Laminated Veneer Lumber – from Forest and Wood Products Research and Development Corporation

Topics Covered

Background

Application

Specification

Manufacture

LVL Beams

LVL Truss Members

Service Classes

Properties

Background

LVL (Laminated Veneer Lumber) is quite similar to the vertically laminated glulam beams, but is made in a similar manner to plywood. LVL is manufactured from veneers that are rotary peeled, dried and laminated together under heat and pressure with an adhesive. The grain on each ply is usually oriented in the same direction as the length of the member. Because LVL is a manufactured product, it can be made to very large dimensions. Standard Large sheets of LVL can be ripped or crosscut to give a very large range of products.

Application

For most applications, LVL is intended to provide a structural function only. The surface finish is usually of low architectural quality with rough glue lines often visible. However, where required, products can be fine sanded and painted to give a similar finish to solid painted timber.

LVL can be used wherever sawn timber is used, but because of the long continuous lengths and the high structural reliability of the product, they are often used in the following applications:

•        High-strength applications such as the flanges in timber I-beams and ply-webbed box beams. The major advantages of I and box shaped sections are their straightness, uniform depth, dimensional stability, comparative lightness and the ability to cut holes in the web of the beam through which services can be installed.

•        Structural LVL can also used as a structural beams – floor joists and bearers, wall girts, lintel beams and roof framing beams. Structural LVL has many advantages when used as a framing member –including high structural reliability and strength, dimensional stability with little tendency to warp, bow or twist.

•        Formwork – The uniform stiffness and reliable strength properties of glulam make them very useful as formwork bearers. Some LVL members are graded and marketed specifically as formwork members.

Although LVL is manufactured using a fully weatherproof glue, it is not recommended for use in permanent exposure to weather. Where such exposure is planned, products can be specially manufactured with a preservative treatment for the particular application.

Specification

The following should be included in the specification for LVL:

•        Product brand name

•        Grade

•        Service class

•        Pre-finishing/protection

•        Handling and storage

•        Preservation

Manufacture

The diagram shows the structure of manufactured LVL members. The grain can be seen running essentially in the same direction. Some LVL members can be made with a few laminations laid up at right angles to enhance the shear strength of the member. These are known as cross-banded LVLs and may need to be specially ordered, as it is not a commonly stocked item.

 LVL is manufactured as sheets, generally 1.2 m wide and with a maximum thickness of around 65 mm. (some manufacturers can produce 75 mm thick sheets in some species). The sheets can be any length at all, as the manufacture is a continuous process, though “off-the-shelf” LVL is generally a maximum of 7.2 m for transport convenience. Longer lengths can be manufactured as special orders if required.

Ripping (longitudinal cutting of the sheets) can produce prismatic members with standard depths such as 100 mm, 150 mm 200 mm, etc. The range of thicknesses and depths of prismatic members varies from manufacturer to manufacturer.

Check with your local suppliers for the range of sizes, species and grades that are commonly available.

LVL Beams

LVL prismatic members can be used as beams. The possibility of long length, large depth and reliable strength with uniform stiffness, makes LVL an appropriate material for carrying loads over long spans. LVL beams are normally straight, as the most cost-effective production is to cut a number of straight members out of one sheet, but it is possible (though costly) to cut curved or tapered members out of a single sheet. If this is done, care with sloping grain is needed in determining performance.

It can be used as primary roof members such as strutting beams, hanging beams, valley or hip rafters and ridge beams. It can also be used for long span joists and floor bearers.

When combined with stiff connections at the knee and ridge, LVL can be used in portal frames. Deep members may need fly bracing to prevent lateral torsional buckling of beams under wind uplift loading. Where large shear forces need to be carried (particularly in the connections), then it may be necessary to use cross-banded LVL.

A number of manufacturers publish span tables that facilitate the selection of appropriate member sizes in common beam applications. Otherwise beam design of LVL members is the same as for other timber bending members.

LVL Truss Members

LVL has high tensile strength relative to sawn timber and high reliability. This makes it ideal for use in trusses.

Truss members can be made from single pieces of LVL, but for very large loads, LVL can be glue-laminated to make large cross-section square or nearly square members. In all truss applications, the limiting factor is the connection, so for very large loads, complex high capacity connections will be required. This may require testing of prototypes of the joints to verify the capacity.

Service Classes

LVL, in common with most of the other manufactured wood products uses glue as a key structural element of the assembly. In order to give appropriate performance under all conditions, the glue must be of suitable durability.

LVL is quite appropriate for use in protected environments, but if an external use is planned, then the supplier or manufacturer will advise on the most suitable LVL product for the application.

Properties

The properties and dimensions of each LVL product are peculiar to each manufacturer. Designers must access the manufacturer’s information on the product they wish to use in order to obtain the properties for that particular product. In most cases, this is provided on a web site, but the supplier has a role in providing supporting documentation for the products they sell.

LVL has reliable and consistent structural properties, and have strength and stiffness capabilities greater than the individual veneers from which it is manufactured. The maximum effect of a single defect in an LVL laminate is very small as the laminates are so thin compared with the thickness of the whole member. The properties of LVL show much less variation than those of both sawn timber and glulam. In fact, LVL has a coefficient of variation in structural properties that is similar to Australian-produced hot rolled steel sections.

LVL has the vast majority, if not all, of the laminations oriented with the grain parallel to the longitudinal axis of the member. This gives orthotropic properties in a similar way to the properties of sawn timber, rather than the nearly isotropic properties in the plane of plywood.

There are no generic grades for LVL. Each manufactured product has its own grade. Only products that conform to <AS/NZS 4357> can be used with <AS 1720.1>. There is a limited (but growing) number of products that comply and are commercially available in Australia and New Zealand. A designer would specify these by their product name.

The structural design of LVL is very similar in principal to the design of other timber members. The same clauses of AS1720.1 are used for sawn timber and LVL.

Source: The Forest and Wood Products Research and Development Corporation

For more information on this source please visit The Forest and Wood Products Research and Development Corporation

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