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Introduction
Wood products have a very long history of providing safe,
comfortable shelter. Over the past 60 years, the development of gluing,
connection and grading technology has resulted in new engineered wood
products that extend even further the possibilities for wood
construction.
It is estimated that over 90% of existing houses in North
America are wood-frame construction For duplexes, row houses and
three-storey apartments, about 85% of buildings are wood-frame
construction.In addition, many low-rise commercial buildings are built
with wood. Engineered wood products (EWPs)open up the possibility of
using wood in more of these buildings due to their increased span
capability and performance characteristics.
Products manufactured to achieve targeted engineering
properties are known as engineered wood products.
They use manufacturing techniques, mechanical evaluation and special
connectors or adhesives to vastly increase the reliable load-carrying
capability.
The performance of engineered wood products is based on
testing and engineering to meet productstandards. These standards result
in both generic and proprietary products to meet market needs.The use of
engineered wood products has grown dramatically over the past 20 years,
and is still increasing.Designers and builders are attracted to the
strength, quality and reliability of these products, with higher
purchase costs offset by ease of construction and increased system
performance in span and loadcarrying capacity.The focus of this brochure
is on these innovative engineered wood products that are used for
structural applications like beams, joists and columns.
Products and Manufacturing
To understand the special properties of engineered wood
products, ithelps to begin with solid-sawn wood products that are still
commonly used for joists and beams.Modern saw mills have equipment for
scanning logs to select the cuts that will extract the best value and
quality from a log, and minimize the effects of naturally occurring
knots and grain directions that reduce strength.The logs have a wide
range of quality and moisture content that can extend well above 100%,
expressed as the ratio of the weight of water to the weight of oven-dry
wood fi bre.After sawing, the lumber is typically dried to a moisture
content of 19%.
Next, the lumber is planed and graded, based on visual
observation by a trained grader who, based on strict rules, assigns a
grade based on the size and location of knots and other characteristics.
Because of variability in the wood material, dimension lumber is
assigned a strength value that is well below its average capability.
Using lower strength values providesa level of comfort that the material
is adequate for a certain application, but does not allow the designer
to take maximum advantage of the actual strength each piece would
exhibit if it were, for example, proof-tested to determine its actual
capability.
Engineered wood products differ
from visually graded solid-sawn lumber products in a number of ways.
First, the manufacturing processes for engineered wood products
generally require the wood have a lower moisture content, usually less
than 15%, resulting in a fi nished product that is more dimensionally
stable and less prone toshrinkage. Second, the manufacturing processes
generally remove strength-reducing characteristics or at least
distribute them so that their overall effect is more predictable. Third,
EWPs are subject to structural property qualifi cation testing and daily
quality control.
General Information
Engineered wood products are similar to solid sawn wood products in many
ways, but there are some fundamental differences that designers and
builders must consider.
This section provides a brief summary on several topics, and more
detailed information
is included in the product sections.
Product Acceptance
All the wood products used for construction are specifi ed for use
in one way or another. For small buildings, dimension lumber can be used
as prescribed by the applicable Building Code.
For larger building applications, engineering design is required to
provide assurance of structural adequacy.
In the case of engineered wood products, there are several avenues for
ensuring their structural adequacy. Some products are covered by generic
standards that stipulate manufacturing, quality control and quality
assurance procedures that are linked directly to published design values
for all such structural products.
Other products are proprietary – they
have company-specific design values that are based on both product
standards and company procedures, and third-party evaluation services.
Structural wood products must use glues that meet stringent
water-resistance criteria. This is to ensure that the structural
performance of products that are accidentally or unavoidably exposed to
rain or moisture will not be adversely affected. This does not mean
these structural products are suited for continually wet conditions, but
does provide a safety margin in the event of temporary exposure to rain
and moisture.
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