Cold-Formed Steel Diaphragm Design

The analysis and design of floor and roof diaphragms intersect with cold-formed steel (CFS) structures in two unique ways.

First, the shape of diaphragms in mid-rise CFS structures (commonly five to 10 stories) are often complex and irregular due to the most common occupancies of these types of structures – namely, residential occupancies such as dormitories, hotels and apartments. Such occupancies necessitate long, narrow floor plans organized around corridors, often incorporating many turns.

Second, a significant number of structures have diaphragms composed partially or wholly of cold-formed metal deck. Examples include bare metal deck at the roof level, gypsum concrete fill over metal deck and concrete over non-composite or composite metal deck. The specifications for the design of these components are found in AISI S100, North American Specification the Design of Cold-Formed Steel Structural Members and AISI S310, North American Standard for the Design of Profiled Steel Diaphragm Panels.

Engineers working in the design of CFS will likely be required to design diaphragms. This presentation will inform that process through discussion of the basic principles of diaphragm analysis, code requirements and industry guidelines, as well as through some practical examples for approaching unique design and detailing scenarios. The definitions of diaphragm rigidity — rigid and flexible idealization and semi-rigidity — and diaphragm regularity or irregularity from ASCE 7 form a starting point. This will also require investigation of seismic-specific requirements such as diaphragm design forces and use of load combinations including the over-strength factor.

The presentation will cover the basics of the design of diaphragms such as determination of internal forces and design of components of the diaphragm including chords and collectors. At this point, a discussion specific to metal deck diaphragms will be appropriate and will include an introductory-level approach to AISI S310, metal deck analysis, strength determinations and common testing procedures. How these become the shear strength and stiffness capacity tables that engineers are used to seeing will be briefly demonstrated.

The presentation will conclude with special design and detailing case studies. Addressing conditions such as large openings, reentrant corners, nonstacking elements of the lateral force-resisting system and/or unique drag strut conditions will be the focus of this portion. Reference will be made to resources such as AISI D310 and the Steel Deck Institute Diaphragm Design Manual (DDM4).

Zane Clark, P.E., S.E.
McClure

Zane Clark is the Structural Technical Lead for McClure, where he has been designing cold-formed steel (CFS) structures since 2015. Through his time at McClure, Clark has gained specialized expertise in the design of mid-rise, load-bearing CFS buildings. Clark's current role involves providing quality control and code compliance reviews of design documents produced by McClure’s structural team as well as promoting technical education and training for the engineering staff. He is active with the ASCE/SEI Committee on Cold-Formed Steel Members, which is producing a design guide for CFS structures. His contribution is on the design of CFS lateral force-resisting systems.

Ian Micklethwaite, P.E.
ASC Profiles

Ian Micklethwaite is the primary Design and Technical Engineer for the Steel Deck Division of ASC Profiles, which is actively involved in the Steel Deck Institute (SDI). Micklethwaite has been working in the cold-formed steel (CFS) industry for the past eight years. Currently, he works on composite floor and roof deck design, which includes design modification to reflect ongoing building code updates as well as new product research and development. Previously, Micklethwaite has been a design engineer for CFS-framed buildings throughout the United States.