CFSRC Reports

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Plastic bimoment determination for cold-formed steel open sections
(2023-12) Glauz, R.S.
The warping torsion strength of cold-formed steel open sections can be much higher than first yield. Recent developments in the prediction of warping torsion bimoment strength demonstrate a dependence on the plastic bimoment magnitude. This paper explains elastic and plastic bimoment stress distributions, presents the calculation of first yield and plastic bimoment magnitudes, and provides the derivation of plastic bimoment formulas for some common shapes.
Shear Response of Fastened Assemblies of Cementitious Panel to Steel Deck for FastFloor Residential Project
(2023-12) Caswell, H.L; Torabian, S.; Schafer, B.W.
The goal of the FastFloor Residential project is to create a new steel floor system that is lightweight, fast to construct, and nonproprietary. FastFloor Residential strives to achieve this by exploring prototypes such as the one shown in Figure 1, employing 18 gauge 3 in. deep steel deck fastened back-to-back to create a cellular deck, then topped with ¾ in thick cementitious (structural) panel screwed to the steel deck. This report summarizes a series of push-out tests that were conducted in the Thin-Walled Structures Lab at Johns Hopkins University. The push-out tests provide the shear response of the fasteners used to attach the cementitious panel to the steel deck. Repeatability of response, fastener spacing, and installation conditions (overdriven screws) are all explored in the testing.
Ductility of Steels Listed in AISI S100-16 Supp. 2 - 2020
(2023-03) Bodwell, P.
This listing provides a database summarizing the ductility of all steels listed in AISI S100-16 with Supplement 2 - 2020.
Analytical Equations for Critical Local Buckling Stress of Lipped Channels
(2023-05) Ding, C.; Schafer, B.W.
The objective of this study is to develop accurate analytical equations for calculating the critical local buckling stress of lipped channels with or without punchouts under compression and bending. Finite strip analysis is conducted on over 1000 lipped channel sections under four different loading conditions. Simple polynomial ratio equations are fit to the finite strip analysis results. The performance of the developed expressions are compared against the finite strip analyses and the element method currently provided in the AISI S100 Specification. The developed expressions are intended for use by engineers in future versions of the AISI S100 Specification.
Structural Design for Fire Conditions of a Prototype Metal Building using the New Proposed Appendix to AISI S100
(AISI, 2022-05) Yan, Xia; Gernay, Thomas
Performance-Based Design (PBD) for fire conditions has been increasingly adopted for different structural systems over the last decade. The objective of this project is to exemplify through a case study how the newly proposed Appendix 4 to AISI S100 on 'Structural Design for Fire Conditions' can be used by the metal building industry. The report describes a procedure to complete the structural fire design of cold-formed steel structures and applies it to column assemblies taken from a prototype one-story metal building designed following U.S. building codes. The fire design procedure includes the definition of performance objectives, design fires, heat transfer analyses by the finite element method, and structural analyses using both the Direct Strength Method and finite element modeling. The results show that the design by analysis achieves the required performance while enabling flexibility and efficiency in design. Thermal analyses provide steel temperatures consistent with prescriptive fire ratings and are also used to demonstrate adequacy of unrated assemblies such as protected double C-shaped sections. The FE model confirms the applicability of the Direct Strength Method with appropriate retention factors to evaluate the elevated temperature response. The CFS columns maintain stability well beyond their prescriptive rating owing to the low applied load resulting from the ASCE/SEI 7 load combination in the fire situation, which does not include wind. Therefore, for structures which are lightly loaded in the fire situation, such as these metal building columns, analysis methods can be used to demonstrate superior performance compared to the applied fire resistance rating.
Compressive Capacity of Cold-Formed Steel High Strength Low-Alloy Lipped Channels with Intermediate Stiffeners
(2023-01) Akchurin, D.; Torabian, S.; Ding, C.; Schafer, B.W.
