Calcs.com expert hours with David Hourdequin (Pt. 1)

Moment frames in place of triangulated trusses

The Saint Joseph Adoration Chapel at Belmont Abbey College is a small structure, roughly 25 feet wide and 40 feet long, built almost entirely of glass with a grid of 6-by-6 timber members on a 5-foot by 5-foot module. Because the architect wanted a square-grid appearance rather than a triangulated geometry, the frame could not rely on truss action to carry load. Instead, every connection in the grid was treated as a moment connection, and the frame was analyzed as a 2D moment frame with gravity and lateral loads applied across all relevant load combinations.

The lateral force-resisting system for the chapel could not rely on shear walls because glass filled nearly every wall bay. Diagonal tension rods in both directions, connected to steel brackets at top and bottom using swaged fittings, provided the required lateral resistance. David noted that keeping those rods snug is critical: a slack rod observed during a site visit years after construction had to be reported back to the owner for re-tensioning.

A proprietary connector called a TimberLnx was used wherever a grid member was interrupted by a crossing piece. Each connector consists of an all-thread rod with expansion sleeve-anchor pins at each end. Once installed, the pins are tightened to pre-tension the joint, creating a degree of moment resistance at every intersection. David described this as what made the structure's behavior interesting: quasi-moment connections distributed through every node of the grid.

Glulam design for an intersecting-gable church

Harrison Community Church in Cincinnati used glued laminated timber throughout, making it one of the largest glulam jobs David had undertaken. The sanctuary featured intersecting gables over the crossing, with arch members framed into steel knife plates slotted into the timber. The knife plates, either 3/8 or 1/2 inch thick, were invisible from inside the finished building but carried the primary connection loads at the king post and at each of the four arch directions radiating from it.

The intersecting gable geometry created a tension ring at the bottom chord of each truss. When vertical load is applied at the peak, every hip rafter pushes the supporting column outward. The bottom chord of each truss acts as a tie that resists that outward thrust, forming a continuous tension ring around the square perimeter. David estimated the tension in those bottom chords at roughly 70,000 pounds, carried by approximately 10 bolts across four or five shear planes.

A 30,000-pound cupola sat at the center of the intersecting gable, concentrating approximately 15 tons of vertical load at the four corner columns through the cupola's own framing. The original design called for an all-brick steeple weighing 45,000 pounds; the owner removed 15,000 pounds during design development, bringing the final load to 30,000. Distributing that load as a trapezoidal pattern across each face of the cupola framing and working it down into the four corner columns was a significant design challenge.

For designers working with curved glulam members for the first time, David flagged two items. First, the lamination layout differs between flexural and axially loaded members: flexural members should have the highest-grade laminations at the tension face, while uniaxially loaded arch members use a balanced, symmetrical layup. Second, a curved glulam member under tension tries to straighten, which puts radial tension across the glue lines between laminations. That radial tension must be checked against the limiting value using the formula in the Timber Construction Manual, sixth edition.

Cascading cupola framing at Saint Thomas the Apostle

Saint Thomas the Apostle Church in Lancaster County used a stack of three square roof forms nested under common walls at progressively smaller sizes. Each level of the stack creates its own ring of hip rafters pushing outward, and the plate at each level alternates between tension and compression depending on whether load above is pushing down into it or hanging from it.

At the tension plates, steel straps were fitted into corner housings cut into the post tops. David's approach to sizing those straps was to resolve the hip rafter forces into components parallel to each plate line and design a strap in each direction to resist the resulting tension. Quarter-inch plate steel was generally sufficient. He noted that simplicity has practical value: a contractor once pushed back on a set of drawings that specified ten different strap sizes around a single roof, and David adopted the lesson of using the worst-case strap everywhere to avoid placement errors in the field.

Contractor Josh Coleman described the assembly sequence: each cupola was assembled on the ground, the four lower hip rafters were set individually and temporarily tethered at their tails to hold position, and then the assembled cupola was lowered and nested into the waiting hip rafter tips. The peak used a reciprocal framing configuration Josh called a pinwheel, with each member nesting into the next and secured by two toe screws per joint, one angled in each direction, so that every member was tied to its neighbors on both sides.

Lateral systems and moisture management in timber frames

For the lateral system, David described 2-by-6 tongue-and-groove roof sheathing as the most common diaphragm in timber frame construction, with an allowable diaphragm capacity of about 70 pounds per foot under allowable stress design. When shear demand exceeds that limit, structural wood sheathing is overlaid on the decking to develop the required capacity. Diagonal knee braces from post to plate are used in open pavilions where no roof diaphragm is available to transfer lateral load.

The exterior wall assembly in a timber frame is typically a light-frame stud wall built outside the timber posts rather than between them. The reason is moisture-driven movement: timber posts shrink as they dry, and any joint formed between the post and an interior element will open up over time. By running the stud wall outside the post line and connecting it to each post at those locations, the shear wall load transfers into the post while the wall itself accommodates post movement. A continuous strip of oriented strand board, about 4 inches wide and as tall as the post, is set between the post face and the inside face of the stud wall as a spacer, allowing interior finishes to run behind the post without gaps opening as the post shrinks.

David closed by directing attendees to the Timber Frame Engineering Council, part of the Timber Framers Guild, as a source of example drawings, example calculations, design reports, and a professional community for engineers entering the heavy timber field.