What is Diaphragm Design?

What is diaphragm design and why is it important?

A diaphragm is a flat structural unit acting like a deep, narrow beam. The term ‘diaphragm’ is usually applied to roofs and floors. A shear wall, however, is just a vertical diaphragm. Shear walls provide support for the roof and floor diaphragms transmitting forces into the foundation. 

A diaphragm structure results when a series of diaphragms are properly tied together to form a unit. When diaphragms and shear walls are used in the lateral design of a building, the structure is termed a box system. With good common construction practice, most sheathed elements in a building add considerable strength to the structure.

Does the structural diaphragm always work?

We wouldn’t be in business for as long as we have been if we weren’t honest with our customers. Like many things in life, there are exceptions with what functions correctly and not correctly. With a diaphragm structure, it’s important to know that some elements will not add strength to your structure.  Those elements are:

• Standing seam metal roofs
• Corrugated asphalt paper roofs
• Other low-stiffness sheathing materials

If the walls and roofs are sheathed, adequately tied together and attached firmly to the foundation, many of the requirements of a diaphragm structure are already met. This explains the excellent performance of sheathed buildings in hurricane and earthquake conditions. 

Applying the principle of diaphragm structure action results in reduced post size and embedment (foundation) requirements consistent with actual building performance. A firm foundation consisting of properly installed footings to support the intended structure and its design load is essential to the structural integrity and performance of your building. Footers must be: 

• Adequate size to support the intended structure and its design load
• Extend below exterior grade sufficiently to be free of frost action during winter
• Properly installed over well-compacted soil
• Consistent with the requirements of local building codes 

This makes post frame construction more economical and competitive with other construction alternatives in code-enforced construction. In measurable terms, the post size (also known as columns) for a typical post frame building can be reduced by more than one nominal size if the diaphragm contribution of roof cladding is considered.

Columns are integral parts of a pole barn. They support the entire roof system and transfer all vertical loads (such as snow) directly to the footings. Columns are the backbone of your walls and assist in resisting horizontal loads most commonly occurring from winds. Keep in mind that your building’s diaphragm carries the majority of the wind loads so “oversized” columns will not necessarily make your building stronger, it is the engineered system that will ensure peace of mind.

For example, the post reduces from a 6 x 10 solid-sawn (or 3-ply, 2x10 nail-laminated) column when no diaphragm action is used, to a 6 x 6 solid-sawn (or 3-ply, 2x6 nail-laminated) column when diaphragm action is considered for a 40’ wide x 80’ long building with a 16’ eave height.

When principles of diaphragm action are not applied, the total lateral wind load must be resisted solely by the wall columns. Each side wall post then behaves like a cantilever beam, resulting in a higher post bending moment at the ground line which requires a greater embedment depth.

Diaphragm action also requires that all pieces of the structure work together, so connecting these pieces together is very important. This includes the correct size, type, and spacing of fasteners. 

When comparing post frame builders, it is important that you pay attention to the fastening systems being used to attach the steel siding and roofing; how and when the system is applied. You want to evaluate the different offerings before you buy. We know that it sounds silly, but there’s a reason why this is important.

Because much of the post frame building strength comes from the diaphragm of steel on the sides and roof, the more secure the steel is affixed to the wood framing, the stronger the building. In addition, the ability of a shear wall to resist lateral loads requires a well-constructed roof diaphragm. The two work together to transfer lateral loads through the shear wall to the foundation. The effectiveness of the system is only as good as the quality and quantity of connections.

Diaphragm sheathing materials are typically a structural wood panel, such as plywood or oriented strand board (OSB), or architectural steel. 

Steel is measured in two ways:

• Thickness: Steel thickness is specified in inches or gauge.
• Yield strength: The amount of force required to cause permanent deformation. Hardness or resistance to denting is a function of yield strength.

Structural wood panels have the following features:

• They are used where a traditional roof or wall appearance is desired.
• Diaphragm tables are referenced in the codes, allowing for easy design and application.
• Exterior adhesives are used in their manufacture to resist the effects of moisture during job-site construction, maintaining an attractive appearance.
• They can easily be painted or shingled.

Wood diaphragms have a large capacity to absorb impact loads, resulting in excellent performance in high wind or earthquake situations.

Post frame construction produces stronger buildings than other methods for the following reasons:

• Columns in the ground add to the building's stability and wind resistance.
• Horizontally connected posts form a tremendously strong box that adds to wind and seismic resistance.
• The direct attachment of trusses to the post frame makes it virtually impossible for the roof to detach from the building.
• The diaphragm effect created by the post-frame structures allows them to flex under stress instead of cracking, crumbling or collapsing like many other structures.

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