We report herein a set of compression tests on high strength low alloy (HSLA) cold-formed steel steel sections intended for use as piles. The specimens are lipped channels with outer cross-section dimensions of 6 in. wide and 4 in. deep, and include 1 in. lips and feature two intermediate flat hat-shaped stiffeners in the web, one intermediate stiffener in the flange, and return lips. The HSLA employed has a measured mean yield stress of 100.5 ksi and a nominal thickness of 0.075 in. The compression tests were conducted between rigid steel end platens and resulted in highly repeatable strengths at all studied lengths: 12 in., 24 in. and 48 in. Mean peak strength for the shortest 12 in. long specimens was 85.3 kips, followed by 80.1 kips for the 24 in. long specimens, and 74.1 kips for the 48 in. long specimens. Tested strengths were compared to predictions using the Direct Strength Method of AISI S100 and assuming either simply supported or clamped end boundary conditions. Although the test conditions only use end bearing, best agreement is still found when assuming clamped end boundary conditions, mean test-to-predicted ratio is 1.16 assuming simple ends and 0.97 assuming clamped ends. In addition, work to summarize measured geometric imperfections, comparisons against a set of companion tests using advanced high strength sheet steels, and potential future work items are all discussed.
FastFloor Residential Testing Report
(2022-11) Caswell, H.L.; Torabian, S.; Schafer, B.W.
The goal of the FastFloor Residential project is to create a new floor system that is lightweight, fast to construct and nonproprietary. FastFloor Residential strives to achieve this by using 3 in. deep steel deck of 18 gauge that is fastened back-to-back to create a cellular deck. The cellular deck is then topped with a cementitious panel that is screwed to the steel deck. A series of physical four-point bending tests on this unique cellular steel deck composite with cementitious panels floor system were conducted in the Thin-Walled Structures Lab at Johns Hopkins University. The goal of the testing is to understand the behavior of the composite action between the steel deck and cementitious panel, identify the failure modes, and evaluate the strength and stiffness of the composite floor system.
Steel Sheet Sheathing Options for CFS Framed Shear Wall Assemblies Providing Shear Resistance
(2007-10-31) Yu, Cheng; Vora, Hitesh; Dainard, Tony; Tucker, Jimmy; Veetvkuri, Pradeep
In cold-formed steel construction, stud walls covered with steel sheet sheathing is an available option to resist lateral loads such as those caused by wind and earthquakes. The current American Iron and Steel Institute (AISI) Standard for Cold-Formed Steel Framing – Lateral Design 2004 Edition provides nominal shear strength for a limited range of steel sheet sheathed shear wall configurations. This report presents a research project developed to add values for 0.030-in. and 0.033-in. steel sheet sheathed shear walls with 2:1 and 4:1 aspect ratios and 0.027-in. sheet steel shear walls with 2:1 aspect ratio and 6-in., 4-in., 3-in., and 2-in. fastener spacing at panel edges. For all specimen configurations, the steel sheet sheathing was installed on one face of the wall. The test program consisted of two series of shear wall tests. The first series focused on determining the nominal shear strength for wind loads for which monotonic tests in accordance with ASTM E564 standard were performed. The second series of tests addressed the nominal shear strength for seismic loads for which the reversed cyclic tests using CUREE protocol were conducted. The research was sponsored by AISI and SSMA, and was performed at University of North Texas.
Load Bearing Clip Angle Design
(2015-04-08) Yu, Cheng; Yousof, Mohamad; Mahdavian, Mahsa
The report presents a comprehensive research project aimed at developing design methods for three limit states of cold-formed steel clip angles: shear, compression, and pull-over of the screw connections. For each limit state, a test program was conducted to investigate the behavior, strength, and deflection of the clip angles. The test results were compared with existing design methods for members similar to, but not exactly the same as, cold-formed steel clip angles. It was found that none of the existing methods worked well for the tested clip angles, therefore new design methods were developed for each of the three limit states studied in the project. LRFD and LSD resistance factors and ASD safety factors were provided to apply to the proposed design equations for nominal strength.
