Expert hours with David Hourdequin: heavy timber design (Pt. 2)

Two heavy timber projects: a Florida wedding barn and a Universal Studios ride

David Hourdequin presented two projects from his heavy timber engineering practice. The first was a wooden barn in central Florida, built as a wedding venue and representative of the commercial barns being constructed across the country. The second was the Harry Potter / Hagrid's Magical Creatures Motorbike Adventure ride structure at Universal Studios Florida, one of the most structurally unusual heavy timber projects David had encountered in a sixty-year career.

Chris Miller, the architect on the barn project, and Ben Brumgrave, the structural engineer of record on the Universal Studios structure, both joined the session to discuss their perspectives.

The central Florida wooden barn: scissor trusses, K-trusses, and high-wind design

The barn sits in a 140 to 150 mph wind region. The structure uses scissor trusses with metal plate connected joints for the main roof framing. At the upper level, a strip of windows ran along the end walls, leaving no continuous path for shear walls to carry lateral load down to the foundation. David resolved this by using K-trusses along those window wall lines to transfer the wind shear.

The foundation was a monolithic slab on grade with interior drop footings for the posts. The high wind uplift forces required HDU11 hold-downs at the shear walls. The main floor framing used 10 by 20 Douglas fir girders spanning across the floor with a centre support.

David and Ben discussed the connection detailing at the scissor truss heel and at the scissor junction. Ben's key point: the lines of action of the bolts at each connection must pass through the centroids of the connected members. Where that alignment is missing, the side plate is subject to a net twisting force that can split the timber ends. The barn drawings show bolt patterns at the heel and at the scissor junction arranged so their lines of action pass through the relevant centroids. A stub tenon at the junction and a bearing landing on the top chord plate further resist rotation and carry the vertical component through bearing rather than through the fasteners.

The Harry Potter ride structure: twelve 70-foot cantilever trusses and a laminated king post

The Universal Studios structure is twelve-sided, with twelve timber trusses each spanning 70 feet and meeting at a central king post. Two of the twelve trusses were cantilevered: each cantilever end was free, and the opposite end was anchored to counterbalance the uplift. Tension rods at the top chord of the cantilevered trusses carried the chord tension from one side to the other through the king post. The structure was in the same 140 mph wind region as the barn.

No single Douglas fir timber of the required size was available. The king post is 26 feet high and 3 feet in diameter, built from stave segments bonded with G-flex epoxy under controlled conditions, similar to mast construction. Internal diaphragm plates of double-laminated Advantech, each approximately one and a half inches thick, were let into the inner face of the staves at intervals: two near the bottom, two in the middle, and additional plates near the top where axial loads concentrated. These diaphragms stiffen the assembly against the unbalanced compression loads from the trusses and prevent the circular section from deforming to an oval.

The axial compression in a typical top chord required a 5-inch bird's mouth bearing, which also had to be checked for horizontal shear parallel to grain in the bearing plane.

Durability, fire resistance, and maintenance access

The structure is built from untreated Douglas fir and is exposed to the Florida climate, with high humidity and periodic wetting. David recommended pressure-treated timber for exposed exterior conditions of this kind. The Universal Studios maintenance regime required access between 3 and 6 in the morning after the ride stopped running. A persistent issue was the ride operator's early practice of misting the timbers and guests as part of the experience: David advised stopping the misting.

On fire resistance, David and Ben agreed that heavy timber is well-suited to fire conditions because the outer surface chars and the char layer insulates the remaining section. In this structure, all connections were buried well within the timber depth: in 12-inch-wide members, the connections are 6 inches in from the face, putting them well inside any credible char depth.

The erection sequence for the Universal Studios structure required a temporary support tower in the centre, because the floor had a large opening for the ride machinery. The tower was self-stabilising and allowed the king post and trusses to be assembled in place rather than lifted by crane.

Connection design principles from David and Ben

Several connection details from both projects were discussed in depth. The key principles that came up repeatedly:

Lines of action of fasteners at any joint must pass through the centroids of the connected members. Offsetting fastener patterns introduces a twisting moment that the plate or timber must resist, leading to potential splitting.

Stub tenons at truss junctions resist rotation and reduce dependence on fasteners for moment resistance.

Bird's mouth bearings at top chords should be checked for horizontal shear parallel to grain in addition to the vertical bearing stress.

For plate washers in tension applications, David noted a practical sizing rule: the washer size in inches corresponds to the required thickness in eighths of an inch.