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The Design Possibilities of Engineered Wood Products
Time:2012-03-02

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 product
standards. 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, it
helps 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 provides
a 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 to
shrinkage. 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.