Cold-Formed Steel Bolted Connections Using Oversized and Slotted Holes without Washers Phase 2
(2010-03-18) Yu, Cheng; Xu, Ke
In cold-formed steel (CFS) construction, bolted connections without washers for either oversized or slotted holes may significantly expedite the installation process and lower the cost. The North American Specification for the Design of Cold-Formed Steel Structural Members requires washers to be installed in bolted connections with oversized or slotted holes. A research project (Phase 1) sponsored by American Iron and Steel Institute was recently completed at the University of North Texas (UNT) that investigated the performance and strength of bolted CFS connections with oversized and slotted holes without using washers. The research presented in this report is the Phase 2 project in which the bolted CFS connections were studied in a broader respect in terms of the failure mechanism, the material thickness, and the hole configurations. Combined with Phase 1 results, the Phase 2 report gives a comprehensive evaluation of the behavior and strength of bolted CFS connections with oversized and slotted holes without using washers. Revisions to the existing AISI North American Specification requirements for bolted connections are proposed to account for the reduction in the connection strength caused by the oversized and slotted hole configurations without washers.
Steel Sheet Sheathing Options for Cold-Formed Steel Framed Shear Wall Assemblies Providing Shear Resistance - Phase 2
(2009) Yu, Cheng; Chen, Yujie
Monotonic and cyclic tests on cold-formed steel shear walls sheathed with steel sheets on one side were conducted to (1) verify the published nominal shear strength for 18-mil and 27-mil steel sheets; and (2) investigate the behavior of 6-ft. wide shear walls with multiple steel sheets. This project is the continuation of a completed project titled “Steel Sheet Sheathing Options for Cold-Formed Steel Framed Shear Wall Assemblies Providing Shear Resistance” by Yu (2007). This Phase 2 project confirms the discrepancy in the published nominal strength of 27-mil sheets discovered by the Phase 1 project, and proposes new values. The project also finds disagreement on the nominal strength of 18-mil sheets for seismic design, which requires further research. For the 6-ft. wide shear walls, this project indentifies special seismic detailing to prevent potential damage on studs while improving the strength and ductility of the shear walls. This report provides detailed information on the test setup, test results, and analyses.
Cold-Formed Steel Bolted Connections Without Washers on Oversized and Slotted Holes Phase 1
(2008-07) Yu, Cheng; Sheerah, Ibraheem
In cold-formed steel construction, bolted connections without washers for either oversized or short slotted holes may significantly expedite the building process and lower the cost. However, the current design specification does not include provisions for such connections, and washers are required to be installed for oversized holes or short slotted holes. The research presented in this report aims to experimentally investigate three typical failure modes in cold-formed steel bolted connections with the non-washer and oversized configurations. The three failure modes have been observed: sheet shear failure, sheet bearing failure, and fracture failure of the net section in the connected sheet. The research project consists of two phases. In Phase 1 the sheet shear and bearing failures will be studied, and in Phase 2 the fracture failure will be investigated. The test matrices include a full range of connection configurations covering various steel sheet thicknesses from 30 mil to 118 mil, different connection types of single and double shear using ASTM A307 and A325 bolt types, and high and low ductile steels.
Semirigid Diaphragm Analysis of Archetype Floor Using Design Office Models
(2022-05) Swecker, J.; Gengrui, W; Avellaneda-Ramirez, R.E.; Eatherton, M.R.
Diaphragms are the floor or roof structures in a building that transfer lateral load horizontally to the braced frames or shear walls. For many types of buildings, the building code requires semirigid diaphragm analysis whereby a three-dimension computer model of a building is created with diaphragm elements that can deform elastically. As a companion to a research project called the Steel Diaphragm Innovation Initiative (SDII), this report describes the semirigid diaphragm analysis of typical floors from archetype buildings. In other publications, the archetype buildings are analyzed using advanced simulation techniques that capture nonlinearity and failure of the diaphragms. The models in this report use software and modelling techniques typical in structural engineering practice and therefore allow the evaluation of design office models for use in understanding behavior of diaphragms during extreme loads such as earthquakes.
