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Ametal building is a system with complex behavior that, when properly accounted for, provides an efficient structural solution. To help the engineer properly design for this behavior, the Metal Building Manufacturers Association (MBMA) has recently released the Roof Framing Design Guide for Metal Building Systems, 2nd Edition (Roof Framing Design Guide).
A metal building roof system is comprised of primary frames, secondary framing, which includes roof purlins and wall girts, exterior cladding and additional bracing elements. These components work together to provide a structural system that can resist the various environmental loads while providing for large open spaces. An example of a metal building system that provides an open floor space for an industrial application is shown in Figure 1.
The interactions between the different roof system components and the resulting strength and stability benefits are not obvious and the mechanisms are not typically taught in undergraduate engineering programs. Therefore, it is the goal of the Roof Framing Design Guide to thoroughly explain how a metal building roof system behaves and to provide guidance on how to design it. The Roof Framing Design Guide provides updates and additional guidance from the first edition to reflect changes in standards and brings the latest available information to the design of these systems. The original Design Guide for Cold-Formed Steel Purlin Roof Framing Systems, AISI D111, was published in 2009 by the American Iron and Steel Institute (AISI). The new edition also incorporates the information from another AISI design guide, A Guide for Designing with Standing Seam Roof Panels, CF97-1.
The Roof Framing Design Guide provides a comprehensive description of the design requirements of roof systems supported by cold-formed steel purlins, with emphasis on the design of the system anchorage. All design provisions are aligned with the 2016 edition of the North American Specification for the Design of Cold-Formed Steel Structural Members, AISI S100-16, referred to as AISI S100.
This second edition of the Roof Framing Design Guide, consisting of six chapters, was reorganized to highlight the nuances in design between systems clad with through-fastened panels versus those with standing seam panels, and to provide improved guidance on evaluating the strength and stiffness contributions of these panels. Because the roof panels help stabilize the cold-formed steel purlins, a roof system constructed with cold-formed steel purlins and metal panels truly acts as a system.
Chapter 1, Introduction: Chapter 1 of the Roof Framing Design Guide outlines the various components of the roof system including types of bracing systems and how they contribute to the system behavior. This chapter provides a quick overview of these systems to help orient a designer who is unfamiliar with cold-formed steel purlin supported roof systems. An example of a common configuration for roofing components is shown in Figure 2.
Chapter 2, Design Method for Purlins: For the design of purlins, the flexural strength depends on the extent to which they are braced by the panels and any additional discrete braces. Chapter 2, Design Methods for Purlins, breaks down the design process into three distinct pathways that account for these differences in bracing provided: through-fastened systems, standing seam systems designed by the Base Test Method, and discrete braced systems. Through-fastened panels effectively provide restraint from movement, so the process for the design of the purlin is simplified. Standing seam panels are typically more flexible, often by design, to accommodate thermal expansion and contraction, so their design must account for this increased flexibility. A designer may take a simplified approach ignoring the contribution of these panels altogether and design the purlins as discrete braced, relying solely on the stability provided by lateral braces along the span. A more sophisticated approach for standing seam systems is to utilize full scale tests, known as Base Tests, to determine the strength of the system. These tests account for the complex forces at play and provide a more accurate method for determining the flexural strength of the purlin.
The Base Test method is a standardized test outlined by AISI S908, Test Standard for Determining the Flexural Strength Reduction Factor of Purlins Supporting a Standing Seam Roof System. Chapter 2 of the Roof Framing Design Guide supplements this standard and provides guidance developing a strategic test program with a reduced number of required tests that can still accurately predict the strength of the system while still accounting for all of the potential variables that could be introduced. For example, AISI S908 requires that six tests be performed for each combination of purlin profile, panel profile, clip type, intermediate bracing configuration and loading. A manufacturer wishing to test a Z-section purlin with two flange widths in combination with a standing seam panel with two thicknesses and three possible clip types would be required to perform 72 tests (2 flange widths x 2 panel thicknesses x 3 clips x 6 tests). By strategically addressing each variable in the tests, such a test program could be reduced to as few as nine tests. A detailed example outlining such a test program is provided.
