Want to know more about building floors out of CLT?

CLT floor

This extract from The CLT Handbook – free to download for all TDUK members – will introduce you to the main types of CLT floor.

There are various ways to construct a CLT floor, which can be grouped into three main categories: slabs; ribbed slabs and cassettes; and CLT concrete composite floors. All three of these are suitable for prefabrication.

CLT floor slabs

A CLT slab is the simplest form of CLT floor. It’s a simple CLT panel that, if necessary, can have finishes and insulation added. The CLT panel alone carries the load and distributes it to the underlying structure. The build-up of the panel – in terms of number and thickness of layers – is determined by the structural, dynamic, fire and acoustic requirements.

To meet noise and fire safety requirements, there will usually need to be additional layers on the top, bottom or sometimes both. The cross-laminated boards of the CLT slab give transverse stiffness to the floor structure, and also little in the way of moisture-related movement.

Wood City Office Helsinki, Finland; (c) Stora Enso and Woodcon

Ribbed slabs and cassettes

A ribbed CLT slab is a CLT slab with glued ribs that provide extra stiffness. To enable the floor structure to handle larger loads and longer spans, glulam joists are bonded to the underside or top of the CLT slab. The floor structure comprises a CLT slab and web joists with or without flanges.

Additional details/layers are often required to meet noise and fire safety requirements. The voids in the floor structure can be filled with mineral wool insulation, for example, and the underside can be fitted with a sprung suspended ceiling of plasterboard on battens. Even better acoustic separation can be achieved if the sub-ceiling is kept entirely separate from the floor structure above. Pipes and wiring can also be run through the voids.

In a cassette floor, spaced web joists are sandwiched between two CLT slabs to create a stressed skin.

CLT concrete composite floor

A CLT concrete composite floor mainly comprises two parts: a CLT slab on the underside and an in-situ concrete topping. The ‘composite’ refers to the fact that CLT slabs are working in composite with the concrete.

Usually, notches in the CLT or some form of shear connector are used to join the CLT to the concrete and so increase the bending stiffness of the structure.

From a structural point of view, this type of structure is highly efficient, as you make optimum use of the materials’ properties – i.e. the compressive strength of the concrete and the tensile strength of the wood. A CLT concrete composite floor has higher bending stiffness than an equivalent wooden floor structure of the same depth. This means that longer spans can be achieved.

In addition, the dynamic performance is generally better, since the damping is often greater. Another positive aspect of this type of structure is the stiffness of the concrete slab in-plane, which means that horizontal loads caused by wind can be spread evenly between the shear walls. The acoustic performance is also often better with a CLT concrete composite floor.

Stora Enso Hylte Mill, Sweden; (c) Stora Enso

Designing floors with CLT – some important considerations

When designing wooden structures, the engineer needs to take a number of factors into account. These include:

  • load duration
  • service class
  • the load’s direction in relation to the grain.

The last of these is particularly important, since CLT is made up of layers of boards in different directions.

CLT and dynamic effects

Dynamic effects that occur, for example, when people walk on a floor, affect our perception of the quality of a building. The performance of the floor depends on the floor’s stiffness, mass and damping. And the floor’s stiffness depends on the floor’s span and panel layup.

Finishes such as partitions also affect the stiffness of floors, but as their effect is very hard to estimate accurately it is conservative not to take into account the contribution of partitions to the stiffness of a floor.

The mass of the floor depends on the density of the floor panel, its size, the mass of the finishes, and a realistic proportion of live load (generally 0.1 × live load for standard residential or office floors). The damping ratio of the floor depends on the build-up of the floor and finishes such as partitions.

Vibrations caused by people are a problem that occurs in the use phase. People walking around on a floor often tolerate greater vibrations than people who are sitting still, reading or writing. Vibrations may be felt by the person causing them, or it might be the activities of others that cause the vibrations.

WoodCity Office, Helsinki, Finland; (c) Stora Enso and Woodcon

It is also important to note that CLT panels that are continuous between rooms can transfer vibrations between different rooms. Vibrations from a neighbouring room are often felt to be more irritating than when the source of the vibrations is in the same room. This is a particular problem where the neighbouring space is under different ownership. One way to prevent this is to stop the CLT slab at the party wall line.

Fire safety and acoustic performance

The fire safety design of CLT floors needs to be carefully coordinated. A combustible material demands a different approach to fire safety: the increased fire load and the risk of collapse if it continues burning need to be considered. While in some cases it may be possible to leave the CLT visually exposed, in many cases it will be necessary for the CLT to be fully encapsulated for fire safety.

As well as fire safety, the acoustic performance will often determine the floor build up. For residential buildings in multiple occupation, where there are obviously tighter acoustic requirements than a single family house, most of the CLT will usually need to be covered with additional layers to achieve suitable acoustic separation.

This is a condensed and edited extract from the ‘Floors’ chapter in The CLT Handbook. The Handbook is free for TDUK members to download, and provides a comprehensive, in-depth investigation of CLT buildings – with information on fire safety, worked examples, sample build-ups and much, much more.