Cross-laminated timber (CLT) structures

In a CLT building the connections do the work that welds and bolts do in steel, and they define both the strength and the stiffness of the structure. Panels are joined with self-tapping screws, angle brackets, hold-down straps and spline joints, and these have to transfer floor loads into walls, tie walls down against wind uplift and overturning, and stitch floor panels together so they act as a single diaphragm. We design the screw patterns, brackets and hold-downs for the forces at each junction, paying particular attention to overturning of shear walls, where the tension hold-down at the leading edge often governs.

Two behaviours need specific attention in CLT. Vibration and deflection frequently govern floor panels rather than raw strength, because occupants notice a floor that feels lively long before it is anywhere near failing, so we check the fundamental frequency and control deflection tightly. Compression perpendicular to the grain at bearings, where a wall sits on a floor panel that sits on the wall below, can crush the soft cross layers, so bearing stresses are checked and, in taller CLT buildings, the cumulative crushing and shrinkage across many floor zones is allowed for so the building does not settle unevenly.

CLT suits residential blocks, schools, offices and any building where a fast, dry, low-carbon structure is wanted. Panels are cut to size off site from digital models, including openings for windows and services, then craned into place and screwed together, so erection is quick, quiet and light on the foundations because timber weighs a fraction of concrete. The exposed timber can be left visible as the finish, and the low self-weight often allows a simpler substructure or an extra storey on the same ground.

The limits are real and are designed for, not ignored. Timber burns, but CLT chars predictably, so panels are sized with a sacrificial charring allowance to keep the residual section strong for the required fire period, and encapsulation with plasterboard is used where exposed timber is not acceptable. Moisture must be controlled during construction and in service, since wetting swells and degrades the panels, and acoustic separation between flats needs careful detailing because bare CLT transmits sound. We resolve fire, moisture, acoustics and movement at design stage so the panels perform for the life of the building.