When designing a building for lateral loads, such as those generated by wind or earthquakes, designers have several alternatives, including braced or rigid frames, diagonal rods, ‘X’ bracing, etc. Buildings can also resist extreme lateral loads through the application of a principle called “diaphragm design.”
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.”
An accurate method for engineering diaphragms (see ASAE EP 484.1 and APA Design/Construction Guide: Diaphragms) has evolved to allow the design of a building to be wind and earthquake resistant at very little cost.
With good common construction practice, most sheathed elements in a building add considerable strength to the structure. Exceptions include standing seam metal roofs, corrugated asphalt paper roofs, and other low-stiffness sheathing materials. Thus, 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.
This makes post-frame construction more economical and competitive with other construction alternatives in code-enforced construction. In quantitative terms, the post size for a typical post-frame building can be reduced by more than one nominal size if the diaphragm contribution of roof cladding is considered. 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 groundline 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. Once properly connected, post-frame construction technology creates a highly optimized structure. Diaphragm sheathing materials are typically a structural wood panel, such as plywood or oriented strand board (OSB), or architectural steel.
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.
For more information about structural wood panels, contact APA-The Engineered Wood Association at 253/565-6600, or visit www.apawood.org.
Architectural steel has the following features:
- It has high strength.
- It is lightweight, making it easy and quick to install.
- It is painted with weather-resistant paint to maintain its color and integrity for many years.
- All steel sheathing and fasteners are galvanized to prevent rusting due to weathering and to provide a long-lasting architectural finish.
To illustrate the effects of diaphragm action, Dr. Kifle G. Gebremedhin conducted the following series of tests:
- Building framework only (no sheathing diaphragms).
- Sheathing attached to end walls only.
- Sheathing attached to all four walls.
- Sheathing attached to all walls and one side of the roof.
- Sheathing attached completely (full diaphragm in place).
Shown below are the results of this testing, which illustrate the building’s increasing stiffness as more sheathing was put in place.