Cross laminated timber-XLAM

What is cross laminated timber "XLAM"? The English abbreviation Xlam means cross-laminated timber, whereas the term glulam means glued laminated timber.

Cross laminated timber can be mark in several different ways:

- XLAM (Cross-Lam),
- CLT (Cross Laminated Timber)
- KLH (Kreuz Lagen Holz)

10 advantages of solid timber houses made of cross-laminated panels

  1. Structures made of cross-laminated panels are able to successfully withstand earthquakes or fires as the CLT panels exhibit great stability and stiffness in the plane.
  2. The high level of prefabrication ensures the greatest possible freedom in architecture and the highest possible execution quality.
  3. The construction process is very quick and simple, with no need for highly-qualified workforce.
  4. The normal wall thickness for buildings ranges from 9 to 12 cm, which enables around 10% of additional living space, i.e. greater net floor surface of the building with the same external dimensions in comparison with other types of structures, which is of great advantage for the investor.
  5. The price of RC foundation structure/basement is lower on account of up to 5 times smaller loads in comparison with traditional construction.
  6. Since timber is an insulation material in itself, a solid timber house is virtually devoid of thermal bridges; instead, it only has certain spots where the insulation rate is lower.
  7. A solid timber structure does not cause phase delay times in the heating dynamics and does not accumulate heat; as a result, the structure does not overheat in the summer and the costs of air conditioning are lower.
  8. In comparison with other structures, solid timber structures have significant advantages also in terms of seismic safety as such buildings can withstand even the greatest seismic forces without any substantial damage.
  9. The solid timber panels are fireproof (i.e. are difficult to ignite) and burn away only on the surface, while the interior timber core retain its load-bearing capacity; the fire progresses no more than two to three centimetres into the structure, after which it runs out of oxygen needed to burn.
  10. The solid CLT panel construction system represents the optimum alternative to the traditional type of construction, which is why users otherwise more favourably inclined towards concrete and masonry structures find it easier to decide for this method.

Technology

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Structural CLT (cross-laminated timber) elements represent a contemporary composite material which exhibits more levelled and superior mechanical and deformation features in comparison with the structural elements made of solid and glue-laminated timber, especially in cases when the layers are glued perpendicularly to wood fibres. The cross-laminated elements are composed of cross-stacked timber lamellas or planks bonded together under great pressure along the surface, thus producing a thicker, solid panel.

The basic raw material is usually coniferous tree wood that has been technically dried to achieve 12% moisture content (±2%), which ensures the natural protection of timber against fungi, insects and moulds. Depending on the intended use and load-bearing capacity requirements, the panels are made in an odd number of layers, namely 3, 5, 7 or more, up to the maximum thickness of 60 cm.

The individual layers of lamellas are oriented towards one another at an angle of 90°, while the transverse section of the panel is symmetrical. In order to meet the specific load-bearing needs of different types of cross-laminated panels in particular directions, lamellas may be oriented in the same direction in a maximum of two successive layers. Such panels are usually made at a customer's request and are different from standard panels. In special cases where greater bending strength needs to be achieved, panels can be made with a double exterior load-bearing layer. Alternatively, panels with a double central layer can be produced to accommodate the need for greater shear strength.

Remarkable mechanical properties

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Cross lamination enables the load-bearing capacity of timber panel elements in both directions, increases the load-bearing capacity and prevents splitting in the direction perpendicular to the wood fibres, while also ensuring dimensional stability of panels, which means that the timber activity (such as swelling and shrinkage) can be reduced to negligible values. The deformations in the plane of such a panel are therefore indeed negligible, while amounting to app. 2.4 mm/m in the direction perpendicular to the panel's plane for every percent of the change in the timber's moisture content. Load transfer

Due to cross-oriented lamellas, the components can bear the load in two perpendicular directions, which enables their use both as elements of walls and ceilings, while the primary direction of load transfer corresponds to the orientation of fibres in the outer layers of boards. The panels may reach dimensions to accommodate the construction of floor walls in a single piece, with previously cut out openings and surfaces prepared for jointing, with the maximum lengths of components adapted to transport requirements.

By properly joining components together to form a complete structure, the panels used as wall components can take on a large horizontal load both in the wall's plane as well as in the direction perpendicular to it. Suitable architectural and structural design of the building coupled with adequate load-bearing capacity of the Xlam panels and satisfactory jointing of walls and panels can thus ensure excellent resistance of the cross-laminated panel structure to the impact of horizontal loads such as winds and earthquakes.

High degree of fire resistance

The solid CLT panels are also exhibit better fire-resistance than usually witnessed in timber structural elements. In fact, their fire resistance can even surpass that of steel and reinforced concrete components. The ability of timber to conduct heat is very low as it conducts it 300 to 400 times slower than steel. The components char slowly from the surface towards the inside, thus reducing heat conduction and preventing the oxygen from accessing the timber.

In view of the speed of wood burning, the computational method applied to cross-laminated panels shows that the normal fire resistance of 3-layer panels is 30 minutes; 5-layer panels exhibit fire resistance of 60 minutes, and the fire resistance of panels with greater thicknesses and multi-layer walls can amount to 90 minutes or more. In line with the fire regulation requirements, even higher values of fire resistance (90 min, 120 min etc.) can be supported by computation, but depend mostly on the thickness and layers of panels and their cladding.

Safe adhesive

For purposes of adhesion, CLT panel manufacturers mostly use a one-component polyurethane adhesive called Purbond that utilizes the natural moisture of timber to bond elements together without the need for solvents and formaldehyde.

The adhesive is applied along the entire length of the timber boards, around 200g/m2, while perfect adhesion is achieved in the press. Since none of the adhesives used contain formaldehyde, the glued structures are entirely comparable with natural solid timber. Due to its high bonding ability, this odourless adhesive enables spare consumption.

Fasteners

CBD 05 detajl temeljna ploscaCBD 04 detajl medetazna ploscaThe cross-laminated wall panels are anchored into the floor/foundation structure by means of metal anchors. Hold down anchors are used in order to take on the uplift force resulting from the wall rotation in the event of a horizontal load, while shear connectors are used to take on the shear force appearing when the wall slips. The anchors are screwed into/fastened onto the foundation by means of foundation bolts; they are usually fastened onto timber panels with wood screws or ring shank nails with a minimum diameter of 4 mm and ranging between 40 and 60 mm in length.

Mid-floor jointing detail A similar method is applied when it comes to fastening walls to mid-floor panels, the only difference being that only wood screws or ring shank nails are used here to fasten the metal angle bracket both to the wall and the mid-floor panel. The joints between individual panels are usually accomplished by means of bolted tension splices. The types of anchors used, the sizes of screws and nails as well as recommendations as to the construction execution itself differ somewhat from one CLT panel manufacturer to another.

The manufacturers thus suggest the application of certain joints and details within the system of jointing CLT panels, but the proposed solutions must undergo computational evaluation and confirmation by the structural engineer.

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