We Are All Odd in Our Own Little Ways …
We Are All Odd in Our Own Little Ways…
Buildings are no different, metal buildings especially.
There was a time when metal buildings were only used for warehouses or commercial structures. Now, they are more mainstream. Often the base structure for a retail or office building. They really shine in affordability and speed of construction. Most commercial metal buildings can be purchased out of a catalog; this long, by that wide, and this tall. Those type buildings are often referred to as “pre-engineered” metal buildings. Custom metal buildings are becoming more common and are also designed by computers that optimize the size and shape of the framing members, supplying the minimum amount of material to meet the design needs.
For the sake of this discussion, let us assume our metal building is located somewhere that might experience high winds and would seldom be subjected to significant snow. In high winds (think hurricane) metal buildings want to do two things; blow over or fly away. They are very lightweight for their size and very flexible, essentially they act like a tent. They are also not rigid like their cousin the log cabin.
As the wind pushes on the side of any building, the amount it moves or leans is known as “drift”. Drift is expressed as a ratio of the height over a given number. The higher that number, the more rigid the building. With masonry or concrete, the number may be in the range of 500. A standard metal building is specified with a drift of H/75, meaning the roof of an 18’ tall metal building would be allowed to move lateral almost 3” inches in a hurricane. Not a huge deal if the building is all metal and used for storage. What if the metal building has a masonry front wall or interior partitions that attach to the side walls? Most retail or office buildings would be like this. The masonry or drywall would not move near as much as the metal building and there would be considerable damage along the intersections.
The most common method to provide the metal building some rigidity or lateral support is with cables placed in an “X” pattern in the side walls. Unfortunately, these cables are thought to be in the way or don’t look important to some erection contractors, and they leave them out. Sometimes, the X-bracing is removed when a building is renovated, or an exterior door is added. The metal building was flexible with all its components installed. Omit or remove bracing and there can be real trouble.
Metal buildings also have issues with uplift. Think of a tent and the tent stakes. Without the stakes, wind could pick up the tent and carry it away. A metal building is not much different. Metal building frames are secured via anchor bolts to concrete footings. For a conventional structure, the footings transfer the weight on the building into the underlying soil. For a metal building, the footings need to be large (heavy) enough to hold the building down when the high winds try to pick it up. There are also often requirements for lateral support for the anchor bolts, often with the use of “hairpins”, which are “V” shaped rebar pieces located around the anchor bolts and extending out into the floor slab. To the untrained eye, the extra concrete for the footings and extra rebar seems overkill. The unknowing or unscrupulous contractor may be tempted to leave it out. You can’t see it and no one would know until the hurricane blows.
Another possible issue with the foundations of the metal building stem from their overall “speed of construction”. The use of a metal building tends to speed up the whole process with the contractor wanting to get the foundations poured and ready for the building when it arrives. The problem is that the size and shape of the foundation is primarily based on the uplift numbers supplied by the metal building manufacturer. There are some rule-of-thumb estimations, but the actual loads may vary – sometimes by a lot!
A couple of things can go wrong at the roof level. Computers designed to minimize materials mean there is little extra capacity, thus every brace or fastener is critical. The typical metal building design includes braces connecting the bottom chord of the main roof beams to the purlins (rafters). These braces appear insignificant but provide considerable uplift capacity to the roof assembly. Without the braces, the roof could fail under moderate wind loads.
That lack of extra capacity also applies to gravity loads. The purlins are designed for the minimum roof loads. Hang a few fluorescent lights from the roof and it should be okay. A 300-pound HVAC unit or a system of iron sprinkler piping would likely cause the purlins to sag. Perhaps the nearly-flat roof would deflect enough to collect some rainwater. The weight of the water would deflect the roof some more, collecting more water… This is known as ponding, and it is bad!
Metal buildings can be great. All parties involved in their use just need to understand their quirks and make the necessary allowances. If there has been a wind event and a metal building appears damaged in an odd way, it may just be the nature of metal buildings.
After all, we are all odd in our own little ways…
About the Author
David Horne, P.E. is a Consulting Engineer in our Charleston, SC Office. Mr. Horne offers consulting services in the following areas: investigating and evaluating structures of all kinds in commercial, industrial, and residential settings. You may contact him for your forensic engineering needs at email@example.com or (843) 832-1052.