Diaphragm Design using Lightweight Steel Cladding

Introduction 

Wind blowing across a typical gable-roof building produces forces that act directly on the cladding and structural members, as well as overturning forces for the building as a whole. Discrete wind bracing or rigid pole-framing are methods for resisting these forces. Another approach is to utilize the in-plane strength of the steel roof cladding to act as a shear diaphragm to transfer these loads to the foundation. The basic forces acting on the building are illustrated in Figure 1 below.

Diaphragm Design Method 

A design aid is available to assist in the selection and detailing of the sheet steel cladding and fasteners, needed to create a lightweight steel cladding diaphragm. This aid is a publication of the Metal Construction Association titled “A Primer on Diaphragm Design”, First Edition 2004, and is available through their website at www.metalconstruction.org. This manual provides a collection of design charts like the one re-produced below (see Figure 2) as well as worked out examples and a commentary. Unfortunately the profiles used for the tabulated design values are not typical of Canadian products. However, a general design expression is included in the manual that can be used to develop design shear strength and stiffness values for other cladding profiles. 

Scope 

The manual provides tables for a variety of different roof and wall systems that include the following components: 

Cladding profiles: 

• 1.5”deep x 7.2”rib spacing x 36”panel width in 33 and 50 ksi yield strengths
• 1.25”deep x 12”rib spacing x 36”panel width in 80 ksi yield strength 

Insulation: 

• R19 fiberglass 
• 3-1/4”polyisocyanurate or thermal spacer blocks Fastener pattern: 
• 3, 4 and 5 screws configurations across the sheet width Fasteners: 
• #12 or #14 screws 

Use with Limit States Design in Canada 

The tables provided in the MCA manual give design shear values that are allowable loads for use in the US with the Allowable Strength Design methodology. In Canada we use Limit States Design, and so the design shear values need to be converted. The tabulated allowable loads are multiplied by the appropriate safety factor (FAC=2.35 in the sample table below) to determine the nominal resistance. The nominal resistance is then multiplied by the appropriate resistance factor to get the factored resistance for use in an LSD design. The resistance factors for cold-formed steel diaphragms are given in Table D5 of CSA-S136-07 North American Specification for the Design of Cold-Formed Steel Structural Members. These values are reproduced below. 

Click to download Fact Sheet #25: Diaphragm Design using Lightweight Steel Cladding