Last edited 05 Dec 2016

Glulam

Glulam.jpg

Contents

[edit] Introduction

Glued laminated timber, popularly known as glulam, is a manufactured timber product. It is made by bonding together individual laminates (layers) of solid timber boards with durable, moisture-resistant structural adhesives.

Glulam can be used for a wide range of purposes, from joinery timber to large-span structures. Glulam structural members can be used as vertical columns or horizontal beams, and their adaptability enables them to be used for a variety of cross-sections and curved, arched shapes to meet varied end-use requirements. Glulam beams are often used as an alternative to steel or concrete for constructing the roof of buildings such as swimming pools, sports halls and supermarkets.

The laminating process allows a single, large structural member to be manufactured by laminating a number of smaller pieces of timber. It also allows the timber to be used for longer spans with heavier loads and more complex shapes allowing more design flexibility than with traditional timber construction.

Tree species that are particularly suited to glulam manufacture include Siberian larch, Douglas fir, spruce and oak. As with other engineered timber products, the overall amount of timber used is reduced in comparison with solid sawn timbers. This is achieved by reducing the negative impact of knots and other defects.

[edit] Laminates

The laminates of timber boards are layered and glued together so that the direction of the grain of each board runs parallel with the longitudinal axis of the member being fabricated.

Vertical laminations are not often used for structural timber members and are unsuited to curved members. Horizontal laminations are the most commonly used method.

The laminate thickness is determined by the depth of the member, but there should be enough layers to allow the end joints to be properly staggered. Laminate thickness for curved members is determined by the radius to which the laminate is to be bent, the tree species, and the quality of the timber.

Joints

The joints in consecutive layers should lap whichever of the following is the greater:

  • Twice the board thickness.
  • One quarter of the board width.

Scarf (two diagonal faces fitting together) and finger joints (interlocking cuts fitting together) can be used. Scarf joints should have a minimum slope of 1 in 12, but in the compression edge of a beam this can increase to 1 in 6. Finger joints are generally more economical in the use of timber than scarf joints.

[edit] Production

Production of glulam is standardised by EN14080 Timber structures. Glued laminated timber and glued solid timber. Requirements, and all standard beams must be CE-marked.

During the production of glulam the following factors should be tightly controlled:

  • The strength of the purchased timber, whether pre-graded or visually graded in-house.
  • Moisture content of the timber. The timber should have a moisture content equal to that which the member will reach in service.
  • Temperature and humidity in the plant during production, the curing chamber whilst glue sets, and in the area where the timber is stored.
  • The strength of glue lines and finger joints.
  • The age and storage conditions of the glue.
  • The mix proportions of the glue.

Laminates should be planed before gluing. The depth of the planer cutter marks should not be greater than 0.025 mm. The gluing process should be carried out within 48 hours of the planning to reduce the risk of the planed surfaces becoming contaminated or case hardened.

Typically, curved members have their feet secured in purpose-made steel shoes and bolted to a reinforced concrete foundation.

[edit] Benefits of glulam

The benefits of using glulam include:

  • Sustainability: The manufacturing process consumes less energy than steel or concrete.
  • Appearance: Designers often choose glulam over steel or concrete for its ‘natural’ appearance.
  • Adaptability: The lamination process is very flexible allowing a wide range of sizes and shapes to be produced.
  • Strength: It has a good strength-to-weight ratio in comparison with steel and concrete which improves the buildability of glulam structures.
  • Durability: The timber species as well as the type of adhesive and preservative (chemical treatment) used all impact on the durability. With the right treatment, glulam can provide effective low maintenance solutions to problematic environments such as swimming pool roofs.
  • Fire resistance: Large section timber elements perform well in fires as they char at a known rate and unlike steel resist deformation. Fire protective finishes can be applied to glulam members to further increase performance.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] External references

  • Glulam
  • Buckland Timber
  • ‘Building Construction Handbook’ (6th ed.), CHUDLEY, R., GREENO, R., Butterworth-Heinemann (2007)