Enhancing Seismic Resiliency of Steel Buildings through Three-Dimensional Modeling of Diaphragm System Interaction with Braced Frame
(2021-03-29) Foroughi, Hamid
The design objectives of early seismic codes were mainly achieved by using acceptable construction materials, and minimum levels of strength and stiffness largely derived based on roughly estimated demands. Early seismic design provisions resulted in sometimes adequate levels of performance for buildings, efficient and reliable building performance was not accomplished with the simplistic demand-based design process adopted. Consequently, capacity-based design approaches and later performance-based design approaches have been adopted and developed for new seismic design provisions. To achieve a certain performance level for conventional steel buildings during seismic events, all the members should be designed to meet the component deformation and force capacities. The lateral-force resisting system, key for developing successful seismic performance, has two main parts: 1) the Vertical Lateral-force resisting system (VLFRS) of which a common example in steel buildings is the braced frame, and 2) the Horizontal lateral-force resisting system (HLFRS) consisting of the floor and roof diaphragm. The floor diaphragm acts as a critical element that distributes the demands developed in a building during an extreme event to the vertical lateral-force resisting systems and eventually to the building foundation. Compared to the VLFRS, far less attention has been paid to the role of the diaphragm in seismic building response, particularly for steel framed buildings. The first part of this dissertation contributes to developing fundamental understanding of steel deck diaphragms as structural systems integrated within the overall building performance and improved strategies for accurate modeling of floor systems within three-dimensional building models to enhance the overall structural resilience of a building. In this study, the effect of different diaphragm designs on the behavior of steel buildings is investigated using three-dimensional computational building models that consider nonlinear behavior in both the vertical and horizontal elements of the seismic force resisting system. Different diaphragm design scenarios based on ASCE 7-16, are investigated for a series of 1, 4, 8, and 12-story archetype buildings with special concentrically braced frames (SCBFs) as the vertical lateral force resisting system: 1) traditional design, 2) alternative design with Rs = 1.0, 3) alternative design with Rs = 2.0 or 2.5, and 4) alternative design with Rs = 3.0 are all considered. For the studied buildings first, modal analyses were conducted to study their basic dynamic properties. Second, nonlinear pushover analyses were investigated to analyze their static overstrength and ductility. Third, nonlinear response history analyses were conducted to evaluate building seismic performance. Finally, the FEMA P-695 methodology is used to assess the seismic performance and propose a reasonable Rs values for conventional SCBF steel buildings. The second part of this dissertation addresses rod bracing, which has a wide application in metal buildings as a truss diaphragm including the sidewalls to provide the lateral stiffness in longitudinal direction, and also the roof by providing the lateral bracing for the rafters in the out-of-plane direction. In this part of the dissertation, an experimental program is conducted to establish the stiffness, strength and applicable limit states for rod bracing. Twelve rod brace assemblages with differing details related to the geometry of the primary frame members, anchorage of the rod into the frame member, and angle of the rod relative to the framing member were tested in tension until failure. Finally, a framework is presented for future evaluation of seismic performance for rod-braced metal buildings by implementing the experimentally established stiffness and strength values and three-dimensional computational modeling of metal building systems.
