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Architects and engineers reach out to us all the time for some advice on how best to lay-out a metal building for cost effectiveness. Today’s metal buildings can be customized down to an inch in most cases, but there are some general rules of thumb that help make some lay-outs more economical than others.
Note: It will help to have a basic understanding of metal building terminology before digging into this blog. Here is a link to one of my past blogs that addresses metal building definitions: Metal Buildings 101.
In most cases it is most economical for the building width to be the lesser dimension. For example if you have a 50′ x 100′ building, it is more economical if the building is 50′-0 wide x 100′-0 long. This is because the width dimension is the span of the roof rafters. A rafter that spans 50′-0 will be less expensive than a rafter that spans 100′-0
For this same reason, given the same square footage, a building that is rectangular will be more economical than a square building. If a customer comes to us with a request for a certain square footage but no specific length & width requirements, we will always design a rectangular shaped building.
The most important question to answer when specifying the height is what height are you looking for? Eave height, which is measured to the top of the eave purlin, or clear height, which is measured to the lowest obstruction.
That being said, if you specify a clear height, it is important to specify whether or not it is acceptable to increase the eave height to achieve the clear height requirement. Allowing for the eave height to change to accommodate the clear height is the most economical solution. However, if there is a situation where the eave height cannot increase (for example when tying into an existing building) the roof framing can be reasonably restricted to accommodate the clear height. But, this is more costly than just raising the eave height.
Metal buildings are most cost effective when they utilize low slopes. Keeping the roof slope to 2:12 or lower is best. Most metal roofs are designed so they can be effective and water-tight where a slope is as low as 1/2:12 so there is no functional reason to increase the roof slope. If a higher roof slope is desired to meet an aesthetic need, keep the roof slope below 4:12.
The material costs increase as the roof slope increases, but probably more importantly, the labor costs rise significantly. It is extremely difficult to install a metal roof on a slope greater than 4:12. We have heard erectors say they double the cost to install a roof when the slope is greater than 4:12. If we have complete freedom to design a roof slope we will typically select a 1:12 slope.
The end-bays of the building should be designed to be the shorter bays of the building wherever possible. Shorter end-bays work better for the roof purlin design and lay-out.
In our climate in Western New York 25′-0 bays tend to be the most economical. Anything greater than 30′-0 requires the use of a truss purlin or bar-joist which can really increase the cost of a building. Having bays less than 25′ is not a bad thing, it could just be slightly less economical. For example if you have a 100′-0 long building with 25′-0 bays that will work out to four bays, utilizing a total of five frames. If the bays were 20′-0 that would be five bays with a total of six frames. So you’ll have an additional frame, albeit a lighter frame because it is carrying less tributary load, but an additional frame nonetheless which will make it the more expensive option.
Clear span frames as the name implies are wide open and utilize no interior columns. Multi-span buildings utilize interior columns. It is more economical when interior columns can be utilized in wide buildings that have large frame spans.
My simple rule of thumb is when the width of the building is greater than 80′-0 it makes sense to consider an interior column. The obvious reason utilizing an interior column is more economical is because the unsupported span of the frame is shorter, which allows the frame to be designed lighter. Additionally an interior column makes erecting the main frames easier.
Another reason to utilize an interior column is to mitigate the need to use tie-bar or grade beam assemblies. Tie bars or grade beams run parallel with the frame and are installed under the floor. They connect the base of the column’s concrete pier on either side of the building to offset the kick-out force on the frames. These systems can really increase the foundation costs of the building.
As I mentioned in the beginning of this article, these are just some simple rules of thumb to make a metal building most cost effective. A metal building can be extremely customized and in most cases can achieve what a conventionally framed building can achieve for less expense with a shorter project duration.