12 Structural Applications

David R. Bohnhoff, P.E., a Professor of Engineering at University of Wisconsin-Madison discusses the features of post-frame construction, and structural applications best-suited for the construction method. We invite you to read his essay.

Any salesperson looking to expand their market must first identify potential markets. Identifying potential markets begins, in turn, by identifying the unique characteristics of the product being marketed. Once a product’s unique characteristics have been identified, markets that will benefit from the characteristics can be defined.

Post-frame construction provides optimal proficiency in the design, use, and look of interior spaces. Interior design is a critical factor in the utility and enjoyment of a building. Not only do occupants and visitors want a space that functions efficiently, but also one that is aesthetically pleasing. Because of the flexibility inherent in post frame, virtually any interior finish can be achieved.

Application of this process to post-frame buildings means that the unique characteristic(s) of the post-frame building system must be defined, and then post-frame building markets identified. Although this may seem obvious, it appears to me that as a building organization, we have not clearly defined the unique characteristic or characteristics of the post-frame building system, nor have we identified all potential market opportunities.

It’s Just a Structural Framing System

Prior to identifying the unique characteristic(s) of the post-frame building system, one thing needs to be clearly understood; IT’S JUST A STRUCTURAL FRAMING SYSTEM. This means that a post-frame building system is NOT an interior or exterior wall finish system, nor is it a roof, ceiling, or floor finish system, nor is it a thermal insulation system, nor is it any other type of building system that you can think of. It is also extremely important to realize that when you select a post-frame building system as your structural framing/support system, you are NOT locking yourself into any specific wall, roof, ceiling or floor finish, nor are you committed to any particular thermal insulation system.

While it is clear to most post-frame builders and designers that “a post-frame building system is just a structural framing system”, it evidently is not very clear to a number of individuals sitting on zoning boards across the country. If it were clear to these individuals, they would not partake in the utterly ludicrous practice of zoning out a particular framing/support system when their real desire is to control some other building characteristic/system. Nevertheless, when a zoning board votes to disallow post-frame buildings within a particular area, this is exactly what they are doing.

The problems posed to the post-frame building industry by zoning boards occurs in large part because zoning boards associate the “post-frame” framing system with less expensive exterior finishes – finishes that they feel will negatively impact surrounding home values. The mindset that framing systems dictate finish systems needs to be altered. To this end, there is a need to demonstrate that a post-frame building system, like any other major framing system, can support virtually any exterior finish system. In a nutshell, many existing, architecturally-stunning, non-post-frame buildings could have easily, and in many cases more efficiently, been framed using a “post-frame” framing system.

To promote the fact that “a post-frame building system is just a structural framing system”, it often helps to overview other structural framing/support systems. In general, a structural framing/support system is either concrete-based, steel-based, wood-based, or a combination of these three. Concrete-based systems include buildings supported by either masonry walls, precast concrete components, or steel-reinforced, cast-in-place concrete components. Steel-based systems range from the high-rise, three-dimensional steel-framed structures to buildings entirely framed with cold-formed, light-gage steel components. In between these two steel-framed assemblies are low-rise metal building systems, which are more commonly referred to as pre-engineered steel buildings (even through all buildings of note are pre-engineered). Wood-based framing/support systems include log, timber-frame, light-wood frame, structural insulated panel (SIP), and of course, post-frame.

Selection of an appropriate structural framing/support system is one of the first steps in building design. When selecting a framing/support system, the building designer is looking for some particular balance of the following seven desirable building traits or characteristics: Functionality, Affordability, Comfort, Aesthetics, Durability, Environmental-friendliness, and Safety. Note that after I developed this list, I found that when I arranged these traits in the order just given, the first letters of these seven traits spelled the word FACADES.

The Uniqueness of the Post-Frame Building System

A significantly greater market share for post-frame will be achieved primarily by exploiting structural framing advantages that the system has over other framing/support systems. This, in turn, requires that the unique structural characteristic(s) of the post-frame building system first be identified.

From time-to-time over the past quarter century, I have addressed the question “What makes post-frame buildings uniquely different from other framing systems?” Not long ago, during a meeting of the NFBA Technical and Research (T&R) Committee in Utica, IL, I again found myself addressing this question. It came during a mid-morning discussion led by Dr. Harvey Manbeck on his latest draft of a Model Specification for Post Frame Building Systems.

As the focus of the draft specification turned to its overall organization, it became apparent that we needed to identify those characteristics of a post-frame building system that made it a unique framing system; this since location of such core characteristics needed to be located in the beginning of the document to ensure they would be the first building items specified during actual document use.

As the discussion wore on, two things began to come into clear focus from an academic design professional’s perspective. First, wood posts are the main vertical framing element in all post-frame buildings. Without wood posts, there is no post-frame building. Second, there is no roof framing system that is unique to post-frame. Any roof framing system used in post-frame could be (and likely has been) placed on a light-wood frame wall (a.k.a., wood stud wall), a log wall, a SIP wall, a masonry wall, a light-gage steel-stud wall, etc. This fact became apparent when it was agreed that “a building featuring 24-inch on-center trusses/rafters attached to post-supported headers”, is a post-frame building just as much as a building in which all trusses or rafters are attached directly to posts. That said, it should be noted the roof of a post-frame building is framed using nominal 2-inch thick lumber or similar-sized engineered lumber components, and this makes it different from timber-framed and post-and-beam systems in which larger glulams and solid-sawn timbers are used as primary roof framing members.

The discussion at the NFBA T&R Committee led me to draft the following simple, yet rather effective, definition for a post-frame building system.

The term “post-frame building system” refers to any structural framing/support system whose main, load-bearing, vertical framing elements are wood posts. These posts may be embedded in ground, supported by concrete piers, or attached to a concrete wall or slab. They support trusses, rafters and/or headersmade from nominal 2-inch thick lumber or similarly-sized engineered lumber products. They are often laterally supported by horizontal members called girtswhose spacing can be varied to accommodate applied loads and sheathing needs. Post size and post spacing are dictated by such factors as: size of wall openings, wall heights, spacing of primary roof framing, and type and magnitude of structural loads.

In most post-frame buildings, almost every truss (or rafter) is directly attached to one or more wood posts. When a single truss is attached directly to posts, a large two-dimensional post-frame is formed. The connection of a series of post-frames with girts and purlins produces the structural skeleton of what many people visualize, and would even define, as a more “pure” or “true” form of the post-frame building system. I would recommend that as an industry we refer to any post-frame building in which virtually all trusses (or rafters) are attached directly to wood posts as a classical post-frame building system.

Note that it is this classical framing system that is the mirror structural image of the framing system used in low-rise steel frame (a.k.a. pre-engineered steel) buildings; in fact, the two framing systems use the same nomenclature. Both framing systems have large 2-dimensional primary frames, that are connected with secondary framing members called girts and purlins. As an aside, I would point out that the low-rise steel frame building industry refers to any sidewall member that simultaneously functions as both a wall girt and purlin as an eave strut. The use of the word “strut” recognizes (1) the importance of the member in handling and transferring axial forces, and (2) that bending stresses induced in such members are significantly lower than in regular girts and purlins. I have used the term “eave strut” in some of my writings when referring to a sidewall girt to which both roof and wall sheathing are attached in post-frame buildings, and I would recommend broader adoption of the term by our industry.