Seismic Performance and Topology Optimization of Building Diaphragms
(2021) Fischer, Astrid Winther
This dissertation investigates the seismic performance of steel deck diaphragms through the effects of rigid and flexible diaphragms on the seismic response, the diaphragm and wall interactions, and the improvements to the diaphragm design using topology optimization. The diaphragm is part of the lateral force resisting system (LFRS), which consists of two main components: the vertical LFRS, i.e., braced frames, shear walls, etc., and the horizontal LFRS, i.e., the diaphragm. With the use of mass-spring models, the diaphragm and wall interactions can be studied, therefore, mass-spring models of a single-story building model and multi-story building models were developed that include a degree of freedom for the diaphragm and two degrees of freedom for the vertical LFRS for each story. The seismic response was studied through a parametric study that considered variations of the diaphragm and wall stiffnesses, mass distribution in the model, and different levels of inelasticity in both the vertical and horizontal LFRS. It was observed that the force demands in both walls and diaphragm(s) depend on the diaphragm and wall stiffness, mass distribution, and the inelasticity levels. Secondly, dynamic amplification occurred in the diaphragm force demands when diaphragm and wall periods are similar. Thirdly, diaphragm forces are observed to be reduced by reducing the capacity of both horizontal and vertical LFRS. Finally, large ductility demands arise in the component of the LFRS with the larger inelasticity level. Three diaphragm examples are optimized for minimum compliance and then modified for a more constructible design. The elastic and inelastic behaviors of modified optimized diaphragm designs were compared to designs using traditional diaphragm design methods. Findings include that the modified designs have stiffer responses in the elastic and inelastic range compared to the traditional diaphragm designs. Furthermore, the modified designs reached a higher capacity at failure and had a better ability to redistribute stresses after initial yield, resulting in a higher amount of dissipated energy through plastic deformations.
Stiffness of Concrete-Filled Steel Deck Diaphragms
(2022-01) Avellaneda-Ramirez, R.E.; Eatherton, M.R.; Easterling, W.S.; Schafer, B.W.; Hajjar, J.F.
In structural analysis of building structures, the in-plane stiffness diaphragms is needed so that lateral loads will be properly distributed to elements of the lateral-force resisting system. In US building codes, diaphragm stiffness is used to determine whether a diaphragm can be assumed rigid or flexible and is also used in semi-rigid diaphragm analysis. For concrete-filled steel deck diaphragms, methods provided in AISI S310 (AISI, 2020) to calculate stiffness have relied on empirical formulas while past research by Porter and Easterling (1988) suggests that mechanical models and theoretical formulas can accurately capture stiffness. Recently, eight cantilever diaphragm specimens were tested with variations in depth of concrete cover, deck depth, perimeter stud anchor configuration, concrete type (normal weight (NW) and lightweight (LW)), and the presence of either welded wire mesh or reinforcing steel. This report summarizes the results of this testing program as they relate to initial stiffness. The initial stiffness results of this testing program are used in conjunction with the results of a testing program performed Porter and Easterling (1988) to form a set of 25 specimens that are then used to validate a proposed prediction model for the initial stiffness of concrete-filled steel deck diaphragms. The proposed prediction model is based on a theoretical framework proposed by Porter and Easterling (1988) which concluded that the initial stiffness of a concrete-filled steel deck diaphragm is a combination of 1) the diaphragm shear stiffness, 2) the bending stiffness of the concrete-filled steel deck diaphragm combined with the chords, and 3) the stiffness of the shear transfer connections between the concrete-filled steel deck diaphragm and the supporting steel frame. The proposed stiffness predictions using this approach resulted in an average ratio of predicted stiffness to measured stiffness equal to 0.95 with a standard deviation of 0.21. Based on this comparison for 25 cantilever diaphragm specimens, it was deemed that the prediction model accurately represents the initial shear stiffness of concrete-filled steel deck diaphragms. This report also includes two examples to illustrate of how the proposed prediction model can be used to calculate diaphragm deflections for two different diaphragm configurations. The results of these examples showed that for the cantilever diaphragm configuration, the deflection of the free end is mostly due to the shear deformation of the concrete-filled steel deck diaphragm or to the deformation of the shear transfer connection, depending on the spacing of headed stud anchors, with the bending deformations contributing the least to the total deflection. For the case of a simply supported diaphragm, the mid-span deflection was attributed primarily to bending deformations of the diaphragm (78% of total deflection), with shear deformations contributing to approximately 25% of the total deflection and the deformation of the shear transfer connections contributing less than 1% of the total deflection.
