By Sharon Thatcher and David R. Bohnhoff, PE
Throughout the post-frame building industry there is the belief that this uniquely simple and adaptable framing system is on the verge of measurable expansion into residential housing markets. But going back through the archives at Rural Builder, it is evident that the industry has been hopeful for years.
Unfortunately, despite continual overall growth, industry expansion into residential housing has remained fairly flat. What is holding it back? Is it a lack of knowledge on the part of residential builders, architects, building code officials, bankers and insurance companies? Is it a belief that the post-frame building system is not applicable to residential construction? Is it simply a lack of design innovation?
When this question was posed to University of Wisconsin-Madison professor and registered engineer David R. Bohnhoff, his response was three simple words “prescriptive building codes.”
What are prescriptive building codes?
We turned to the internet for a definition: “Prescriptive codes are like a type of cookbook for building, simply follow the recipe.” (canvas.instructure.com)
Bohnhoff offers more depth: “Prescriptive codes tell the builder exactly what to use in terms of material sizes, types, orientation, etc. and exactly how to connect various components (e.g., size, type, number and location of fasteners). Prescriptive codes are largely comprised of sets of selection tables. While these tables are extremely useful, their finite number and size generally restricts their application (and hence use) to residential and smaller commercial buildings.”
What many people fail to recognize is that virtually every home in the United States must be designed in accordance with a prescriptive code. In 49 states, this prescriptive code is some version of the International Residential Code (IRC)— a model building code developed by the International Code Council (ICC). In the State of Wisconsin (the only state not to adopt the IRC), residences must be designed in accordance with the state’s Uniform Dwelling Code—a prescriptive code written by Wisconsin state government employees.
“The problem,” says Bohnhoff, is that provisions in the IRC are primarily associated with light-wood frame, light-steel frame and SIP construction. The IRC does not contain design provisions for major elements of post-frame building design and construction. Bottom line, if the IRC is law in basically every jurisdiction of the United States, and the law doesn’t directly address post-frame building systems, you have a huge barrier to the construction of post-frame buildings for human habitation.”
What codes cover post-frame buildings?
In addition to a residential building code, state and local governments enforce a commercial building code. Across the United States, this commercial code is some version of the International Building Code (IBC). The IBC is a performance-based code. All building systems, including post-frame, can be designed to meet the performance provisions of the IBC.
“Performance-based codes are codes that require a building to meet certain performance requirements. For example, depending on where you live, the code may require your building be designed to withstand a certain snow load,” Bohnhoff explains. “Performance-based codes generally always require the involvement and seal of a licensed engineer or architect. In short, a knowledgeable person (as implied by licensure) must show (via appropriate calculations) that the design meets the performance criteria.”
To assist engineers and architects in the design of post-frame buildings, the American Society of Agricultural and Biological Engineers (ASABE) has developed standards addressing mechanically-laminated wood assemblies, metal-clad wood-frame diaphragms, and shallow post and pier foundations. These standards have been recognized by the American National Standards Institute (ANSI) as American National Standards and have been adopted by reference in the IBC.
Who would benefit from a prescriptive post-frame building code?
Given that U.S. residential building is largely controlled by prescriptive codes, the obvious beneficiaries of a prescriptive post-frame building code are those post-frame building designers, contractors and suppliers interested in home building.
But the advantages extend well beyond the obvious. “The most immediate beneficiaries,” claims Bohnhoff, “are likely to be companies and DIYs who currently put up smaller post-frame buildings without the assistance or support of engineers. Such non-engineered structures are generally associated with applications that have been exempted from local building codes. This almost always includes agricultural buildings, and in many jurisdictions also includes detached residential garages as well as storage units and shops built for private use.”
Since buildings that are designed in accordance with a recognized building code are generally superior to those that have been erected without engineering input, the real winners in all of this, notes Bohnhoff, would be the consumers of smaller post-frame buildings as well as the companies that insure them.
Bohnhoff knows that some people are under the impression that “if there is no code, I can save money.”
But this often flies in the face of reality. “You have guys trying to copy fully-engineered structures without understanding what’s actually all involved in the design, and hence critical to a properly functioning building system,” he said. “Engineering is all about creating balanced designs. In many non-engineered structures you will find half the building components have been over-built, and the other half under-sized.” Oversizing wastes money and materials (and thus is not environmentally friendly). Under-sizing increases the probability of a failure. “And we’ve seen some horrendous failures,” he said. “If anyone loses when we provide a prescriptive post-frame building code, it will be the lawyers who make a living off building failures.”
Bohnhoff also believes a prescriptive post-frame building code, if properly developed and incorporated into the IRC, would expose many more architects to the economic and ecological advantages of various post-frame building elements. He adds that this would subsequently result in more creative and aesthetically pleasing post-frame homes as well as commercial buildings.
So why aren’t there prescriptive post-frame building codes?
“In truth, a prescriptive post-frame building code that is extremely limited in scope, quite inflexible in overall application, and that does not use standard industry terminology, was developed a couple years ago by a particular jurisdiction,” points out Bohnhoff. “Around 2010, the National Frame Building Association (NFBA) was approached about getting involved in the original development of this particular code but the NFBA Board declined the invitation. This is not to say that there was not support within the NFBA for prescriptive code development; I and a couple other members of the NFBA Technical and Research Committee pushed for it.”
