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Is your barn engineered to carry the load?

Progressive Dairy Editor Dave Natzke Published on 27 November 2019
Collapsed dairy barn

Dave Bohnhoff’s frustration still surfaces when he recalls the dates of April 14-15, 2018. On those days, a heavy spring snowfall resulted in the collapse of numerous dairy barns in Wisconsin.

Bohnhoff, a registered professional engineer and an emeritus professor in the University of Wisconsin – Madison Department of Biological Systems Engineering, addressed farm lenders and advisers at the Farm Management Update for Ag Professionals held last September in Kimberly, Wisconsin. The meeting was held not far from where many of the barn failures occurred.

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A post-frame building researcher for more than 30 years, Bohnhoff authored the National Frame Building Association (NFBA) Post-frame Building Design Manual and some major post-frame building standards. With his expertise, he’s often called in to investigate post-frame building failures.

When Bohnhoff investigated the April 2018 barn failures, there were lots of questions: Why did agricultural buildings collapse and not other large commercial buildings? Was it reasonable to expect these barns to handle an event like the April 14-15 snow? Were the barns designed but not built to handle the snow loads – and, therefore, was it a construction issue and not an engineering issue? Did farmers think their barns were built to handle this snow load event?

The answer to ‘why?’

What Bohnhoff found during his on-site investigations was: Almost without exception, the buildings that failed were not structurally engineered. Prior to construction, no one estimated the variety of loads to which the building would be subjected; no one determined what magnitude of forces the applied building loads would induce in various framing members, mechanical connections and cladding of the building; and no one sized any of the framing members, the mechanical connections and the cladding to meet expected building loads.

The lack of structural engineering of agricultural buildings is not a new problem. In fact, agricultural buildings – unlike other commercial buildings – have been failing en masse for decades due to a lack of structural engineering. The simple fact this does not need to be the norm is what irritates Bohnhoff.

“Many farmers assume when they are given an elaborate set of construction drawings that list the size and grade of framing members as well as the size, type and location of all fasteners, their building has been structurally engineered. In so many cases, this is far from the truth,” laments Bohnhoff. “Anyone can draw up a building and put labels on the various elements. This in no way means loads (to which the building may be subjected) have been calculated and the building elements have been sized to withstand the calculated loads.”

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Why agricultural buildings and not others?

Wisconsin, like many other states, exempts agricultural buildings from the state’s commercial building code. In other words, state law does not require buildings used for farming purposes to be structurally engineered.

The special code exemption for farm buildings is historic in nature. Building codes are primarily fire codes. They were initially developed to limit the large fires that in some cases destroyed entire towns and killed hundreds of people. Farm buildings were exempted from these codes because they were relatively small, low-cost, presented a low risk to humans when they did catch fire and were isolated in rural areas.

The farm building exemption was also instituted at a time when the largest agricultural buildings were a fraction of the size they are today. In Bohnhoff’s opinion, the current exemptions granted to agricultural facilities are being abused.

“The exemption is being used to skirt structural engineering,” he said, “which was never its true intent. Frankly, it’s sheer lunacy, if not criminal in today’s society, to construct any building several hundred feet in length if it has not been fully engineered.”

Structural engineer emphasized

“Engineering does cost money, but in many cases it pays for itself in reduced building costs,” Bohnhoff said. “A fully engineered structure saves money in construction because of the efficiency of the design, and it’s much safer. It’s balanced for the loads for which it is subjected and relies on a systems approach to resist loads. A fully engineered structure will have a low probability of failure if all the performance criteria are met.”

Buildings that are not structurally engineered may have a mix of oversized and undersized components. Oversized components unnecessarily drive up construction costs, and undersized components increase the probability of a failure.

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Structural engineering is especially critical for large buildings. “When you add in the fact a large building has a higher number of components (and many more weak components if not properly engineered), and larger buildings are bigger targets for extreme wind and snow loads, put together you can appreciate the probability of failure increases as the size of the building increases,” Bohnhoff said. “Always keep in mind, a building is only as strong as your weakest component.”

One of the frequent failures Bohnhoff sees in agricultural buildings that are not structurally engineered is a “progressive roof collapse.” In that situation, the failure of a single component triggers a chain reaction of failures, resulting in a large-section – or total – roof failure. Some progressive collapses occur rapidly, while others can occur over a period of time due to wood creep. That latter type of collapse is especially dangerous to people attempting to remove animals or work inside a structure that has exhibited a roof failure and is still heavily loaded with snow/ice. 

Education over regulation

Calling himself “just enough of a libertarian,” Bohnhoff stops short of calling for increased regulatory oversight. There hasn’t been a push at any legislative level, and large regional builders don’t embrace increased regulation because they believe their on-staff engineers give them a competitive advantage as part of their package. Most large regional and national farm construction companies have structural engineers on staff because they can’t afford failures.

Bohnhoff also said he isn’t out to eliminate hardworking entrepreneurs in the business of constructing agricultural facilities. He is insistent, however, that persons trained and licensed in structural engineering be involved in the design and construction of those buildings.

He urges bankers who lend money for ag buildings, and insurance agencies who insure them, to become involved. Bankers should insist on seeing a complete set of plans and consult with a structural engineer to ensure they are designed properly. Most insurance providers do not differentiate coverage rates between an engineered and non-engineered structure, spreading the costs across all structures.

A building is only as strong as your weakest component.

Look for calculations

Bohnhoff said he believes producer awareness can reduce the number of building failures. Farmers should understand the structural differences and realize structurally engineered buildings will be economical.

Plans for structurally engineered buildings will include a stamped/sealed set of calculations by a qualified licensed professional, containing the loads used in the structural analyses, the forces resulting in all structural elements due to the loads and checks that show each of the structural elements can handle the forces induced in them by the loads.

Larger post-frame building companies typically employ their own structural engineers. Builders who do not have their own structural engineers will contract with an independent consulting engineer or firm specializing in post-frame building design to obtain a stamped/sealed set of plans that contain the proper structural calculations.

Be sure to make sure the calculations do not pertain only to trusses, Bohnhoff warned. Truss designs performed by local lumberyards or truss manufacturing companies are seldom applicable for post-frame buildings used in agricultural settings, he said. That’s especially true for wider structures in which non-uniform loads and complex load combinations control design, and loads passed to the truss by posts and other framing members must be taken into account.

Story not over

With another winter approaching, buildings damaged during the April 2018 snowfall may have another story to tell.

“Unfortunately, many of the buildings suffering failure or damage were repaired using the same standards used in their original construction, so they have the same inherent weaknesses – some of which have not yet manifested themselves,” Bohnhoff said.  end mark

PHOTO 1: When Bohnhoff investigated the April 2018 barn failures in Wisconsin, he found that – almost without exception – the buildings that failed were not structurally engineered.

PHOTO 2: Buildings that are not structurally engineered may have a mix of oversized and undersized components. Oversized components unnecessarily drive up construction costs, and undersized components increase the probability of a failure. A building is only as strong as your weakest component. Photos courtesy David Bohnhoff/UW – Madison Biological Systems Engineering Department.

Dave Natzke
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