Cross-laminated timber: structural principles

Figure 2 shows the six key connections used to join together cross-laminated timber wall and floor panels.

Figure 2: Typical connections

Typical details for these connections are presented in the next six sections:

The ground floor wall panels typically sit on a level soleplate (over a DPC) fixed to the concrete floor slab. The soleplate is fixed to the slab using fixings appropriate for the substrate and designed to resist the applied horizontal loads. The CL panels are then positioned onto the soleplate.

A soleplate (Figure 4) will ensure that a level surface is provided before the CL panels arrive on site. The soleplate can be positioned, fixed and grouted to enable erection of the CL frame to progress unhindered by the need to level wall panels or allow grout to cure.

Alternatively, where there is no risk of standing water during construction, the ground floor wall panels can be bedded directly onto a grout bedding on a DPC to ensure a full bearing surface (Figures 3 and 5).

Figure 3: Wall panel – foundation

Note that the bolt installation is incomplete in Figure 3.

In Figure 4, fixings to foundation would be specified in the same way as timber frame construction.

Figure 4: Wall-foundation, with soleplate

Figure 5: Wall-foundation, with grout bedding

This connection may use an engineered timber jointing piece and offers the potential to embed service runs.

Some systems use connections like those shown in detail 4 to join wall panels.

Figure 6: Wall – wall, straight

The outer panel is screw fixed into the end grain of the abutting panel (Figures 7 and 8). Precautions for airtightness and/or sound transmission may be needed.

Figure 7: Screw-fixed connections

Figure 8: Wall – wall, junction References Acknowledgements

This connection is usually a half-lapped joint milled in the factory (Figures 9 and 10).

Figure 9: Half lapped floor joint on concrete substructure.

Figure 10: Floor – floor

These are similar to corner wall connections (Figures 11, 12 and 13).

In this situation, the terms ‘floor’ and ‘ceiling’ may be interchangeable.

Figure 11: Wall – floor (ceiling)

Figure 12: Floor panels screwed to walls below

Figure 13: Installing angle brackets to fix next level wall; the sealing strip is for airtightness.

Cantilevered wall panels, for example parapets, can be formed effectively using continuous (balloon-framed) wall panels with the floor structure supported inside the wall on a ledger (either steel or timber) screwed to the wall panel to support the floor panel (Figure 14).

Figure 14: Balloon frame options

Figure 15: Wall-joist, balloon frame method

Engineered timber floor structures can be supported on top of the CL wall panels (platform frame approach) or inside of them (balloon frame approach). Figure 15 shows a decking on joist system supported in a balloon frame configuration.

Figure 16: Wall – I-joist, platform frame method

Where CL wall panels are combined with engineered timber joists built into the wall (in a platform frame approach), vertical load can be transferred via solid blocks between the joists (Figure 16).

In this detail, the solid blocks and ring beam should be an engineered timber product, eg laminated veneer lumber (LVL), parallel strand lumber (PSL) or laminated strand lumber (LSL), in order to minimise shrinkage.

Figure 17: Wall – engineered timber floor, platform frame method

An alternative would be to use top-hung open web joists to avoid the requirements for solid blocking beneath walls (Figure 17).

The construction sequence usually follows the platform frame approach, meaning that walls (starting from corners) are temporarily braced before floor panels are lowered onto them and fixed. The next level of walls is then constructed off this working ‘platform’.

Connections are secured with self-drill woodscrews of up to 400mm in length and proprietary mild steel angle plates.

Pre-compressed foam tape may be inserted between adjacent panels to produce airtight joints.

Where designed for inplane loading, for example in a floor plate transferring lateral forces to shear walls or in shear walls resisting overturning, the connections between adjacent CL panels must be capable of transmitting the shear forces at the panel interface.

CL has high dimensional stability, similar to plywood and does not need movement joints.

Deformation is related to the direction of the grain. Because the panels have boards running in two directions, there will always be at least one layer in either direction. The maximum deformation in service (per % timber moisture change) is 0.02% within the plane of the panel and 0.24% perpendicular to the panel.

The comparative frame shrinkage of a CL structure, due to change in moisture content, is much less than expected in a conventional timber frame structure. However, the effects of creep become significant above about 12 storeys and must be considered in the detailing of claddings, linings and vertical runs of building services (see GD10).

WIS 4-11 describes how wood-based panel products and timber behave in fire. The specifier must consider the product’s reaction to fire as well as its fire resistance. See GD10 for an application to cross-laminated timber.

Reaction to fire. Plasterboard linings or fire retardants may be needed to satisfy Building Regulations requirements for reaction to fire.

Fire resistance. Fire resistance of CL panels can be determined using the design method in Eurocode 5, or by providing fire-resistant linings, or by a combination of both. CL panels have a charring rate comparable to softwood of 0.7 mm/min.

Check tall and long-span structures for vibration. GD6 sets out the principles using Eurocode 5. See GD10 for application to cross-laminated timber.

The Building Regulations Part A3 requires all buildings to be able to sustain a limited extent of damage or failure without collapse.

CL structures, with wall panels able to span in two directions, are inherently robust and the structures usually exhibit a high degree of redundancy with multiple possible load paths.

It is therefore unlikely that disproportionate collapse would be a critical factor for specifiers. See GD10 for application to cross-laminated timber.

TRADA Technology acknowledges the assistance of CCB Evolution in preparing this information sheet.

Photo credits: Figures 1 and 7 Re-Thinking (Willmott Dixon); Figures 3 and 9 Stora Enso/DMH; Figures 12 and 13 Stora Enso.