Evaluation of Metal Building System Seismic Response Modification Coefficients
(2019-04) Moen, Cristopher D.; Torabian, Shahab; Schafer, Benjamin W.;
A seismic evaluation was conducted of common metal building configurations within the probabilistic framework defined in FEMA P695 “Quantification of Building Seismic Performance Parameters” with the goal of evaluating the applicability of current seismic design procedures in the ASCE 7-10 “Minimum Design Loads for Buildings and Other Structures”. The evaluation began with the definition of a performance group of index archetypes which was guided by industry steering group-led design studies that explored the influence of seismic load combinations and geographical location on the primary frame design. It was observed that taller, shorter span buildings in the western U.S. were most sensitive to seismic demands. This led to the definition of a performance group covering a range of natural periods and seismic weights designed to ASCE Seismic Category D and with the seismic response modification factor of R=3.5. The seismic parameters calculated for the performance group, designed by industry with ASCE Equivalent Lateral Force procedures, were the system overstrength, ductility, and probability of system collapse when exposed to the Maximum Considered Earthquake (MCE) which has a 2% probability of exceedance in 50 years. The system overstrength and ductility for each index archetype were calculated with simulated experiments using thin shell high fidelity simulation, where all metal building components were modeled including the built-up primary frames, the girts and purlins, the exterior metal facade and screw down roof, the rod bracing, and the primary frame flange braces that are important for controlling lateral-torsional buckling. The high fidelity simulation protocol was extensively validated with research between 2006 and 2013 that included monotonic and cyclic primary frame subassembly tests and shake table tests at the University of California, San Diego. The simulated pushover experiments revealed significant system overstrength in the index archetypes and a post-peak ductile response that was sensitive to the controlling limit state. When primary frame lateral-torsional buckling was controlled by the intermediate flange braces, post-peak deformation was available out to large drifts. Panel zone buckling at the knee of the column/rafter resulted in steady post-peak strength degradation. The heavy wall buildings had a higher pushover strength than the light wall buildings because the seismic design load combinations were more influential on the heavy wall primary frame design. The same high fidelity models were used to characterize the quasi-static cyclic response for each index archetype using an accepted American Institute for Steel Construction (AISC) industry loading protocol. The cyclic response, including strength and stiffness degradation from local and global buckling and column-rafter knee panel zone tension field yielding, was fit to a nonlinear Single-Degree-of Freedom (SDOF) material model used for incremental dynamic analysis (IDA). The IDA performances from 44 far-field ground motions required by FEMA P695 led to a cumulative distribution function of spectral intensity of the far-field record set which could be used to calculate the median collapse probability for each index archetype. 4 Uncontrolled collapse was never observed for these light buildings in the simulations or the shake table experiments, however fracture was, in the knee-rafter panel zone from shear buckling and in the rafter taper joints after lateral-torsional buckling. Both drift and fracture studies were conducted to settle on a drift-based collapse limit of 4.5% for the performance group. The collapse margin ratio, defined as the spectral acceleration at median collapse probability to the spectral acceleration from the Maximum Considered Earthquake (MCE) was on average across the metal building performance group higher than the acceptable collapse margin ratio corresponding to a 10% collapse probability, with no outliers greater than 20%. This confirms the viability of the existing ASCE 7 equivalent lateral force seismic design procedures for metal buildings in the performance group considered. With the seismic evaluation process now established and validated, the metal building industry can now investigate other performance groups with potentially large commercial impact - for example, heavier roof buildings that are outside the limits of current ASCE 7 procedures. A modular metal building seismic performance group also becomes available for study with the verified high fidelity modeling protocol used to perform simulated pushover experiments and to quantify cyclic performance.
Cantilever Testing of Concrete-Filled Steel Deck Composite Diaphragms Using Various Types of Steel Reinforcing
(2021-11) Avellaneda-Ramirez, R.E.; Eatherton, M.R.; Easterling, W.S.; Schafer, B.W.; Hajjar, J.F.
