Structural Design

Wood Structural Design

Wood can be used in many popular structural forms from the light duty repetitive small structures to the larger and heavier framing systems used in commercial projects such as arenas or storage facilities.

Because wood has a high strength to weight ratio, dead load is a smaller component of the total load factor than for heavier materials. Usually the lightest or least involved construction type appropriate for a given span that is capable of carrying the design load is the most preferable.

A table of typical spans is presented below to aid the designer in selecting an appropriate structural system.

table

 

Design Considerations

Wood is a naturally occurring renewable material affected by species, natural growth characteristics and moisture content all of which contribute to variability of its structural properties. Because of its cell structure, wood has different strength properties in different grain directions and is therefore categorized as an anisotropic material.

figure5.3Wood standards provide strength values for various wood products and connections. Links to some of these values can be found under the relevant Wood Product category.
Like all building materials, wood has unique design properties. By understanding the nature of these properties, designers are able to maximize the positive attibutes of materials and account for other effects. Unique properties that affect wood design include:

  • Hygroscopicity – The tendancy for wood to absorb moisture. This can affect both dimensional stability and strength.
    figure5.4 A

figure5.5

  • dimension calculator to account for shrinkage can be found under the Resources menu in Online Tools.
  • Duration of Load effects – Wood has a greater capacity to carry short term loads compared to sustained loads enhancing its performance in seismic and high wind zones.
    figure5.11
  • System effects – Wood systems have the ability to distribute and mutually support loads increasing the efficiency of wood framing systems.
  • Size effects – Research has shown that smaller wood members are stronger per unit area than larger members.

Detailed information on wood design and wood properties can be found in Introduction to Wood Design

Structural Design Standards

CSA Standard O86-09 Engineered Design in Wood is the current referenced wood design standard in Canada. This consensus-based standard is referenced by Part 4 of the National and Provincial Building Codes and is written in the limit states design (LSD) format. It provides resistance equations and specified strength values for lumber, glulam, panels, composite components such as stressed skin panels, shearwalls and diaphragms, fastenings, poles and piles and proprietary structural wood products such as I-joists.

The Canadian Wood Council’s comprehensive Wood Design Manual includes member and fastenings design examples, tables and reference material as well as a copy of CSA O86.

Wood designers in the US can use either an Allowable Stress Design (ASD) format or a Load and Resistance Factor Design (LRFD) approach. The referenced design standard is the ANSI/AF&PA National Design Specification (NDS) for Wood Construction.  For further information on this and related wood publications visit the American Wood Council (AWC).

 

Additional helpful tools

  • WoodWorks software (Canadian and US versions available, includes Sizer, Shearwalls, Connections, Editable database, NDS and SDPWS (pdf, US version) or CSA O86 (pdf, Canada).
  • The Span Book (Cdn)