Bohnhoff believes the NFBA Board decision to stay out of the prescriptive code development business was primarily driven by larger post-frame building companies (i.e., companies who employ their own engineers) and by engineering firms specializing in post-frame buildings. “Both of these groups have concerns about increased competition and misuse of a prescriptive code,” states Bohnhoff. “A prescriptive code largely removes the engineer from the design process, and this is not always a good thing. In fact, it becomes a bad thing when continual use of a prescriptive code emboldens someone to construct large and complex buildings for which the code was not intended.”
Bohnhoff understands the position of NFBA. As a registered professional engineer, and an engineering educator, Bohnhoff was a longtime opponent of prescriptive post-frame building code development. To a large extent, he felt that removing an engineer from the design process would, more often than not, compromise the structural integrity and threaten the safety, health and welfare of the very public he was sworn by his professional canon to protect.
But now he is a strong advocate. Why?
With a passage of time that has brought with it investigations of numerous non-engineered building failures, Bohnhoff now believes that a properly written prescriptive code covering smaller post-frame buildings is just what everybody in the industry needs. “From a public safety perspective, you can argue that the current system is broke. A prescriptive code for smaller post-frame buildings is as much about educating users about critical structural design elements, as it is about streamlining design, reducing failures (and associated litigation), reducing insurance rates, opening up residential markets, protecting consumer investments and ensuring public safety.”
To some extent, a prescriptive code for smaller post-frame buildings underscores the importance of performance codes for larger buildings. “By default, a prescriptive code for smaller post-frame buildings establishes clear size limits for agricultural as well as non-agricultural buildings that no person or company should ever exceed without the proper involvement of engineers,” notes Bohnhoff.
In Bohnhoff’s opinion, the current exemption from building codes that is granted to agricultural facilities by most jurisdictions is being abused. The exemption originated at a time when the largest agricultural buildings were a fraction of the size they are today. “The exemption is being used to skirt structural engineering,” he laments, ”which was never its true intent. Frankly, it’s shear lunacy, if not criminal in today’s society, to construct any building several hundred feet in length if it has not been fully engineered.”
Inasmuch as a rising tide raises all boats, Bohnhoff believes that any market expansion associated with implementation of prescriptive codes will be a boon for larger post-frame building companies and engineering firms, especially if it helps clarify when structural engineering is critical.
How easy is it to develop a prescriptive post-frame building code?
Development of a prescriptive code for post-frame buildings can get very complex in a hurry if the goal is to build in as much design flexibility as possible while also working to match the format of prescriptive residential building codes.
Modern residential buildings rely almost exclusively on diaphragm action to handle horizontal components of applied loads, and this is reflected in the format of prescriptive residential building codes. “From a structural design perspective, you can look at prescriptive residential building codes as largely comprised of individual chapters on floor plates, wall plates, ceiling plates and roof plates,” says Bohnhoff.
While the vast majority of post-frame buildings constructed today also rely on diaphragm action, this was not always the case. Prior to the 1980s, post-frame building designers relied entirely on embedded post foundations and/or rigid primary frames to resist horizontal forces—as was done when pole buildings first dotted the landscape.
Providing the ability to rely entirely on embedded post foundations or rigid primary frames to resist horizontal loads is something that Bohnhoff feels is fundamentally important to include in a prescriptive post-frame building code. Currently, the only way he sees this aspect of post-frame building design effectively finding its way into the IRC is by adding an appendix to the IRC that specifically provides designs based on embedded post foundations and rigid primary frames.
Regardless of how it is accomplished, inclusion into the IRC of designs using embedded post foundations and rigid primary frames has the potential of significantly expanding residential building designs. For example, openings between rigid primary frames enable incorporation of infill panels, doors and windows of virtually any type and size, and post and pier foundations enable construction of innovative stilt homes on steep terrains and in locations prone to flooding or high annual snowfalls.
Who should create a prescriptive post-frame building code?
The best approach, in Bohnhoff’s opinion is to assemble a standard development committee (SDC) within ASABE and have this committee draft a stand-alone prescriptive post-frame building code as an ASABE standard. One reason for ASABE involvement is that all current standards specific to post-frame building design are ASABE standards. More important, however, is that ASABE is an ANSI-approved standard development organization (SDO)—a critical factor when seeking approval of the prescriptive code as an American National Standard, and when seeking inclusion of all or a portion of its provisions in existing residential codes.
An excellent alternative to ASABE is the American Wood Council, which is also an ANSI SDO, and has considerable experience interacting with the International Code Council on changes to the IBC and IRC.
Regardless of what organization oversees code development, it is an essential ANSI requirement that a standard development committee be comprised of a balance of participants from diverse interests. With respect to a prescriptive post-frame building code, this means assembling a committee with a blend of engineers, builders, suppliers, building code officials, researchers, insurance company representatives, etc.
If past work on a prescriptive post-frame code has demonstrated anything, it is the importance of insuring that experienced post-frame building engineers are involved as code development committee members. Experienced post-frame building engineers are familiar with and understand the tremendous design options associated with the framing system, and the advantages and disadvantages of these design options. More than anybody, they understand when reliance on a prescriptive code becomes either inefficient or potentially dangerous, and thus are in the best position to establish limits at which performance-based codes should dictate design.
Finally, Bohnhoff stresses that assembling an initial draft of a code and moving it through the process is a huge undertaking that will only occur with major financial support. His estimate—the price of one very large post-frame building. RB