This report provides a summary of nonlinear response history analyses conducted on a three- dimensional model of a series of steel buildings with special concentric braced frames (SCBFs). The models are conducted in OpenSees and include appropriate nonlinear response for the braced frames as well as the concrete-filled steel deck diaphragms and bare steel deck roofs. Additionally the buildings are designed considering traditional diaphragm design as defined by ASCE 7-16 12.10.1 as well as the new alternative diaphragm design procedures of ASCE 7-16 12.10.3. These alternative procedures have a seismic response modification coefficient, Rs, which is specific to the diaphragm system. Rs values between 1 and 3 are investigated herein. The results indicate that SCBF building performance is sensitive to the diaphragm design, and that traditional diaphragm design does not lead to acceptable levels of performance. Use of the alternative diaphragm design procedure with Rs=2.0 for concrete-filled steel deck floors and Rs=2.5 for bare steel deck roofs is recommended. Future work is needed to continue to refine collapse criteria for 3D building models and to allow the engineer greater clarity in the extent of expected inelasticity in the vertical system vs. the diaphragm system when different combinations of R and Rs, i.e. different combinations of vertical and horizontal lateral force resisting systems, are employed.
Seismic Behavior of Steel SCBF Buildings Including Consideration of Diaphragm Inelasticity
(2021-04) Foroughi, H.; Wei, G.; Torabian, S.; Eatherton, M.R.; Schafer, B.W.
This report provides a summary of nonlinear response history analyses conducted on a three- dimensional model of a series of steel buildings with special concentric braced frames (SCBFs). The models are conducted in OpenSees and include appropriate nonlinear response for the braced frames as well as the concrete-filled steel deck diaphragms and bare steel deck roofs. Additionally the buildings are designed considering traditional diaphragm design as defined by ASCE 7-16 12.10.1 as well as the new alternative diaphragm design procedures of ASCE 7-16 12.10.3. These alternative procedures have a seismic response modification coefficient, Rs, which is specific to the diaphragm system. Rs values between 1 and 3 are investigated herein. The results indicate that SCBF building performance is sensitive to the diaphragm design, and that traditional diaphragm design does not lead to acceptable levels of performance. Use of the alternative diaphragm design procedure with Rs=2.0 for concrete-filled steel deck floors and Rs=2.5 for bare steel deck roofs is recommended. Future work is needed to continue to refine collapse criteria for 3D building models and to allow the engineer greater clarity in the extent of expected inelasticity in the vertical system vs. the diaphragm system when different combinations of R and Rs, i.e. different combinations of vertical and horizontal lateral force resisting systems, are employed.
CFS Bracing Design Using Combinations of Discrete and Sheathing Bracing
(2020-08) Schafer, B.W.
The objective of this report is to summarize efforts related to establishing the behavior and design of cold-formed steel wall assemblies potentially braced by discrete steel bracing and wall sheathing. Cold-formed steel wall assemblies are typically designed with discrete steel braces; however, the predicted accumulation of brace forces in these systems can lead to demands at odds with past successful practice. At the same time, testing on sheathing braced wall studs demonstrate the effectiveness of sheathing bracing, but questions persist about their effectiveness in extreme conditions. It is hypothesized that a combined bracing system could provide adequate strength from the steel system under extreme (environmental) conditions and benefit from the effectiveness of sheathing bracing in service and other conditions. A focused series of experiments on wall stud assemblies in compression with different combinations of discrete bracing and sheathing bracing were completed. The testing indicated that sheathing bracing dominates the braced stud response and that brace forces in discrete braces do not accumulate when sheathing is present. A series of spreadsheets were developed to provide engineers with a clear and efficient means to predict the strength of studs with both discrete bracing and sheathing bracing. The proposed strength calculations agree with the limit states observed in past and current testing; however, unlike in previous work where predicted strengths were conservative, in the testing conducted in this project the predicted strength is from 7 to 19% unconservative depending on the details of the tested specimen. Enabling combined bracing in cold-formed steel design requires modifications to AISI S100 and AISI S240 and a series of ballots that would be necessary for adoption of the new methods are detailed and priorities for adoption given. It is recommended that additional testing and design aids be developed.

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