Structural watchpoints

1. Avoid transfer structures. If not avoidable, loads to be light and spans short.
   
2. Meeting floor vibration criterion generally governs floor slab sizes (unless fire criteria is onerous).
   
3. Charring due to fire scenario will result in sharp reductions in capacities as char depth increases, particularly when passing a glue line.
   
4. Consider how exposed sections can be connected and what can/can’t be seen.
   
5. Timber connections can be unforgiving. Front load the design intent during early stages.
   
6. Low density CLT is not particularly effective for resisting acoustic transmissions (additional means of mass or isolation may be required).

Building layout

CLT is a great timber product for supporting uniform loadings e.g. roofs, office floors or residential floors. The material will however struggle with large concentrated loads such as transfer loads. They may be possible but can significantly increase sizes of the elements, have large creep deflections and generally require stiffening elements (e.g. steel).

Efficient layouts can be created by maintaining a simple load path to the foundations/podium structures e.g. building setbacks to be aligned with the structural grid or wall panels. It’s not uncommon for the ground floor to require a more open plan structure to the layout. A podium to this area is quite regular and generally formed from concrete.

Floor spans

The main driving factor for a CLT floor structure will typically be the vibration criteria to be achieved. This should be looked at early in the scheming process and can define the setting out of the structure/architecture/material volume.

From a timber volume regard, shorter spans will significantly reduce the volume of material required and hence the cost. The sizes in Table 9.1 give an approximate indication of expected size ranges.¹