Structural Systems – Laminated veneer lumber
Panels of LVL are cut into structural members which have high strength and stiffness.
LVL is suited to structural applications such as beams, rafters and columns in a wide range of buildings including houses, commercial, industrial and rural structures. Some special LVL has a small number of veneers laid perpendicularly (cross banded).
LVL has a number of advantages over other wood-based materials:
- Small logs can be made into large dimension LVL products.
- Long lengths of LVL are available; up to 12m ex stock from merchants (or 18m by arrangement).
- The wood resource can be optimised by grading and selecting veneer for different parts of an LVL cross section and making a range of products with different properties.
- Structural properties of LVL are very uniform because the randomised layers of thin veneers are pre-graded for stiffness (coefficient of variation for modulus of elasticity less than 5%).
- LVL members have high strength because of the low variability and randomised wood properties in thin layers.
- LVL can be cut and machined with normal woodworking tools.
LVL is often used to complement the use of sawn timber in domestic construction.
In commercial or industrial structures it is often used as a wood-based alternative to structural steel or reinforced concrete.
Generally LVL is chosen when sawn timber is not strong enough to do the job, or long lengths are needed.
- LVL is particularly well suited to the following applications
- Rafters and joists
- Lintels, beams and framing members
- Truss chords
- Portal frames
- Scaffold planks
- LVL is manufactured in Australasia in mills operated by Carter Holt Harvey Futurebuild (at Marsden Point in Northland and Nangwarry, South Australia), Nelson Pine Industries (Nelson), WESbeam (Perth, Western Australia) and Juken Nissho Ltd (Gisborne).
The main species of wood used for the manufacture of LVL in New Zealand is Radiata Pine. Other species such as Douglas fir, larch, and eucalyptus, can pose manufacturing problems. WESbeam makes LVL from maritime pine.
The .pdf below describes the manufacturing process for LVL, including:
- The Adhesives used in manufacture
- The option of special orders of cross-branded veneer to reduce cupping in deeper sections or where strength is required perpendicular to the grain
- Specification considerations
- Low variability
Manufacture of LVL (.pdf)
- Supply of LVL is usually through merchants, timber suppliers or the manufacturing companies.
Delivery from mill to merchant usually takes 5-7 working days for standard lengths, but may be longer for preservative treated material.
The nominal 1200 mm wide billets are usually made in thicknesses from 35 to 105 mm. The billets are sawn into smaller sizes which are integer factors of the 1200 dimension, such as 150, 200, 240, 300, 400 and 600 mm. A summary of available stock brands and sizes is shown in Table 1. Other brands with specified properties also available (e.g. hyCHORD is a product specially designed for use with truss plates in light timber frame buildings).
- The pdf below provides the following performance and design information for LVL:
- The importance of specification of particular brands of LVL to identify the actual properties assumed for design
- Design standards
- Strength modification factors
- Design of joints
- A design method for engineered I-joists
- Fire resistance information
- Effects of moisture
LVL Performance (.pdf)
- LVL is not normally treated with any preservative chemicals or waterproofing agent. Exposure to weather during normal construction periods is not cause for concern because the adhesive is a waterproof phenol formaldehyde resin.Untreated LVL may decay if the moisture content is above 20% for extended periods.
LVL is not recommended for permanent use where directly exposed to the weather, or where exposed to high humidity interior conditions such as in aquatic centres.
LVL scaffold planks are used untreated, but are plant items and should be subject to strict rules on inspection, maintenance and proof testing. The surface veneer in LVL will respond quickly to cycles of drying and wetting to form surface cracks if exposed to weather for extended periods.
Deeper sections, common with LVL, will cup if unevenly exposed to moisture. For these reasons, LVL members should not be used on outdoor structures such as pergolas or as joists for exposed decking, even if they are preservative treated.
Treatment of LVL is difficult because glue lines are a barrier. Most treatment providers use light organic solvent preservative (LOSP).
