Design Load Reductions & Risk
Design Load Reductions & Risk
A building owner has a number of choices when it comes to structural design. In making those choices, he or she needs to decide upon a fundamental question: Should I build to code minimums or exceed them? Component manufacturers (CMs) are in a position to provide information and share experiences with customers that can help them answer this question, making it clear that a truss package designed to a code minimum may be less expensive initially but can end up costing a customer far more in the event of an unintended building performance deformation, or worse.
An example of where a customer might make construction choices without a sense of their potential consequences involves agricultural buildings. This is because agricultural buildings are very often viewed as not requiring life safety design considerations, which in turn often means they are not required to undergo building department plan reviews or site inspections.
Making use of load reductions that are allowed by the building code when designing trusses for agricultural buildings is legitimate, and may allow the owner to obtain the most cost-effective design in a very competitive market. However, since agricultural buildings are not subject to the double checks that most buildings go through, this may also mean that there is less room for error or unanticipated loading conditions.
The Risk of Load Reductions: A Costly Example
A real-world example of unintended downstream vulnerability and risk due to prior upstream decisions can be seen in the widespread collapses of agricultural buildings during a record-setting Wisconsin snowfall in April 2018. Between the evening of Friday, April 13 and late Sunday, April 15, areas of Wisconsin’s Fox Valley saw over 30 inches of snow in addition to rain, ice, and sustained winds. Unable to withstand the combination of weather extremes, the roofs of numerous agricultural buildings in the area could no longer sustain the applied loading conditions and deformed – some severely.
In 2006, Truss Systems, Inc. of Little Chute, Wisconsin designed and manufactured roof trusses for one of these recently collapsed buildings: a large barn housing milking cows and two milking parlors. Truss designer Terry Schneider remembered designing the roof trusses for the building. “It’s a free stall barn and it was about 113 feet wide and a thousand feet long. It [was] comprised of two mono trusses on either side of it and then in the center was a 61-foot clear span truss, 12 foot on center,” says Terry. In the original bid, the building owner had specified a design using one-ply trusses. Terry preferred a design that provided greater rigidity and left more room for any unknown downstream issues or loading conditions during the construction and use of the building. “A 61-foot clear span truss at 12 foot on center – I just don’t like doing that,” says Terry. “That truss is just being maxed out.”
Consequently, in bidding for the project, Terry decided to present the contractor and building owner with two designs: the originally requested design and a more robust truss design option. Terry bid it “the way they wanted it, but then I also presented it to them [that] these trusses should actually be two ply, but in this case it was an extra $33,000,” he says. “The contractor went back to the building owner and then presented it to them that way.”
Given its use as a free stall barn and milking parlor, the building owner decided to purchase the cheaper designs, asserting that the extra performance capacity wasn’t necessary for a building merely housing cows. Truss Systems acquiesced and was awarded the bid, providing the truss package the building owner had originally proposed.
Unfortunately, during the Fox Valley’s severe snowstorm on Saturday, April 13, a number of the building’s single-ply trusses did not perform well, and the results of the actual loads applied to the roof are shown in the adjacent photos. Terry says he received “a text that Sunday that they had some problems” and that they “wanted to talk to me Monday morning.” On Monday, Terry learned that “probably about 40 percent” of the roof collapsed, he says. “It wasn’t one continuous [area] where it was down, it was hit and miss throughout the whole building.”
The cleanup project began right away “so they could get their parlor operating,” says Terry. “The contractor went over there and they started pulling down some of the collapsed areas,” he says. “They had two parlors, and one of the parlors was down,” says Terry. “They couldn’t get access, so they shipped out like 1,500 cows to the slaughterhouse and then a lot of the other cows that were in that free stall barn got shipped out to other operations that they owned throughout the state,” he says. “They lost half of their production for a couple days.”
In response, the building owner requested an even more resilient design than Terry had proposed as an alternative 12 years ago. Terry recommended rebuilding the collapsed trusses as two-ply trusses, but the building owner did not want the roof to have an inconsistent appearance. In addition, says Terry, the building owner “didn’t trust the single-ply trusses that were left,” and proposed a design that added further reinforcement to the remaining, undamaged trusses. In a matter of a few days, Terry created a new design, and shortly thereafter Truss Systems’ production plant was busy manufacturing components for the repair.
Truss Systems first replaced trusses in the area where the single-ply trusses had been removed. “We rebuilt 43, 61-foot single-ply trusses just like there was,” says Terry. The final design also added additional columns underneath the clear span trusses and scab trusses to both the new and existing clear span trusses for extra reinforcement. “They actually went back in and added more columns, so now that 61-foot truss is actually a four point bearing truss, and maximum span on that’s now like 20 feet,” says Terry. After installing the new trusses with the additional scabs, “they reinforced the other existing trusses” Terry says, with scab trusses spanning about 20 feet “the entire length of the building.” In all, Truss Systems supplied approximately 126 new trusses, including the replacement clear span trusses and 83 scab trusses.
Now that the repair is complete, “they’re bringing the cows back from some of the other operations,” says Terry. Sadly, this situation cost the building owner far more in repairs and lost production time than the $33,000 extra for Terry’s initially proposed two-ply design. In terms of risk management and truss design best practices, this case study helps put into perspective the real value of taking the time up front to consider all the consequences that could occur in moving ahead with the least expensive truss solution. While the future is always hard to predict, 20/20 hindsight says that $33,000 would have been cheap insurance in this instance.
Though an unfortunate way for it to happen, this ordeal has led to greater respect for Terry on the part of the project’s general contractor. “He understood back then why I wanted to do the two-ply trusses, but now he’s more apt to push [my ideas] stronger to the owner,” says Terry. “He kind of takes my advice a little bit more to heart.”
The building owner also continues to do business with Truss Systems, who recently provided valley trusses for one of the other barns on the same property.
In the end, Terry’s decision to provide and make a case for a more robust design that he preferred reflected well on his company. “I don’t like to go down to code minimum just to save a few bucks,” he says, knowing that his reputation is on the line if one of his proposed designs does not perform as expected.
A more robust truss design that considers the downstream risks, ASCE 7 loads, and allowable building code adjustments may be worth the initial cost for its ability to mitigate the risk of unintended poor performance. Beyond this, it helps build confidence and trust in a CM’s expertise and the quality of their products.