Over the last decade, Legacy Building Solutions has advanced fabric buildings for a wide spectrum of applications. The Minnesota-based company is currently involved in projects ranging from horse racetrack facilities in Florida to mining structures in Chile.

“At any given time, we’re working on projects in five to seven different key industries across the world,” says Nathan Stobbe, general manager at Legacy. “We’re continually looking at new variations of exactly what a client is looking to accomplish with their facility.”

Legacy’s fabric building technology combines a rigid steel I-beam frame with ExxoTec PVC-based fabric panels that are individually bolted to the frame using a patented fabric attachment system. The resulting biaxial tension provides a wrinkle-free finish to the fabric panels that in turn demonstrates a grab tensile strength of 745 pounds per inch. The steel frame itself has an extended lifespan, and Legacy’s fabric buildings boast high structural integrity.

Designing Clear Span Buildings

Legacy’s process begins with the estimation phase where a building’s engineering requirements are evaluated. The engineering team finalizes a custom design, which then goes through the drafting stage. The building’s structure is mapped out and reviewed by the client, who proceeds to work on preliminary foundations. Legacy completes the drawings in order to secure the necessary permits, and fabrication of the building commences.

Legacy specializes in fabric buildings that showcase open clear span dimensions, some of which are large enough to function as airplane hangars. The foundations and or supporting walls are often designed by local engineers however Legacy is involved with foundation design for its structures on occasion as well. 

“Clear span buildings have a tendency to produce a large horizontal thrust at the wall, pushing out—and so your foundation is going to have to resist those horizontal forces,” describes Dwayne Moench, head of engineering at Legacy Building Solutions.

“Most of our clientele want no obstructions inside the building so that they can maximize their floor space. A lot of times when I get involved in the foundation designs, I will have to take into consideration the large uplifts that are produced from a clear span building because the buildings are so light. It’s usually not the downward pressure that’s the problem with the foundations; it’s the uplift or the horizontal thrust pushing outward. You have to design the footings big enough so that they will resist sliding outward, and large enough so they don’t get pulled out of the ground by the wind uplifts that are there, as well,” Moench says.

“Usually if interior columns are needed or allowed, we’ll do the extra portion of the building as a lean-to,” adds Stobbe. “That can shrink the clear span space a little bit, and optimize the design for the client.”

Legacy’s open structures support a variety of dynamic loads including hangar doors, fire suppression systems, cranes, conveyors, catwalks, mezzanines, HVAC systems and lighting equipment. To further reinforce the robustness of their fabric buildings, Legacy incorporates secondary framing features such as flange braces, purlins, cross cables and bracing rods.

Solid Steel Beams versus Open Web Trusses

In order to achieve the open span dimensions of its fabric buildings, Legacy utilizes large structural steel members to allow for large amounts of interior space. In contrast to open web trusses—which are often used by tension fabric structure manufacturers—Legacy’s rigid steel tapered I-beam frames serve to enhance the strength, design and long-term durability of each fabric building.

According to Moench, one of Legacy’s greatest reasons for using rigid frame structures is that they are easy to manipulate and optimize, and much more straightforward to construct.

“When we originally started out, we were looking at the truss design as well—and there’s just a host of problems with the truss system that we don’t have to worry about with the solid plate system,” Moench explains. “You’ve got all the additional fabrication that’s required when you have a truss system—all the different welds, parts and pieces that have to be assembled to make the truss. With the system of fabrication that we have here, we have machinery to weld plate-to-plate in a web-to-flange rigid frame easily, and the depths of the web can be varied to allow for changing frame loads.”

“The beauty of the rigid frame is that it’s only going to give you as much frame depth as you need to reach that near-100-percent capacity of the frame for any particular site. You can have tapered structures where you’ve got a real deep haunch at the eave and then it shrinks down to a smaller overall depth where the stresses are a lot less. You can’t really do that with the truss because it’s so complicated. With the truss, your depth, column, and web thicknesses are typically all at the same locations. It’s a sliding scale that’s hard to meet compared to a rigid frame structure where you’re just varying the depths of the column or the beam, and then the width or thickness of the flanges can change the capacity of a frame section significantly as is needed,” he adds.

Using rigid steel frames also enhances design flexibility. Legacy uses Metal Building Software (MBS)—one of the most universally recognized design software programs in the pre-engineered metal building industry—for designing the major portion of its fabric buildings.

“MBS designs things mostly two-dimensionally,” says Moench. “The lateral parts of the loads that are applied on the building are designed into the frame system. The software also takes care of longitudinal loads, where we have a bracing system with either purlins, cables or tubes to transfer loads longitudinally through the building, into the sidewalls and down into the foundation.”

The finite element analysis program is often used in conjunction with RISA, a 3D structural engineering design program that Legacy utilizes to design frames, beams and secondary members that may not be handled within MBS.

Legacy uses high-strength 55 ksi steel for its Rigid Frame members, which are manufactured on site using an ISO 9001:2015 certified process. Based on project requirements, plate thicknesses range from 10-gauge steel to one-inch or thicker plates.

“As far as point loads are concerned, we put that in the software,” says Moench. “Whether it’s cranes, conveying equipment, sprinkler systems—anything that might hang from our structure—we can put that all in MBS when designing the structure. The software optimizes around the point loads.”

“One of the biggest advantages is that we can taper the beams and/or columns, which allows us to adjust the frame to the design requirements needed,” says Stobbe. “So, we can easily add point loads, and the frame will automatically—as it goes through the software—enhance so that we get the best usage of steel possible in that design. It enables us to easily customize the structure while minimizing the cost impact.”

