The success of a fabric building project goes beyond the fabric to the steel frame supporting it.
By Dwayne Moench and Nathan Stobbe
Constructing a building with fabric membrane cladding has long been an intriguing, cost-effective option for the engineering community. For decades, fabric has proven its value proposition, delivering a variety of benefits that metal-roofed buildings are unable to provide.
The translucency of fabric allows natural light to permeate the roof material and reduce the need for artificial lighting inside. Fabric is also inherently resistant to corrosion, making it an obvious solution for storage of corrosive materials or high-humidity environments. For good measure, modern fabric installation methods allow for fabric to be applied far more quickly than the laborious process of screwing in steel roof panels.
For building users, consultants, and engineers who have already determined that fabric is the clear cladding choice for their particular application, it’s a little ironic that the most important factor to consider actually goes back to another look at metal—this time specifically the style of metal frame used to build the fabric structure.
Re-Framing the Debate
While traditional fabric building framing approaches—most notably the use of steel web trusses—are still prevalent in the industry today, a new innovation was developed several years ago by Legacy Building Solutions when the company introduced rigid frame, I-beam design to tension fabric buildings.
This marriage of a rigid frame (the hallmark of a metal building) with a fabric membrane (and its unique attributes) quickly transformed the outlook of what users could get from a fabric building; it was the first step toward making customized designs the norm, rather than the exception, in an industry that had been previously dominated by pre-engineered, standard-size offerings. In many ways it created a new market segment of its own.
Of course, the beauty of rigid frame design is that it really is not a new concept at all. Rather, it’s a long-established construction type that is universally accepted in the building industry. Contractors and engineers alike understand exactly how the rigid frame structure works, the software that designs it, and how it will function in a real-world environment. In other words, it provides a level of certainty that was often lacking with web truss structures.
Customization can be a scary word for cost-conscious consumers in any market, as that word is often perceived synonymously with “more expensive.” Perception was reality with old-school fabric structures, where any necessary modifications to pre-engineered designs—such as thicker cords or adjusted jigs—could be done, but usually only at a significant cost.
Rigid frame design turns that traditional fabric building process on its head, always beginning with a clean sheet. Using finite element analysis (FEA) software, engineers work with the client to input precise building dimensions while accounting for risk category codes and site-specific requirements related to wind, snow or seismic loads. The result is an efficiently rendered design that is optimized not only for the building’s intended use and location, but truly for each and every detail of the customer’s specific project.
Creating a customized rigid frame with FEA gives fabric building suppliers much more structural flexibility to add lean-tos, mezzanines, sidewall doors, and other features to a building design. Where rigid frame really separates itself, however, is with its ability to quickly and accurately provide framing models to handle collateral loads and hanging loads for items like fire suppression systems, cranes, or conveyors.
By calculating stresses, pressures, and deflections based on the anticipated loads, the software generates an optimal design with the appropriate I-beam depths and thicknesses. Different frames within the same building are varied depending on their location and expected loads. And because this is all part of the basic engineering, there is no added cost to customize, and material costs are limited to only what is needed in the design.
Rigid frame engineering leads to an end result that is simpler and more durable as well. Common fabric framing alternatives like steel trusses or aluminum I-beams/box beams require a more complex system of web connections to hold the frame together. These frames are also thinner, making them more vulnerable to corrosion than thicker I-beams, particularly in the case of hollow-tube frames that can rust from the inside out. Additionally, while aluminum beams are limited in their span reach, a rigid frame allows for much longer clear span designs since the solid steel beams can withstand greater compressive and tension forces.
These factors all add up to a stable frame that doesn’t move or sway. And it’s not just building manufacturers and end users who recognize these advantages. Many sub-suppliers, such as those providing conveyors or cranes, will often avoid installing their products in truss buildings because flexing of the frame can lead to premature component wear and higher maintenance costs on their equipment. With a rigid frame structure, these companies can stand behind their offerings with an understanding that any wear-and-tear will only occur through normal use.
In situations where a building user wants to relocate a structure, rigid frame structures are also built to be moved and reassembled more easily. Furthermore, the design is amenable to making environmental load modifications should different codes apply at the new location.
Another fabric building industry trend is the transition some have made from being a fabric structure supplier to becoming a true fabric building manufacturer. Companies like Legacy have invested in their own fabric production and steel beam fabrication facilities, allowing them total control over their supply chains and delivery timelines instead of relying on third-party fabricators.
For all the initial planning that goes into a building design, modifications from the customer as the project progresses are a way of life. Having an on-site factory allows far more flexibility to make minor adjustments without major disruptions. For example, if a building user needs electrical wiring to pass through the solid rafter web, holes can be cut right away in the shop, thereby saving time later in the field.
Quality control is another clear benefit. Having the team that designs the building under the same roof as the team that produces it helps ensure proper manufacturing and consistency from start to finish. Some even employ full-time AWS/CWB weld inspectors to verify that every beam leaving the manufacturing facility meets all necessary quality and safety criteria. Those with their own in-house, professional installation crews to erect the building can take this a step further and claim full responsibility and control from concept to completion.
Manufacturers working with rigid I-beam design have commonly used hot-dip galvanizing to protect steel frames against corrosive elements. But at the behest of clients, some are preparing to make epoxy paint a more readily-available option through their typical production lines.
Whereas galvanizing essentially slows down the corrosion process by sacrificing itself over time, epoxy is a coating that creates an actual barrier between corrosive elements and the steel. Another advantage that makes epoxy paint a superior choice is that if an area of a steel beam were to be damaged, the coating is repairable just by cleaning the affected area and re-applying more epoxy.
Epoxy is already prevalent in highly corrosive applications like potash mining, where organizations working in that field typically require epoxy for all equipment and steel structures. Although epoxy paint can sometimes be more expensive, certain industries have determined it is worth the added investment to extend the lives of their equipment and buildings.
While historical fabric structure framing was often characterized by having to make engineering compromises of one kind of another, rigid frame design is universally regarded as providing long-term, time-tested engineering principles. Everyone involved in the construction process—from the client, to engineers and consultants, to the building manufacturer—is able to proceed with more confidence in the long-term viability, durability, and performance of a rigid frame building.
Dwayne Moench is a senior structural engineer and Nathan Stobbe is general manager for Legacy Building Solutions. They can be reached at firstname.lastname@example.org and email@example.com, respectively.