Heavy Timber Cantilever Rack: Western Timber Frame, Lehi, UT

Western Timber Frame builds heavy timber structures in Lehi, Utah: pavilions, pergolas and timber frame kits cut from large Douglas fir and glulam members. The raw material that arrives at a shop like that is the hardest kind of inventory to store. Individual members are long, they are heavy for a single piece rather than for a pallet load, and their surfaces are the finished product, so a scuff is not cosmetic, it is scrap or rework.

In February 2020 Chris Rodgers quoted a structural cantilever rack row for the shop. The quote was deliberately simple: five 16-ft columns, five 4-ft bases, fifteen 4-ft arms rated 3,200 lb each, and brace sets in two lengths. It shipped in March 2020, freight included, with the shop handling installation. The manufacturer’s part list confirms what went out the door, including one detail that makes this project worth writing up: the columns are double-sided, while the bases and all fifteen arms are single-sided.

The component vocabulary used throughout this case study is explained in our cantilever rack systems overview.

The arms on this project are 48 inches long with a 4-inch face and a published rating of 3,200 lb each. That number is the single most misread figure on any cantilever quote, so it is worth being precise about what it does and does not say.

It is a rating for one arm, at that arm length, uniformly loaded. Three things follow. First, capacity is tied to length: the same arm profile cut longer carries less, because the load acts farther out from the column and the bending moment grows with the distance. A 3,200 lb rating at 4 ft is not a 3,200 lb rating at 6 ft. Second, the rating assumes the load is spread across the arm rather than concentrated at the tip. A single heavy timber resting near the outer end of an arm is a harsher load case than the rating describes. Third, an arm rating is not a column rating: the column and base have to carry the sum of every arm above the floor, which is why arm count per column and column height are quoted together.

For timber, deflection usually governs before strength does. A timber that spans two arms set far apart will visibly sag long before anything is close to failing, and in a shop that cuts joinery to tight tolerances, a member that has been stored with a bow in it is a problem even if the rack never complained. That is why bay spacing, not arm capacity, drove the rest of this configuration.

This order included X-brace sets and horizontal braces in two sizes, 72 inches and 96 inches. Brace length is what sets the distance between columns, so a row built from two brace lengths has two different bay spacings along its length. That is not a mistake or a leftover, it is a design choice, and it is the detail that separates a cantilever row planned around real material from one ordered off a spec sheet.

Bay spacing decides how far a stored member spans between supports. Short, stiff, heavy stock is happy on wider spacing. Long, slender or flexible stock needs the columns closer together, or it sags between arms. A timber shop stores both: large section beams that would bridge anything, and lighter, longer members that will take a set if they hang unsupported. Building part of the row tighter and part of it wider gives each kind of material the support it needs without paying for a row that is uniformly tight everywhere.

There is a second reason to mix spacing. Column spacing also decides how many columns stand in the way of the forklift when a long member is slid in from the end of the row. Wider bays are easier to load through. Planning the tighter bays where the flexible stock lives, and the wider bays where the loading happens, is the kind of thing that only comes out of a conversation about what the shop actually handles.

This order shipped as material and freight, with the shop installing the row itself. For a fabrication shop with forklifts and crews who build structures for a living, that is a sensible call. It does mean the configuration has to be right before the truck leaves, because there is no installer on site to absorb a missing brace or a base that is too short for the arm depth.

The build sequence for a structural cantilever row does not change. Mark and square the whole run before setting a single anchor, because a row that starts out of square cannot be corrected at the far end. Set and anchor the first two columns and check plumb. Install bracing as the row grows rather than at the end; an unbraced row is not partly built, it is unsafe. Bolt and torque the arms level by level. Then load from the bottom up with the heaviest material on the lowest arms.

Anchoring is what makes a cantilever base work. The base resists the overturning moment created by a loaded arm, and it can only do that if the anchors match the slab thickness, strength and edge distance, and if bases are kept away from control joints and slab edges. After the row is loaded it needs a real inspection habit: bolt tightness, base plates, column plumb and any forklift impact. General requirements for secure material storage are set out in OSHA 29 CFR 1910.176.