Chapter 3, Continuous Purlin Design: This chapter guides the designer through the various design checks required for cold-formed steel purlin systems. Because purlins are typically lapped across supports to provide efficient continuity with simple connections, the purlin strength must be checked at many more locations. As a result of the slenderness of the cross section of cold-formed steel members, additional limit states must be checked that may not be familiar to engineers accustomed to designing hot-rolled steel. Guidance is provided for both gravity loading conditions as well as wind uplift conditions. Numerous flow charts assist designers to account for all applicable limit states.
Chapter 4, Diaphragm Requirements: This is an entirely new chapter for this edition. It integrates much of the information originally published in A Guide for Designing with Standing Seam Roof Panels, CF97-1, and has been updated in accordance with AISI S100. The strength and stiffness of profiled panels is determined by AISI S907, Test Standard for Determining the Strength and Stiffness of Cold-Formed Steel Diaphragms by the Cantilever Method, known as the cantilever test. Chapter 4 provides guidance for adapting the diaphragm strength and stiffness quantified by the cantilever test to roof systems. Most of the design methodologies for purlins rely on the resistance provided by the diaphragm for strength, therefore there are limits to the lateral deflection of the diaphragm. Also provided are several methodologies with ranges of sophistication that allow the designer to predict the lateral deflections of the purlins to ensure compliance with deflection limits. These special procedures are introduced and illustrated with examples.
For cases where a diaphragm may not provide sufficient strength or stiffness to adequately stabilize a system of purlins, the system of purlins may be designed as a discrete braced system as classified in Chapter 2. The Roof Framing Design Guide provides guidance on calculating these brace forces using the approximate method from Section C2.2.1 of AISI S100 or by using a more refined mechanics-based method. In addition, there is new guidance on the design of strut purlins as it relates to axial loads, along with a design example for a strut purlin in a standing seam system with discrete braces.
Chapter 5, System Anchorage Requirements: To complete the design of the metal building roof structural system, bracing forces must be determined, and their load paths followed to the primary frames. The transfer of these forces to the primary frames is referred to as anchorage. Chapter 5 describes and outlines the procedure in AISI S100 Section I6.4.1 to determine the anchorage forces in roof systems with varying bracing configurations. Additionally, alternative methods to determine anchorage forces are explained, including the Component Stiffness Method, and guidance is provided to model purlin systems either using frame element or shell finite element models. Numerous calculation examples are provided illustrating how to apply the various methods. Figure 3 is a plan of a roof system with multiple anchors for which a detailed design example is provided.
Chapter 6, Miscellaneous Topics: The Roof Framing Design Guide concludes with this new chapter that covers standing seam roofs on steel joists and standing seam roofs with roof-top units or hanging loads.
The Roof Framing Design Guide for Metal Building Systems, 2nd Edition, provides updates and additional guidance from the original Roof Framing Design Guide to reflect changes in standards and brings the latest available information to the design of these systems. It is an essential resource to understand how metal building roofs work as a system. This publication went through a very thorough review by metal building and cold-formed steel engineers. An MBMA steering group reviewed the drafts of the guide, and comments were incorporated. It was then reviewed by the AISI Education Committee and AISI Subcommittee 4, Assemblies and Systems. These comments were incorporated into the document, which was then published as an MBMA design guide.
The new Roof Framing Design Guide is available as a free download from the Design Resources page of the MBMA website, https://mbma.com/design-resources. ■
About the Author
Michael Seek, Ph.D, PE, is an Associate Professor of Engineering Technology at Old Dominion University in Norfolk, VA.
Vincent E. Sagan, PE, F.ASCE, is the Director of Codes & Standards for the Metal Building Manufacturing Association (MBMA) in Cleveland, OH.
