CLT BUILDING DESIGN

Understanding the material properties allows designers to apply any material where it will be working to its strengths and minimise the constraints.

CLT FRAMING

Structural openings, such as large doors and windows will tend to require a lintel of some sort as Figure 9.1.

FIG 9.1 Methods of forming openings, from left to right: 1. Two-way spanning slab over openings (no lintel), 2. Wall panels with punched openings, 3. Lintel support.

UNDERSTANDING MOVEMENT BEHAVIOUR

While CLT reinforces the stability of the product it does so in one direction only. The typical construction method for CLT is to support the floor panel by directly bearing onto the CLT wall (platform construction, as Figure 9.2). The next wall then bears onto that floor panel. This means that the building load is applied across the grain of the floor panel which is more susceptible to the displacement and to moisture drying shrinkage effects.

FIG 9.2 Section through floor slab (platform style construction) indicating direction of movement when loaded.

Understanding the movement of the structure is necessary in ensuring that the details’ design can be developed to either resist or allow for this change in dimension. Areas where this is particularly relevant include:

• Lift shafts: The predicted movement/settlement and tolerance to the lift should be communicated to the lift installation company to ensure enough allowance is made in the connection details and to enable a consideration of potential lift realignment once the building settlement has occurred.
  
• Stacked facades: Where a facade is stacked from the ground floor, consideration should be given to the differential deflection at windows and parapets, e.g. a brick facade will expand in certain conditions while the timber structure will settle over time.
  
• Fixed facades: A rendered solution would likely need a movement joint at the floor level to avoid potential cracking uses due to shrinkage. It is also typical to fix heavier facade solutions (brick/stone) at each floor level. This limits differential movement issues but will add to the load in the timber, which could result in to reinforcement to avoid crushing of the floor.

Detailing for vertical movement tends to fall under one of two approaches:

1. Allow the timber floor to move and detail the design around this.
   
2. Restrict the level of movement that can be experienced in the structure.

In limiting the vertical movement the aim is to reduce the amount of loaded perpendicular grain to the floors, so the load path becomes an end grain to end grain solution. This can be accomplished by using castellated timber walls or grout pockets.

CLT ELEMENT DESIGN

The structural analysis of a CLT element is not as straightforward as reinforced concrete slab or steel section. The structural elements of the section are the layers with the grain direction parallel to the span direction. The layers in between contribute very little to the stiffness other than increasing the lever arm between the parallel layers.

In analysing the stiffness of the CLT panel a critical factor is the rolling shear in the perpendicular grain. This will need to be considered when calculating the moment of inertia (I value), as the timber fibres can ‘roll’ and create a slippage in the element as Figure 9.3. Parallel layers cannot therefore be assumed to act in a fully composite manner. This leads to a counterintuitive situation where the longer the floor spans the more composite the section becomes. This is due to the area of the perpendicular layers increasing longitudinally and hence there is more resistant to rolling shear forces.

FIG 9.3 Floor panel section indicating potential for fibre rolling effect of intermediate layers (cross lamella) when loaded.

There are several methods of calculating such properties, however the most common is the ‘Gamma method’ where an approximation of the shear deflection is calculated. Annex B in BS EN 1995-1-1 gives guidance on the calculation of a mechanically jointed beam.² This gives a method of calculating the slip in between the joint members called the Gamma value. This value is based on the slip modulus of the fastener which it assumes is either a screw, bolt, dowel, staple etc.

On this basis, the sectional properties of the CLT panels can then be calculated by taking into account the fact that the fasteners between the structural panels are perpendicular timbers where shear failure is critical. Once this value has been obtained it can be applied to the structural layers using parallel axis theorem to calculate the stiffness of the panels. From this, the stresses and deflections in the panel can be calculated to ensure an appropriate design.

With the analysis of a beam section the layers in line with the span are taken into account during the design for moment, shear and deflection. The buckling of a beam is not usually an issue as the beam is typically restrained by the support slab. However, in certain situations this may not be the case, therefore a buckling check is required.

The total width of a CLT beam may be used for the critical bending stress calculation if the outer layers are aligned with the span. If they are not, they should not be included in the buckling calculation.

The analysis of walls and columns follows the same principles to that of a beam i.e. selecting the correct structural layers for strength and buckling checks.

Stability to a CLT structure is typically provided by the floor panels acting as a diaphragm and directing the loads to the vertical stability wall elements. Where CLT walls provide the stability to the building, the floorto-floor heights can impact on the capacity of the walls to resist horizontal wind loads or limit building deflection.

FIG 9.4 Stability wall arrangements and potential for deflection under load.

ACOUSTIC CONSIDERATIONS

Sound transmittance and soundproofing remains a key issue between uses, dwellings or areas within the same building, such as adjacent office floors or school classrooms. Timber is significantly less dense than concrete, typically around 20% of the density. Therefore, an approximately 500mm thick CLT floor slab would be required to achieve the same level of sound insulation as a 100mm thick concrete floor slab. Simply increasing the thickness of a timber slab is not often a viable solution.

A better way to increase the level of sound insulation is by acoustically decoupling the elements within a separating construction. This can be achieved by the inclusion of cavities separating two mass elements and/or resilient fixings. The inclusion of an absorbent material within cavities can further help to improve the level of sound insulation by reducing resonances. Adding mass is another commonly adopted mitigation measure, this might be in the form of a screed topping over CLT slabs or dry laid high density boards, typically lap jointed, above a slab.

Manufacturers usually offer detailed guidance regarding sound insulation, focusing on airborne sound (such as music or voices) and structure borne sound, including impact sound (e.g. footsteps or moving furniture). Accessory manufacturers offer separation strips that can be used to improve the acoustic performance of panel joints or fixings and test data is widely published for various elemental build-ups using different materials; this is typically reflected in manufacturers technical approval documents such as European Technical Assessments (ETA) or software tools.