There are many ways to design a building to meet the structural requirements of building codes. In situations where there are very large window and door openings, high walls, large open areas, or high lateral wall pressures, it is generally most cost-effective to go with a post-frame building system.

The term “post-frame construction” relates to the materials and methods used to construct the structural frame of a building. A post-frame building system relies on large primary frames that are connected with smaller wood members (girts and purlins) to resist applied building loads. The large primary frames consist of wood posts and trusses that are typically spaced between 4 and 12 feet apart. How a post-frame building appears is entirely dependent upon how the owner decides to cover the structural frame.

Like any framing system, a post-frame building system is useful for a wide range of projects, and can be optimally designed to meet almost any load requirement. It is as permanent a framing system as is used in any building.

A major advantage of a post-frame building system is that it does not require a continuous concrete foundation. Although some local building regulations may require a continuous concrete foundation for homes or other structures, there is no basis in the International Building Code for such a requirement. A continuous concrete foundation may be installed if desired on a post-frame building.

There are several other foundation options for post-frame buildings, including pre-cast and cast-in-place concrete piers and new products that provide exceptional protection to wood embedded in the ground that serves as a building foundation. For more information on post-frame foundation options, click here.

Due to a proven record of success, and financial and design efficiency, wood foundation systems are found on most post-frame buildings. The wood is protected from insect and microbial attack in accordance with standards defined by the American Wood Protection Association (AWPA). Many people do not understand that there are many different levels of treatment for wood, and presume that all treated wood is the same. They may have an experience replacing a deck after several decades of use, and assume that therefore all post-frame buildings must be replaced in similar fashion. In reality, the US Forest Products Laboratory has been testing wood treated equivalently to what we use today for post-frame buildings over many decades in extremely harsh environments, with excellent results.

AWPA standards are very specific - post-frame buildings must use treated wood that is of a different quality than what is treated for use for decks, playground structures and other typical residential uses. AWPA Use Category UC4A is the minimum allowed for post-frame skirtboards (similar to “sill plates,” these run around the perimeter of a post-frame building at ground level). AWPA Use Category UC4B is the minimum allowed treatment for posts and laminated columns embedded in the ground for post-frame buildings. CCA-treated timbers or laminated columns are presently still allowed for post-frame building posts, but these must be treated to .60 pcf (significantly more treatment than what is required for decks and other typical outdoor structures).

Post-frame buildings were “born on the farm,” so to speak, as from the 1930’s to the 1970’s they primarily were used for agricultural, military and other government uses. Since the 1970’s, however, commercial and residential post-frame has exploded. Today each is about one-third of the industry, with commercial taking the largest portion and agricultural coming in last after residential. However, this would not have been possible had it not been for the American Society of Agricultural and Biological Engineers (ASABE, formerly ASAE), which used a consensus process to establish the engineering practice standard for building post embedment.

This engineering practice standard, ASAE 486.1 - Shallow Post Foundation Design, has been referenced by the International Building Code since 2003. ASAE EP 486.1 describes appropriate post embedment and is much more favorable to modern post-frame design and construction methods than the 2000 IBC was. With the IBC now being adopted across the country, post-frame construction is recognized as a form of construction that is validated by this important building code. Consequently, in the 21st century commercial post-frame market growth has been particularly robust.

Post-frame design is also codified by several important industry groups. Trusses are designed in accordance with the latest Truss Plate Institute specifications. Structural elements are designed in accordance with the American Forest & Paper Association’s National Design Specification for Wood Construction, which is now part of the Allowable Stress Design Manual for Engineered Wood Construction, or the Load and Resistance Factor Design Manual for Engineered Wood Construction. Each structure can be optimized to meet loading requirements as specified by the local building code or code official. The NDS gives design equations and allowable stresses for wood design. It also includes shear values for a unique fastener that was developed specifically for the post-frame industry, “Post-Frame Nails” fabricated in accordance with ASTM standard F1667. The National Frame Building Association supported research conducted by the US Forest Products Laboratory on the unique withdrawal strength of post-frame nails. Designers may use these values to take advantage of the unique properties of post-frame nails to increase the efficiency of the design – which also increases the efficiency of materials and labor used for post-frame buildings.

AF&PA’s American Wood Council has also developed a document called Design for Code Acceptance No. 5 for Post Frame Buildings. The purpose of this document is to provide guidance to post-frame building designers for meeting the requirements of the IBC and to confirm that a properly designed post-frame building is, in fact, code compliant. This document is available on the AWC Web site at www.awc.org/Publications/dca/DCA5/dca5.htm. 

Post-frame buildings have extraordinary design efficiency due to diaphragm design, which allows these lightweight structures to resist lateral loads much more efficiently than buildings that are not amenable to diaphragm design techniques. Codes of engineering practice recognize the effectiveness of the diaphragm design of post-frame buildings in resisting wind or seismic loading. The strength of the particular type of cladding can significantly increase the effectiveness of diaphragm action. In tests conducted at Cornell University, the addition of steel sheathing reduced the maximum deflection of the test building from 6.1 inches to 0.44 inches. ASAE EP484 covers recommended diaphragm design procedures for post-frame structures.

Post-frame is a relatively new type of construction. It evolved from primitive techniques into a modern and innovative solution that is extremely efficient in terms of design, labor, material inputs, and long-term energy efficiency. Code bodies have recognized this progress with appropriate building and engineering standards. Today’s pre-engineered post-frame buildings are code accepted and complaint with building regulations.

Click here for more information on pertinent post-frame related standards and technical documents.

This story was written by John Fullerton and originally appeared Frame Building News magazine, the official publication of the National Frame Building Association. To subscribe to Frame Building News, please visit www.FrameBuildingNews.com.