One key factor that distinguishes rigid steel beams from open web trusses is chord plastification, where thin steel webs within the truss could punch through the wall of the supporting chord of the truss, eventually leading to instability or structural failure.

“In the past, we’ve seen some truss structures where there’s a lot of distress at the web/chord attachments,” says Moench. “Especially in a corrosive environment, you can see where those joints are really taking a beating—and the web will eventually penetrate that chord. Chord plastification has to be accounted for in the design of the truss, and it’s a very difficult thing to get to work on paper. One of the biggest problems I see with web trusses is that there are many challenges in trying to protect those joints over time.”

“The other thing about a truss is, typically you’re going to have panel points, where the web attaches to the chord,” Moench continues. “You cannot vary those as much as you can with a rigid frame structure that’s tapered. You’re kind of locked in to where those locations can be on a truss. If you start manipulating them, you’re going to create a lot of problems for the process when people are fabricating these things. There’s so much more flexibility with rigid steel frames.”

Clients often request modifications to existing fabric structures, resulting in added loads that were not originally included in the design.

“It’s just a lot easier to add on steel or weld something onto an existing rigid frame structure, than it is to try and go in and manipulate all the different things for a webbed truss structure,” Moench points out. “You’ve got all the webs and chords that could be overstressed. You’ve got all these members you have to add, just to make sure you’ve accounted for any new loads that are being placed in this structure. So again, the flexibility of retrofitting is also a benefit of the rigid frame.”

“It’s about all the different joints that are being produced. Every web has a joint at each end, so there are all these potential possibilities for chord plastification on every one of those webs—which are eliminated completely on a rigid frame structure because you have a continuous web welded to continuous flanges.” The I-beam uses continuous plates, resulting in the absence of pressure points which otherwise occur in trusses. “You’re not pushing against a thin wall member,” explains Moench. “It’s a thick plate pushing against another thick plate, and the bearing is essentially near uniform.”

Stobbe adds that another variability arises in the manufacturing processes.

“With the truss frames, every time you create a unique truss or frame, you have to put all new jigging in place,” explains Stobbe. “There’s a lot of setup involved. Whereas with the rigid frame design, the manufacturing tooling allows us to build different-shaped or configured frames with really no setup time whatsoever. And so, it’s enhanced not only from a design standpoint, but also from a manufacturing standpoint, to optimize that structure for our client.”

One important aspect to consider is the issue of corrosion. Legacy applications include bulk fertilizer storage facilities or other facilities that store corrosive materials, where walls have the potential to come into contact with corrosive materials. While fabric has the benefit of being corrosion-free, efforts still have to be put into protecting the building’s steel framework.

“At Legacy, we actually have several different options,” states Stobbe. “The first one is hot-dip galvanizing, which is readily available to most organizations. The challenge with hot-dip galvanizing is that it doesn’t prevent corrosion; it just slows the process down. And so, in 2019, we made a significant investment and added a state-of-the-art epoxy paint line into our facility. This allows us to apply multiple coatings of epoxy paint onto the framework to resist the impact of corrosion.”

“One benefit of epoxy paint is that it’s a barrier coating, so it creates a barrier between the corrosive elements and the bare steel,” Stobbe adds. “The second benefit is that if there’s damage to that epoxy paint, it is easily cleaned up and repaired—whereas with hot-dip galvanizing, the structure is generally rusting continuously. Doing repairs is not really practical because you’re recoating the entire structure. And so, from a long-term standpoint, epoxy paint allows the client to perform maintenance on that coating system and extend the life of the building.”

When it comes to open web trusses, their hollow frame pieces are vulnerable to internal corrosion in caustic environments.

“You would never know that corrosion is occurring inside the truss frames, or to what extent it’s corroded,” says Stobbe. “One of the huge benefits of the rigid I-beam is that everything is exposed. If you need to do a repair, or if you need to do a touch-up on a coating, it’s easy because the product is exposed. With trusses, what you can see is the outside—you can’t actually see the inside of that tubular product, and you can’t see the impact of corrosion. And so, especially in corrosive applications, that’s one of the added risks associated with trusses as compared to a rigid frame design.”

Legacy structures must also withstand hostile climate environments that include extreme wind pressure, snow loads, seismic activity and UV exposure.

“Truss frames tend to have more movement in weather events,” says Stobbe. “This isn’t necessarily a structural issue, but in many applications where you might have cranes, conveyors or different attachments to the building, movement of the frame is detrimental to the operation of that equipment because things get out of alignment, can cause equipment binding, etc.”

“And so, one of the biggest pieces of feedback that we’ve heard from our clients is that they’ll only utilize rigid frames for their design because the movement of those frames is significantly less than in a typical truss frame,” he adds.

The design of each Legacy structure accommodates environmental conditions that are unique to the region, resulting in a customized fabric building for every location.

“Unless a customer has purchased two identical buildings for one site, we’ve never made the same building twice,” Stobbe states. “Every building is designed uniquely to facilitate the loads on that site.”

“If you take a typical, 100-foot-wide building in Florida, or a 100-foot-wide building in a really high snow load area, they’re both going to look different,” says Moench. “Florida has its hurricane winds that are going to produce different stresses on a structure, versus a frame that has heavy snow pushing down on it.  Additionally, there may be a building in southern California where there are large seismic forces producing other types of forces on a structure.”

The fact that Legacy takes all these elements into consideration is testament to the integrity of its fabric structures.

To learn more about Legacy’s fabric buildings, visit their website.