratio of key fasteners, as well as relations between fastener strength and shear wall strength are all explored. Reliability evaluation is provided for four different design methods. The results indicate that shear wall strength benefits from a system effect whereby variability in fastener response is reduced through redistribution resulting in reduced variability in overall shear wall strength. Concomitant with this is a slight decrease, approximately 3%, in the mean system strength that also must be considered.

Introduction

Cold-formed steel (CFS) structural systems are commonly used for low and mid-rise construction. In the design of CFS-framed buildings, shear walls are typically used to provide lateral resistance for seismic or wind load (e.g., see Fig. 1). Commonly, wood sheathing, such as oriented strand board (OSB), is screw-fastened to CFS studs and tracks to develop lateral shear stiffness and strength (e.g., see Fig. 2). As the wall is sheared an incompatibility exists between the CFS framing, which is largely deforming as a parallelogram, and the wood sheathing that remains nearly rectangular and primarily undergoes rigid body translation and rotation because of its large in-plane rigidity. The incompatibility between the deformed frame and sheathing causes a relative displacement that must be accommodated at the fasteners. This displacement causes tilting and bending of the fastener, as well as deformation and damage to the steel and wood sheathing material around the fastener. This damage is the source of yielding and energy dissipation in these systems [1,2] . The resulting overall CFS-framed wood-sheathed shear wall cyclic response exhibits significant hysteresis, degradation, and pinching, as shown in Fig. 3. CFS-framed wood-sheathed shear walls have been tested extensively. In North America AISI S400-15 [3] (previously AISI S213-07 and -12 [4]) provides nominal shear wall strength for different types of sheathing, fastener spacing, and stud and track thickness based on the available testing (e.g., see [5,6]). The shear wall strengths in AISI S400 are based directly on tested capacities, and a / = 0.6 is used for the resistance factor in design. This value was selected initially based on typical / value for steel deck diaphragms (which is based on a connector failure limit state and a target reliability, b, of 3.5) and has remain unchanged as additional entries to the tables in the standard have been included. CFS-framed shear walls may be viewed as a small structural system – and system reliability for steel structures in general [7] and CFS structures in particular [9] has been studied recently. Monte Carlo (MC) simulation of models of steel frames have been used to assess component vs. system reliabilities and explore system-level resistance (/) factors based on target system reliabilities as opposed to component reliability [8] . It has been shown that the system reliability of typical CFS framing under gravity demands far exceeds the individual component reliabilities [9]. Also, the reserve strength of CFS CFS-framed floor diaphragms when considered as a system has been calculated [10]. Recognizing the central role that the nonlinear response of the steel-fastener-sheathing connection has on the overall shear wall response Buonopane et al. developed and validated an OpenSees simulation that adequately predicts CFS-framed wood-sheathed shear wall cyclic response [11]. This model provides the potential to conduct MC simulation of CFS-framed shear walls and explore the variability and reliability of their response. This has the potential to provide improvements to the current reliability assessment in AISI S400 [3],which is essentially based on engineering judgment alone. The work herein employs the validated shear wall model of Buonopane et al. [11],the shear wall tests of Liu et al. [1], and steel-fastener-sheathing connection tests of Peterman et al. [12] to perform MC simulations on a series of CFS-framed shear walls and assess the predicted reliability of the studied shear walls. The fastener testing is characterized in terms of a random variable and used to drive MC simulation of the selected shear walls. The simulation results are summarized and explored to provide insight on the importance of load redistribution, fastener location, and the resulting variability of shear wall strength. Next, the reliability of the peak strength based on the MC simulations is determined. Finally four potential shear wall design methods are considered and the reliability of these methods assessed against the available data both with and without consideration of the system effect as discerned from the MC simulation. The paper concludes with discussion of needed future work and the potential for further incorporating system reliability into CFS-framing design.

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