This treatment can be recognised by its smell, or the waxy appearance of the residual solvent on the surface.
Preservative treated LVL can be identified either by a green dye or, for clear finish, branding attached to the end of each treated member.
The H3 LOSP treatment uses a spirit-based solvent with active ingredients of tributyl tin napthenate (TBTN) and permethrin or other approved chemicals such as propiconazole or tebuconazole.
Because this type of treatment only treats the outer envelope of the product, it is recommended that if the envelope is broken by cutting, drilling or machining then subsequent repair of the envelope be undertaken.
The recommended remedial action is to brush on a generous coating of a suitable timber preservative such as Metalex or similar available from merchants.
Leaching of some treatment chemicals can occur if wood is continually wet and some chemicals are affected by UV rays.
Whilst it is recommended above that LVL should not generally be exposed to weather, there are exceptions.
If LVL elements of structure are exposed to weather, then in addition to LOSP treatment, the LVL members should be protected with details like durable waterproof caps and flashings, cavities for drainage and drying, suitable claddings and well-maintained paint systems, so that the LVL is not subjected to repeated wetting and drying cycles.
LVL which is preservative treated to level H3 with LOSP to the requirements of AS/NZS 1604.4 will provide a level of protection equivalent to H1.2 or H3.1, which gives protection (in accordance with NZS 3602) for members protected from the weather but exposed to ground moisture (not in ground contact) or with a risk of moisture penetration conducive to decay.
NZS 3640:2003 specifies hazard categories H1.2, H3.1 and H3.2, which do not apply directly to LVL because NZS 3640 applies to round and solid timber only.
The New Zealand Building Code B2/AS1 refers to AS/NZS 1604.4 for LOSP treatment of LVL in clause C1.1.2, but this does not currently include categories H1.2 and H3.1, hence the use of H3 treatment.
Note that LVL cannot be treated with CCA to achieve a H3.2 level of protection so it cannot be used for members exposed to exterior weather conditions and dampness applications as defined in NZS 3602. H3.2
LOSP treatment (with copper based preservatives) is not recommended for LVL.
Timber is often used where chemical deterioration eliminates the use of other structural materials.
Since wood is relatively inert chemically, under normal conditions it is not subject to chemical change or deterioration. LVL is resistant to most acids, rust and other corrosive situations including hide curing complexes, fertiliser storage and swimming pools.
The face veneers of LVL may contain minor blemishes such as knots, small knot holes, scarf joints, or glue marks.
The surface may be planed or sanded to enhance its visual appearance. The planed or sanded surface can be left unfinished, or can be painted or stained.
If the surface is sanded or planed, the product is more likely to be susceptible to surface swelling and cracking due to peeler checks becoming exposed in the outer veneer. Any exposure to weather for long periods of time can result in swelling and discoloration.
LVL is manufactured by converting wood resources from sustainable forests to upgraded products.
The manufacturing process results in very few emissions of any kind. Radiata pine veneers are sourced from sustainably managed plantations in which the forestry management companies adopt comprehensive environmental management systems to manage the forestry operations and focus on required environmental outcomes.
LVL is produced in high-technology mills which have strict environmental controls on amenity features and emission levels.
These controls have been developed in consultation with district and regional council staff, as well as tangata whenua, community and environmental groups.
Burning of wood waste generated in veneer preparation and billet processing provides most of the on-site heating for LVL production. This results in a major reduction in CO2 emissions compared with burning fossil fuels, and puts LVL manufacturing into a greenhouse neutral situation. In addition, carbon is stored in LVL wood products while they are used in buildings or furniture.
Emissions from veneer drying operations are contained and fed into the furnaces, providing the air needed for combustion and removing any volatile organic compounds generated in the veneer drier.
The adhesive used in the manufacture of LVL is produced from a phenol formaldehyde resin, which sets permanently under controlled heat and pressure.