Metal Building Minute
What Are Metal Buildings?
Pre-Engineered Metal Buildings (PEMB’s) are buildings that have been designed and constructed to utilize pre-manufactured system components (such as horizontal wind girts, roof purlins and roof metal decks) to economically and efficiently construct enclosed spaces, especially large-spanning buildings in need of open space.
Advantages of PEMB Use:
- New, more aesthetically pleasing facades are now being utilized which make metal buildings more versatile in their uses.
- PEMB’s are now commonly used for schools, banks, strip malls, and other familiar structures.
- PEMB’s repetitive designs and standard components make metal buildings lower in cost than conventional construction and quicker to build. These efficiencies make it possible to obtain larger buildings at a lower cost.
Some Disadvantages of PEMB Use:
- Metal building manufacturers are often never told the “use” of the building, which can lead to components being incorporated improperly.
- Metal building design is often referred to as a “black box” approach where the different design disciplines involved in the project are not involved in the design of the metal building. This lack of communication can lead to mistakes being made in the design and/or purchasing process.
- There can be limited “checks and balances systems” between the design entities involved which limits the protection for the owner. Liability often falls to the foundation engineer-of-record, and sometimes this engineer does not receive or review the PEMB calculations.
- Some failures are not caused by poor engineering or erection; these failures are due to another adjacent building or structure creating step or drift conditions on the new or existing metal building. This can be avoided with communication if your building will be near or attached to another building. Step conditions must be accounted for in the engineering and design phases.
Some Quick Facts:
- Potential building owners would have added benefit from choosing a PEMB manufacturer that is a member of the Metal Building Manufacturers Association (MBMA); members of the MBMA agree to certain standards in the design and construction of metal buildings. Some non-MBMA members manufacture or purchase parts that do not meet required standards, which can lead to problems subsequent to the installation.
- The building typically arrives on the site in one load. It is important to “shake out” the building before the erection process begins. “Shaking out” is laying out all the components and comparing what was delivered to the delivery manifest in order to make sure all the pieces are accounted for.
- During the erection process, the siding panels must be installed before the roof frames to avoid uplift. Even the slightest winds can cause catastrophic damage if this is not sequenced properly.
- Be sure that the erector has the manufacturer’s recommended rigid frame installation techniques, or that the interior frames are installed with guy-wire anchors to resist wind loads and some girts installed between frames. Once interior frames are completed, the gable-end frames can be installed.
- When reviewing the snow loads of a metal building keep in mind that many specified loads are ground loads, not roof loads. The cost difference in constructing a PEMB for 30 pounds per square foot (PSF) ground snow load versus 30 PSF roof snow load can exceed 20%. This is a common theme that will be addressed in future Metal Building Minute articles.
- In a metal building, doors are not typically designed for the same wind load as the walls; therefore, they will most commonly fail first during wind events. Look for distress at the doors when investigating wind damage claims.
Distress Observation / Repair Facts:
- A good way to determine if the insulation needs to be repaired or replaced is to poke the vinyl on the underside of the insulation. If you are able to poke you finger through or crack the vinyl by pinching it, it needs to be replaced.
- When repairing leaks in a metal roof panel keep in mind that roof panels need to move, therefore, penetration in a metal roof cannot be sealed with a typical flashing or sealant. Contact the roof manufacturer to make sure the proper flashings and sealants are used.
- Large doors opened during a wind event can significantly increase the uplift on the roof system. Check the connections at the top and bottom of supporting interior walls to aid in determining if significant winds may have played a role in wind damage claims.
- The ties or straps, connecting the purlins to each other are designed to perform a structural function. They should be installed and maintained to ensure tightness of the assembly. Be aware of the flat-strap bracing at the purlins; these can become loosened over time or even during instillation, with a resulting reduction of the load-carrying capacity of the attached purlins.
- If a fire occurs at a PEMB, keep in mind some of the potential distress associated with these events. According to the American Institute for Steel Construction (AISC), typical fires have a temperature between 1,000-1,800 degrees Fahrenheit. Structural steel (such as rigid frames) begins to experience a loss of capacity at about 800 degrees Fahrenheit. Light gauge steel (less than 3/16” thick) becomes compromised at about 650 degrees Fahrenheit.
- There are several non-destructive tests that can be performed to see if the steel has been compromised.
- Visual tests can be conducted to observe the straightness of the beams and see if any distortion has occurred.
- For beams greater than 12 inches in depth, .25 inch of distortion is allowed during fabrication; any more than this amount of ‘twist’ could indicate damage due to excessive heat.
- A hardness test can also be performed as a simple test to determine whether the structural integrity of steel members has been compromised.