Why Do Some Agricultural Buildings Fail To Withstand Inclement Weather?
November 11, 2016
Everyone wants their agricultural building to withstand inclement weather conditions. But many do fail, and it’s for a simple, but entirely avoidable, reason: They’re not professionally engineered.
To fully understand how inclement weather conditions can affect a post-frame building, we reached out to an authority on the subject: Dr. David Bohnhoff.
Dr. Bohnhoff is a Professor of Biological Systems Engineering (formerly Agricultural Engineering) at the University of Wisconsin-Madison. He specializes in post-frame building design and analysis, and has published extensively on the subject.
In our interview, Dr. Bohnhoff touched on a number of items engineers consider when designing a building to stand up to Mother Nature.
The Key to Avoiding a Building Failure: Quality Engineering
Over the past couple of years, heavy snows have brought down large portions of several agricultural buildings in the Upper Midwest. Surprisingly, it appears that no other buildings in the Upper Midwest failed due to the same snowfalls. No residential and no commercial buildings.
“The only buildings that continuously fail at a high rate are agricultural structures,” Bohnhoff said. “And that’s because the buildings are seldom engineered.”
Failures like the ones in Minnesota, Iowa and Wisconsin ultimately occur because no one has properly sized all the members and designed all the connections in the building.
“Non-engineered buildings fail because they are so unbalanced,” Dr. Bohnhoff said. “It’s not like all elements in a non-engineered building are under-designed. In fact, many building elements in a non-engineered building are much stronger than needed. The problem is that some of the other elements in the same building are not nearly as strong as they need to be and they are the ones that trigger a building failure.”
Not only must every element of the building be well-engineered, but it must be designed based on a variety of inclement weather factors. These include:
1. Weather Conditions: The Best Designs Factor In the Worst Conditions
Good structural design is all about designing a building to withstand Mother Nature, no matter its function. “In the Upper Midwest, agricultural building design is largely controlled by snow and wind loads. Live loads – those due to use and occupancy of the structure – typically only come into play in the design of bulk storage buildings,” Dr. Bohnhoff said. “To a large extent, an agricultural building can be designed without knowing its end use.”
Consider the following weather conditions:
Humidity: Will a mechanical connection be subjected to repeated wetting and drying associated with condensation? Will this result in an accelerated rate of corrosion of the fasteners comprising the connection, or in a loosening of wood surrounding the fasteners?
Snow loads: What amount of snow can be expected to fall at the building’s location, and how will surrounding buildings affect drift patterns on the building’s roof? How exposed is the building to high winds? Are you on hill, or in a valley? Are you among conifers?
Exterior temperature: What is the typical frost depth at the building’s location, and what impact will that have on design of the building’s foundation?
Indoor building temperature: To what extent will the indoor temperature enhance problems associated with condensation or alter loads by melting roof snow and increasing snow slides?
Wind: To what extent can wind pressurize a building and apply uplift forces on posts. How extreme are the wind forces that act on roof edges? On large sliding doors?
2. Building Materials: Using Materials of Known Strength and Durability
“To withstand forces imposed by applied loads, an engineer only utilizes materials of known strength. The bottom line is you can’t put something in a building if you don’t know how strong it is,” Dr. Bohnhoff said.
For example, to resist applied loads, you need to know the lumber’s bending, shear and axial strengths. Or, if you’re worried about hail, you would choose thicker roof steel and/or perhaps a steel with a higher yield strength.
Materials can also fail when the building’s environment is poorly maintained. “Excessive moisture along with nitrous oxide and sulfur dioxide emanating from manure can result in the formation of acids that corrode metals. Truss plates near the open ridge of animal confinement buildings have been quite vulnerable to such corrosion. For this reason, many owners and builders opt to replace metal plate connected wood trusses with laminated veneer lumber (LVL) rafters,” says Dr. Bohnhoff.
3. Failure to Brace During Construction: Disaster in the Making
One of the most critical parts of withstanding inclement weather is to brace properly during construction. It’s also one of the most overlooked.
“Most building failures occur during construction and not after construction,” Dr. Bohnhoff said. Because so much of the overall structure relies on elements to be held in position during the building process, not bracing properly for inclement weather is a recipe for disaster.
In many cases, bracing not only helps maintain the proper position of building elements, but it’s critical to helping resist wind forces until sheathing gets attached.
Dr. Bohnhoff has seen structures come down in winds of low magnitude, primarily because of excessive deflection resulting from a lack or improper use of bracing.
“The deflection of a simply-supported beam that’s uniformly loaded will increase by a factor of 16 every time you double its span. Think about this when you look down on a truss supported on each end by a post. That truss acts as a very narrow simply-supported beam when wind hits the side of it. If the truss clearspans 80 feet and is not laterally braced, it will deflect at least 256 times more than a truss with the same size chords that clearspans only 20 feet,” he said.
You have to keep temporary bracing in place until all critical building elements are completed. Bohnhoff recalled an incident in which people were nearly killed because the builder removed important bracing before completing an endwall that was functioning as a critical shearwall.
In this case, the function of the shearwall could have been temporarily served with some diagonal bracing running from the base of a couple posts to truss chords.
Make Insurance Premiums Higher for Non-Engineered Buildings
So why do ag buildings fail at a higher rate? It’s simple: “Seldom is there an upfront penalty for constructing a building that has a high probability of failure. More specifically, from a premium perspective, most insurance companies do not differentiate between an agricultural building that is properly engineered, and one that is not engineered. This encourages owners to put up the cheapest building they can, even if the probability it will fail is 10 or 100 times greater than a structure that costs only a few dollars more,” Dr. Bohnhoff said.
If your building is destroyed by inclement weather, the insurance company simply cuts you a check to build a new one. There’s no risk on your end.
Bohnhoff advocates making premiums higher for non-engineered buildings. He’s not suggesting there should be more stringent codes, or government oversight. Instead, tie insurance premiums to the level of engineering, which not only will prevent building failures, but it will also ensure that the building isn’t “over-designed or under-designed.”
“An engineer on a larger building almost always saves you money,” Bohnhoff said. And in the case of inclement weather, engineering can also save your building.