Advancements in metal building systems deliver predictability.
The benefits of a preengineered metal building have always related to its speed of construction, predictable and competitive cost, ease of expansion, and energy efficiency. More recently, the quality level gained from this materials option, particularly with the development of high-performance, standing-seam metal roof systems, has even increased their specification. This highly leak-resistant alternative to built-up and other roofing is being specified on a record amount of new construction and as a reroofing method for older buildings with deteriorated original roofs.
Why Use Metal?
A preengineered metal building must be viewed as the sum of its parts. From design through manufacture, building system parts are subjected to quality-controlled engineering and fabrication methods that permit the optimization of materials and accurate size, fit, and assembly of the component package. In most cases, a project's entire structural, roof, and wall requirements are available from a single source manufacturer represented by locally authorized builders or dealers who undergo extensive training in proper application and erection. Where statutes permit, this type of construction is ideal to bid as design/build programs using general performance and warranty criteria. Exceptional warranties are possible because of the predictable performance of a preengineered building. Their assemblies have been subjected to extensive testing and years of field observation. In fact, preengineered metal buildings are among the most thoroughly tested forms of construction in the marketplace.
For purposes of this article, a preengineered metal building system is deemed as a series of factory manufactured components that have been presized and prefitted to erect into a fully-integrated building unit. For most projects, custom manufactured and standard-size components can be combined to satisfy specific span, load, and other design criteria established for a project. The single source supplier for the entire package, coupled with the precise integration (essential to the transfer of loads) of the preengineered material solution (structurals, roof, and wall panels), strongly differentiate a preengineered building system from various free-standing metal components offered by unrelated manufacturers.
Those responsible for public works construction projects and code compliance should fully acquaint themselves with the features of preengineered buildings because they are applied to well over half of all nonresidential, low-rise building programs. The slight differences in engineering practices of manufactures and the scope of their warranties are especially important to understand when evaluating proposals. Although there are a number of industry standards, distinct variances exist among company practices resulting in differences in the quality level of in-place construction.
When comparing quotes for a metal building system, a wide spread in bids should trigger-close evaluation. Always require a "Design Certification Letter" signed by a licensed engineer on staff with the manufacturer. This document should be quite definitive and careful attention paid to explanations and any disclaimers. For example, study the application of live load to frames and secondaries. Also, determine whether the application of snow load is defined as ground or roof snow load. Another important reference is the allowed deflection of the frame, particularly as it relates to sidesway and the method used by the manufacturers' engineers to calculate that deflection (i.e., either bare frame calculations or composite effect of the building system). In projects where structurally-supported cranes are involved, compare the type of unit(s) and service levels influencing the framing requirements. Only by having this information specifically explained can an objective comparison be made about the structures' overall quality.
Structural framing options include the clear-span rigid frame, modular rigid frame (which incorporates interior columns), long bay (up to 50 ft), multi-story, and even a roof truss framing system used to integrate a standing-seam metal roof system with load-bearing hardwall construction. The latter eliminates the need for exterior columns by allowing the roof to act as a structural diaphragm in the transfer of loads directly into the hard-walls and foundations, as bar joists would if fitted with metal decking. There are also self-framing buildings to satisfy small building end uses.
There are numerous insulated and uninsulated metal wall panels and others with synthetic stucco, aggregate stone, or other simulated facings. On many projects, of course, conventional wall materials are integrated on otherwise preengineered structures.
Roof options include the traditional lap-seam metal roof, whose panels are affixed to underlying structurals using thru-fasteners, and more recently-developed, high-performance standing seam metal roof systems. The latter include both structural and architectural type standing seam systems.
The standing-seam metal roof system is superior because the fastening method employs concealed clips instead of thru-fasteners. The very best have an engineering provision in these clips that enables the seamed roof to accommodate thermal expansion and contraction, forces that over time can otherwise compromise a roof's weather tightness. A specifier should compare the method and integrity of the seams, which differ with each manufacturer's roof, and insist on a 20-year "weather tightness" versus just a material warranty. Another difference may exist in the warranty coverage for the roof panel coatings.
The characteristics shared by most standing seam metal roof systems are a minimum 1/4-in. per ft slope to ensure drainage, 24 in. wide metal panels, seams elevated well above the drainage plane, and the aforementioned panel-to-structural connections that eliminate thru-fasteners. Ideally, any vents, flashings, gutters, access hatch openings, and other roof trim should be part of the preengineered package rather than generic "field-fitted" products. Most of the better standing-seam metal roofs carry the U.L. Class 90 Wind Uplift Rating and some satisfy the even more rigid U.S. Army Corps of Engineers (CEGS 07416)/ASTM E 330. In areas where wind uplift forces are a consideration, many architects require both of these tests in performance specifications.
Preengineered metal buildings were applied to more than $7 billion worth of low-rise projects last year. Many of these were for public end uses. As government seeks more quality in its facility investments, indications are that preengineered mend buildings will be increasingly specified.
Mr. Miller is Vice President-Marketing, Buildings Division, Butler Manufacturing Company, Kansas City, Missouri.
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|Date:||May 1, 1995|
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