Glulam Building: Start to Finish – Part Two D
Welcome to our “Glulam Building: Start to Finish” series. Our goal is to provide an overview – from idea to completion – of a glulam building, from the perspective of a glulam manufacturer and designer.
Part 2D: Limitations
With basic framing layout and a lateral system in place, what else is there to consider in the design of a glulam structure? This is where some designs will run into some questions of: can it be done, or should it be done differently. The first is a question of physical limits, the second is typically a question or cost. Both limits can be hit in the factory (curving and size), on they way to the job site (shipping), or in the finished building (clearance).
Curving Limits: Can we make the desired shape? Can it be done cost effectively? The inside radius determines the ply thickness, directly translating to the number of plies and cost.
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| A radius smaller than 7′ being glued with custom plies. |
- 3′-0″ inside radius is a practical limit we typically adhere to, as it requires 1/4″ plies to achieve.
- 7′-0″ inside radius is the minimum to get full use of 1″ nominal lumber.
- 20′-0″ inside radius is the minimum to get the full use of 2″ nominal lumber.
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| A spiral glued in upright position. |
Another curving limit applies to 3D curves and spirals. Spirals need to be glued in their upright position, curving around the vertical axis. We therefore are limited by the height of the forms, where a 10′ tall spiral can be glued.
Size Limits: Can we glue and finish the desired size? We are generally quite flexible here, often running into shipping limits (see below) before production. There can definitely be cost implications, however, particularly with member widths. Standard width are most efficient (3″, 5″, 6-1/2″, 8-1/2″, and 10-1/2″), and any intermediate size can be made for approximately the same cost as the standard size above it. Over 10-1/2″ wide we need to do a more expensive combination layup, which have standard widths of 12″, 14″, 16″, 18″, and 20″. We can do an even more expensive triple combination, until we reach the maximum width of 24″. Beams depths can efficiently be made up to our planer capacity of 76″. Deeper beams are possible (we have shipped 108″ deep beams). Length, for Unalam, is only limited by shipping.
Shipping Limits: Can we legally transport the components to the job site? This is the most frequently hit limitation for projects, in our experience. There are cost implications as one exceeds 8′-6″, 10′-6″, 12′-6″, and 14′-6″ widths, with 15′-11″ being the absolute maximum.
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| Example of maximum shipping width of 15′-11″, shipping length is 125′. |
Shipping weight is almost never an issue, although it can increase the number of shipments. For shipping length, 125′ is a practical limit, although Unalam did once transport a 175′ long beam!
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| Transporting our 175′ long beam! |
Clearance limits for building are typically related to head height, either below beams or
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| How shipping width is measured for Tudor Arches |
trusses, or near arch legs for horse riding arenas. Beam and truss depths can be reduced somewhat by using wider members or reducing the spacing between beams, but that is rarely efficient. Plan on an 8:1 span to depth ratio for trusses, and approximately 15:1 for beams. Tudor Arches can run into a design spiral of increased span to gain interior clearance, which increases the member depth (reducing clearance again), and potentially hitting some of the production or shipping limits. If a preliminary arch design is close to the shipping width limits, check with us to verify the feasibility before progressing too far.
