Before You Buy: Structural Considerations for Your Custom Fabric Structure

When planning the design of your fabric building, every design choice can affect material requirements for strength and durability. That’s why knowing your structural options is key to the success of your design and the lifespan of your building.

To construct a long-lasting fabric structure, all materials included in the structure must be carefully chosen for its engineering quality, and every building must conform to industry standards. You might think that such practices are commonplace, but unfortunately this is not always the case.

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Until Legacy introduced its innovative rigid steel design a few years ago, the use of steel open web trusses was the industry standard for fabric building designs. But open web trusses, despite being considered sufficiently strong while keeping structure costs low, have a number of inherent problems.

The Problem with Open Web Trusses

There is some controversy in the fabric building industry regarding the structural integrity of web truss designs, but the fact is, these designs have a track record of introducing flaws that can weaken the trusses and possibly cause collapse. One such structural failure is known as “cord plastification,” where a thin steel web within the truss “punches” through the wall of the supporting web frame, eventually leading to instability or structural failure.

To prevent such a puncture, open web trusses can be augmented with steel reinforcement during manufacturing. An open web truss design, however, is usually chosen on the basis of lower cost. So by the time added reinforcement or repairs are included, the overall cost of the building can rise well above the cost of a choosing a rigid steel frame at the outset.

Solid Steel Beams or Hollow Web Trusses?

The most important engineering difference between web trusses and steel frame designs might be the most obvious difference of all.

When open web trusses are used, as many other manufacturers do, the frame pieces of the trusses are completely hollow and connected to each other only by welded steel cords. Legacy uses only solid steel I-beams in the construction of our frame designs. We think this fact alone makes a significant difference in the long-term structural integrity of a fabric covered building.

More than Strength – Other Advantages of Structural Frames

In addition to superior structural strength, design flexibility is another major benefit of rigid frame designs. When designing steel frame buildings to custom shapes and sizes, adapting steel beams to the custom shape is accomplished much more quickly, easily and inexpensively than with trusses. And again, rigid steel beam construction contributes significant strength and longevity advantages.

It’s true that web trusses can be built into custom dimensions and building shapes, but these customizations tend to be much more expensive than with rigid frames designs because of engineering limitations intrinsic to the trusses themselves.

Finally, steel frame buildings from Legacy are designed to maximize the usable space inside of the building. By necessity of their engineering, web truss buildings must conform to an arched shape, which inevitably creates space along the walls of the building that cannot be used. The walls of our steel frame fabric buildings are perfectly straight, allowing for full use of every square foot of interior building space.

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To further reinforce our steel frame designs, Legacy includes secondary framing practices that further add to the strength and lifespan of our fabric buildings.

Purlins

Purlins are added to support abnormal roofing loads. This secondary framing feature in roofing structures adds cross-supporting, perpendicular beams between the larger rafter pieces of the roof frame. All of our purlins can handle compression, a claim very few of our competitors can make.

Flange Braces

Imagine looking end-on at the shape of a steel I-beam. The flanges of the beam are the top and bottom portions that extend the sides of the beam. To reinforce the beams, we add flange braces, which span the diagonal distance between beam flanges in the corners where the beams meet.

Cross Cables

Cross cables are yet another way to add strength to the steel frame of Legacy fabric buildings. We provide cross cables that are factory made and have been load tested. This further increases frame tension and strength.

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Fabric pressure and tension play a critical role in the weatherproofing capabilities and durability of fabric covered structures, which is why Legacy takes extreme care in how we tension fabrics in our designs as well as during construction so as to ensure maximum lifespan of the fabric.

Individual Bay Panels

Legacy manufactures its buildings with individual fabric panels that are each attached to their own keder rail. Our special design allows us to individually adjust each panel to its optimal tension, thus more evenly distributing the fabric pressure across the surface area of the building and greatly increasing the lifespan of the fabric. Typically, each bay panel spans 20 feet.

In addition, we further reinforce the building fabric panels with an extra layer of padding anywhere it makes contact with the building’s secondary framing, such as at the peak of the building.

Minimizing Fabric Pressure on the End Frame

Other companies of fabric-covered buildings tend to cover as much of the building as possible with a single, large sheet of fabric. This process saves money and makes construction much simpler for the building company. The problem with this overly simple design is that a larger piece of fabric must bear the majority of its tension pressure at the end frame where it attaches to the building. This design is susceptible to excessive wear or tear of the fabric near this end frame. Again, this problem is mitigated by individual bay panels, which is the only way Legacy constructs its fabric buildings.

Keder Rail Attachment: Screws Vs Bolts

The entire roof membrane of most fabric buildings is secured to the trusses with TEK screws. Experience has shown these connection sites to clearly be weak points on most structures, especially over time as these screws corrode. This is why Legacy only attaches its keder rail system using ½-inch diameter bolts—an industry first. This attachment system provides optimal design strength and longevity.

Fire Rated Fabrics

For building designs that require fire-rated fabrics, we offer a variety of options.

Endwall design in fabric building engineering

How a building company designs the endwall section of a fabric structure has a critical impact on the longevity, functionality and appearance of the building.

Fabric Attachment

All Legacy fabric buildings are constructed so that the endwall fabric panels are kedered to create a perfect, snug fit across the face of the endwall. This extra detail accomplishes two things: it makes the building face weather-tight, and it substantially increases the life of the fabric. Endwall panels are tensioned both vertically and horizontally.

To further secure the fabric panels, we affix these with girts to each horizontal and vertical framing crosspiece in which the panels make contact. Fabric panels in the endwall are double-reinforced in all areas where they touch supporting steel members of the frame.

Reinforced Structural HSS Columns

Legacy offers a column design with specially reinforced hollow structural sections (HSS). This improves the strength of endwalls where necessary spaces need to be added to install ventilation or air circulation equipment. The extra support that we add to HSS columns is often overlooked in the designs of other building companies. Our HSS endwall columns provide impressive strength at 46 KSI while minimizing the need for lateral bracing when compared to open section members.

Materials used in fabric buildings

Steel Strength

Our durable steel frames feature 55 KSI steel welded by our own highly skilled and certified welders. The steel itself is typically treated in a hot-dip galvanizing bath unless a red-primed, gray-primed or powder-coated treatment is requested.

Fabric Strengths

Depending on a customer’s requirements, we offer a 15 oz/yd2 polyethylene (PE) fabric as well as polyvinyl chloride (PVC) coated fabrics of all weights and styles. The quality of PE and PVC fabric has greatly improved in recent years, and these fabrics provide a great overall value for a wide array of applications. In addition to the durability provided by the heavy-duty scrim, we further enhance longevity by applying our fabric so it cannot come in contact with the frames in any way. By keeping the fabric away from the steel, there’s no chance of the material being adversely affected by a frame surface that might not be completely smooth.

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