Today, engineering knowledge is rapidly advancing, providing the opportunity to contemplate analysis-based design as an enabling tool for general performance-based fire engineering of cold-formed steel systems. However, in fires the performance of cold-formed steel systems are assured by prescriptive detailing and standardized testing. Consequently, design specifications, and structural analysis tools have rapidly evolved to facilitate engineering design of these complex thin-walled members. Cold-formed steel systems have become popular in building construction as both load-bearing and non-load-bearing elements, primarily due to their high strength-to-weight ratio and ease of construction. This paper brings together existing research on cold-formed steel materials, members, and assemblages at elevated temperatures and complementary analysis and design methods necessary for the development of analysis-based design for cold-formed steel systems under fire. The American Iron and Steel Institute (AISI) has published three new cold-formed steel framing research reports: 1) RP15-3: Advancing Seismic Simulation of Cold-Formed Steel Framed Buildings, 2) RP17-1: Experimental Study on System Reliability of Cold-Formed Steel Roof Trusses, and 3) RP17-2: Monotonic and Cyclic Response of Single Shear Cold-Formed Steel-to-Steel and SheathinFrom 2001 to 2012, the AISI Committee on Framing Standards developed nine different framing standards to cover specific aspects of cold-formed steel framing.Performance-based fire design for cold-formed steel systems is in its infancy.
The fire-resistance rating is expressed by the number of hours that the assembly can maintain its integrity while containing gases and excessive temperature increases out of the fire compartment. In the United States, the assemblies are required to be “fire-rated” (IBC ) based on their ability to withstand a standardized “fire” test (ASTM ). In a fire, partition walls serve as primary barriers to maintain building integrity, and avoid the spread of fire between compartments (rooms). CFS interior partition walls are framed with studs, have track at top and bottom, and are then sheathed (most commonly) with gypsum wallboard(s). CFS stud and track are used extensively in buildings as the framing for interior partition walls, exterior curtain walls, and more recently as the complete load-bearing system (Allen , Schafer ). The most common members are channels (tracks) and lipped channels (studs and joists).
To date, fire design for load-bearing CFS systems (where the complete structural system is framed from CFS members) has followed the same test-based, prescriptive detail-driven approach that has been previously established for interior partition walls. ASCE/SEI 7-10 , ASCE Standard, Minimum Design Loads for Buildings and Other structures, American Society of Civil Engineers, 1801 Alexander Bell Drive, Reston, Virginia 20191-4400. Cold formed, galvanised purlins made from GALVASPAN steel have minimum yield.AISI S214-07, AISI Standard, Standard for Cold-Formed Steel Framing Truss Design, American Iron and Steel Institute (AISI), Washington, DC, June 2007. These prescriptive solutions are critical to current design and represent an important review of the state of the art in their own right however, the focus here is on enabling performance-based fire design, not additional prescriptive solutions.Timber Frame Truss Roof With Ridge Beam, Purlins, and Common Rafters.
In such a situation, the complete system may be designed for the desired fire performance with interactions between demand, propagation, and capacity fully included through analysis. Ideally, performance-based fire design brings the demand (fire modeling), propagation (heat transfer), and capacity (strength at elevated temperatures) all into the realm of analysis. Fire demands and heat transfer analysis of CFS systemsThis article focuses on AISI S240 which applies to cold-formed steel structural members subject to gravity loading, wind loading, and seismic loading, except.Fundamental to determination of the fire resistance is establishing the fire demand and then propagating that demand to the underlying members. This state-of-the-art review discusses current research and recent findings on the fire performance of CFS. Further, recent research has taken the first steps towards performance-based design for establishing the fire resistance of CFS structures, including temperature dependence of the material (mechanical and thermal) behavior, thermo-mechanical response of members and sub-systems, and temperature dependence of member strength predictions. In addition, hot-rolled steel has demonstrated the possibilities and advantages of enabling performance-based fire design (e.g.
Typically, it is assumed that a building compartment is subjected to a uniform temperature distribution that follows the parametric fire curve. However, standard fire curves provide a consistent benchmark and their use is so pervasive that generally they are regarded as fire demand regardless of the specifics.Parametric fire curves represent a modest generalization of the fire curve approach (CEN ). Typically, the fire curve is only weakly related to the actual time-temperature curve for a fire in a modern building. Similar time-temperature relationships have been implemented internationally. The fire curve was intended to represent a worst-case expected fire scenario, based on empirical data from timber construction (ASCE ). Fire demandOne of the first formal attempts to account for fire action on building structures emerged in 1918, when the ASTM standardized a time-temperature relationship (called the fire curve) to consistently evaluate the fire resistance of buildings.
These models are used to predict the development of fire in a building structure (including fully three-dimensional models), incorporating flames and smoke propagation. Amongst other details, these models account for the fact that higher temperatures are observed in the upper zone of the compartment.The most sophisticated simulations adhere to computational fluid dynamics (CFD) and at some level attempt to model actual fire dynamics. In zone models the compartment is divided into multiple regions, each with its own uniform temperature distribution following a parameterized fire curve. A further evolution of parametric fire curves is the use of “zone models” (Quintiere ). In general, parametric fire curves include a nonlinear heating phase followed by a linear cooling phase, while the standard fire is represented by an increasing curve (Figure 1).
Aisi Cold Formed Steel Framing Software Solutions Are
Heat transferOnce the thermal fire demands are established, the next step is to propagate these demands to the structure itself through heat transfer analysis. Multiple software solutions are in current use, including PHOENICS (Spalding ), FDS (McGrattan et al. Nonetheless, true performance-based design of fires relies on the long-term potential of this approach.
Thermal properties also vary with temperature, e.g. Mechanical properties of steel such as the elastic modulus, yield stress, and ultimate stress degrade with increasing temperature, thus steel members lose strength and stiffness under increasing temperature. Cold-formed steel material at elevated temperaturesDuring a fire, the temperature of structural members increases and, subsequently, material properties change. See Performance of Walls section for further discussion of modeling heat transfer in CFS assemblages. It is also common to make the simplifying assumption that the temperature varies linearly throughout the web of the CFS member in the assemblage, while the temperature of the flanges and lips are constant (Shahbazian and Wang ). Also, moisture migration and hot air flow are ignored, thus thermal gradients along the length of the assemblage (i.e., height of the wall) are ignored.
In general, tested specimens range from 0.50 mm to 2.00 mm thick, with yield strengths from 250 MPa to 550 MPa at ambient temperature. Chen and Young Ranawaka and Mahendran Kankanamge and Mahendran Chen and Ye ). Mechanical propertiesSeveral research groups have studied mechanical properties of sheet steel at elevated temperatures (Lee et al. Quantification of the temperature dependence of thermal and mechanical properties of sheet steel is a fundamental building block for predicting the response of CFS under fire.
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