When it comes to LVL, decisions should be based on solid knowledge. By Craig Kay
The definition of assume from dictionary.com is “to take for granted or without proof”. We all make basic assumptions about things in life, but sometimes these assumptions are wrong. We must never trust in what we assume, only what we know. Perhaps the slang definition that goes something along the lines of “When you ASSUME, you make an ASS out of U and ME” is more apt.
With the current shortage of structural building materials, it is understandable that one product that is out of stock is being substituted with another. It is in this environment that people with limited engineering knowledge are making assumptions that one product can be substituted by another, without fully understanding the full ramifications of that action. Structural building products are subject to Building Legislation as well as Consumer Law.
Given that structural grades of solid sawn timber have codified design strength and stiffness values that can be compared with one another, substitution is a little easier, but unless people understand the engineering jargon that makes up these design values, product substitution can be a trap for untrained personnel. While LVL sold in Australia needs to be manufactured in conformance to AS/NZS 4357 before it can be designed with AS 1720.1 Timber engineering standard, the LVL standard does NOT define standard LVL grades, acknowledging that each LVL has proprietary properties defined by the manufacturer.
The manufacturer defines a suite of design strength and stiffness values for their LVL, and to conform to the standard, these characteristic design values must be published for the purpose of engineering design in accordance with AS 1720.1.
To better understand what the published values mean, below is a brief description.
Stiffness (MOE) E: Stiffness is an indicator of the ability for an element to return to its original form after being subjected to a force. i.e., the greater the stiffness, the less the deflection under a given force. Many structural timber-based products are described by their stiffness e.g., MGP10, GL13 etc. Some manufacturers of LVL also mark their product with a numeric stiffness value descriptor, but in the absence of a codified grade for LVL, the user MUST search that manufacturer’s published literature to determine the actual design stiffness value.
Strength (MOR) f’b: Strength measures how much stress can be applied to an element before it deforms permanently or fractures. While somewhat related to stiffness, there can be circumstances even in wood-based products that one species or LVL layup may exhibit high stiffness but low strength, and vice versa. This difference can be quite marked between species of wood and, as a general rule, LVL has a much higher bending strength than a solid sawn piece of timber with a similar stiffness. This makes substitution from LVL to a solid sawn element more difficult.
Shear f’s: For most serviceability-based designs of timber elements in a domestic structure, the shear forces experienced are rarely close to their maximums. There are some, usually short span heavily loaded elements that may, if not correctly designed, experience a shear failure. In wood-based products where the grain is parallel to the span direction, a typical shear failure is in the horizontal plane evidenced by a translational movement of the upper portion of the element from the lower portion as shown in the diagram above.
Compression perpendicular to the grain (End bearing) f’p: Compression perpendicular to the grain capacity measures the ability of an element to support applied loads without deforming (crushing) more than 2mm, typically at a support. Each species has a different fibre capacity to these stresses, and while there is usually a correlation between stiffness, strength and compression perpendicular to the grain, a piece of hardwood with a similar stiffness and bending strength as some softwoods, will most likely have a significantly larger capacity in this area.
Joint strength Group (J or JD values): The joint strength of the wood-based product is a measure of the load capacity of nails, screws and bolts when fastened into the element. This value is closely related to the density of material but is also affected by the grain direction in EWP. The tooth capacity for type C fastener (nailplates) is a proprietary value developed for each different manufacturer’s nailplate. An experienced timber engineer can demonstrate many examples where sawn wood or LVL may present with relatively high stiffness values but have a relatively low density and thus have a low joint strength group. A good example is Picea abies (White Baltic, Norway Spruce) commonly used as wall framing. While it can correctly be graded to a MGP 10 and MGP 12 grade, it typically has a lower joint strength than other common framing material such as Radiata pine and Pinus sylvestris (Scots pine or Baltic pine).
For the sake of brevity, I will not discuss some other vital characteristic strength properties such as tension and compression parallel to the grain which are important if the element experiences any axial forces.
With sufficient knowledge of the intended function of the timber element, a suitably experienced and qualified person can reference AS 1720.1 or extracts from that document and thus compare the tabulated values for ALL the above strength and stiffness parameters between two solid sawn timbers, standard plywood and GLT grades, but NOT LVL.
As previously mentioned, the characteristic design values of LVL are proprietary to that manufacturer, so it is vital that any person seeking to substitute one LVL for another LVL, solid sawn or GLT, that the full suite of strength, stiffness and joint strength values for the particular LVL under consideration. The product simply marked with an E value (e.g., LVL 13) is telling a fraction of the story, and is actually either quite meaningless, or at worst, potentially misleading.
This is exacerbated by the recent phenomenon of volumes of LVL coming onto the market in response to the current severe shortage. While new entrants to the market adding extra supply is a good thing, there has been unfortunately quite widespread plagiarism of LVL product markings from existing suppliers, use of preservative chemicals that do not conform to any Australian standard and a general lack of supporting documentation mandated by AS/NZS 4357, and therefore creating a non-conformant product. And NO, it is NOT OK to recommend to prospective users that one can simply use the intellectual property and span tables from a competitor product to design the LVL.
Attempting a product substitution based upon only one or two structural properties out of a full suite of required strengths is perhaps best described by a line in An Essay on Criticism by Alexander Pope in 1709 which finishes with “fools rush in where angels fear to tread”. Product substitution for structural materials that affect the overall robustness of any structure is a serious business and should only be undertaken by suitably qualified and experienced people.
Craig Kay is the national product engineer for Tilling. For more information on this topic, contact Craig Kay and the Tilling engineers via email at firstname.lastname@example.org