Architects and insurers are a true odd couple coming together for a building project. Architects concentrate on design flexibility and aesthetic appeal, while Insurers focus on acceptable risk. Despite each concentrating on different aspects of the structure, their ultimate goals are the same. Design and build a building with excellent performance that lasts. In the low-rise sector of the construction market, metal buildings systems present both designers and insurers the best of two worlds.
Modern metal building systems are custom engineered structures that offer levels of flexibility, durability and reliability that cannot be equaled easily by other methods of construction. This is the result of years of research on materials and loads and specialized engineering processes used in creating a metal building today. The advances in metal building technology have been very significant in recent years.
Registered professional engineers staff member companies of the Metal Building Manufacturers Association (MBMA) and bring sound engineering principles to each project. These engineers don’t take their professional responsibility lightly since their employers routinely supply evidence of compliance with code specifications, design criteria and design loads.
Metal building systems, contrary to what many people still believe, are not matters of catalogue-selected parts. Rather, each metal building is custom designed to satisfy customer needs and building codes. Advanced computer methods are used to help design customization and optimization.
There are metal building “look-a-likes” that are not easily distinguished from metal building systems. These structures are often made up of components purchased from one or more vendors and are shipped to the site as a complete metal building. However, they are not designed with regard to the interdependence of the structural elements. Evidence of this difference is often found in investigations of metal building failures due to extreme loading. The look-a-likes simply don’t hold up nearly as well as metal building systems.
More and more architects are becoming aware of what metal building technology has to offer. A broad range of materials and finishes means an increased number of churches, schools, shopping centers and office parks are opting for the very cost-effective metal building approach.
Built to Last
Metal building systems have evolved into an efficient, functional, and versatile alternative to other low-rise construction techniques. While there are many variations of the engineered metal building, there are some basic structural elements in all metal building systems.
A modern metal building is made up of primary rigid frames, secondary members called wall girts and roof purlins, cladding and bracing. While it may seem simple, the engineering involved in optimizing all of these elements into a stable system is quite sophisticated and challenging. MBMA member companies have developed their expertise in metal building design and construction to extraordinary levels. One need only look at the range of building uses where metal building systems are being specified. The technology that makes such structures so attractive to more and more of the low-rise market is their use of structural optimization wherever possible to streamline material without affecting the integrity of the building.
Structural optimization is hardly new. Galileo first looked at optimization of a cantilever beam in the 17th century. In terms of modern metal building systems, structural optimization is a matter of using modern computer technology to remove material where not needed, thereby not affecting the building’s ability to carry the proper loads. A desirable byproduct is, of course, lower cost.
Structural optimization, does not, in any way, produce a weaker building. The result is a far more efficient structure that is as safe as is practical with today’s materials and code requirements.
One way to achieve optimization is through the use of welded plate members instead of hot-rolled sections that have traditionally been used in steel building construction. Welded plate members allow the use of tapered webs with increased depth in areas of higher moments.
Further optimization comes with varying the web thickness and flange sizes as well as using higher strength steel where needed. Sections are also frequently used with unequal flanges.
Cold-formed secondary members (purlins and girts) offer a higher strength to weight ratio and these can be roll-formed by the manufacturer to the required depth and thickness to carry specific loads.
Structural redundancy is another characteristic of metal building systems that should be examined by insurance companies as well as others in the low-rise market. The term structural redundancy refers to multiple load paths within a structure. Metal buildings are extremely redundant structures. All flexural members used in a metal building system are usually continuous members that permit redistribution of loads if an overload occurs. Primary frames, as well, are indeterminate structures that offer additional redundancy.
Still, there have been instances of building collapse or partial roof collapse under abnormally high snow loads. MBMA has always tried to learn from past experience, and investigating collapses due to catastrophic loads is one of the best available laboratories. Based on research sponsored by MBMA and the American Iron and Steel Institute (AISI), it was determined that existing codes should be strengthened to account for snow drifting across the ridge of low slope gable roofs. The latest edition of the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-98) and the International Building Code have incorporated this information that should lead to improved performance under snow loads for all types of buildings with low slope roofs. Research is continuing to provide further answers to this phenomenon of drifting snow which has had much less attention than wind or earthquake loads.
MBMA requires all member companies (about 90% of the entire industry) to become certified by the American Institute of Steel Construction (AISC). This program is designed to insure that all MBMA members have the personnel, organization, experience, design procedures, knowledge, equipment, capability and commitment to produce fabricated steel of the required quality for a given category of structural steel work.
These certification procedures are very rigorous. An independent auditing team visits each company annually and investigates all aspects of the firm’s operations and procedures including the design and manufacturing process. As a result, architects, design professionals, building code officials and the insurance industry can be assured that an AISC-MB certified metal building systems manufacturer is capable of meeting the industry’s highest standards for product and design integrity.
Standing Seam Roofs
A major technical innovation was the introduction of the standing seam roof design. This type of roof has provided the industry with the most effective structural, weather tight system available today. Over 2 billion square feet of standing seam roofing is installed annually. More than 50% of all commercial and industrial buildings are covered with metal using this technique. Yet, there have been some criticisms of this roofing technique, notably in terms of the added complexity it brings to purlin design.
Standing seam roofs are attached using concealed clips that are screwed to the purlins. The advantage over a through-fastened metal roof is that the standing seam roof system permits the building to better adjust to thermal expansion and contraction. However, the designer must determine how much lateral support the clips provide to the purlins. This can only be determined through tests, unless the designer wants to ignore the contribution of the restraint provided by the clips.
MBMA and AISI have sponsored research to determine the requirements and procedures for such tests. These procedures, called the Base Test, are now part of the AISI Specifications for the Design of Cold-Formed Steel Structural Members.
A purlin, like any structural component, needs to be properly designed and constructed to provide the intended service. The purlin must be properly connected to the main framing with adequate overlap where they are continuous. Finally, the purlin must be adequately braced.
Systems Engineering Approach
MBMA is in favor of a systems design approach where all of the components result in a smoothly functioning building which will safely carry all intended loads. Therefore, MBMA and AISI co-sponsored the development of a guide to provide assistance in applying the AISI Cold-Formed Specification procedures to purlin design. It’s called A Guide for Designing With Standing Seam Roof Panels, CF 97-1 and is available from AISI.
Standing seam roofs are also providing new life for many buildings with flat roofs that have proved to be problematic. A standing seam roof with a low slope can be built right over the old roof with minimum disruption of operations. Or, a steep one can be installed for a new building look and some additional attic space for improved energy conservation.
Standing seam techniques are one more example of how sophisticated the engineering is in today’s modern metal building systems. Insurers in particular need to know about the remarkable progress the metal building industry has made in recent years to produce a structure that is not only functional and pleasing to the eye but also a technically advanced system providing one of the best low-rise construction alternatives available.