Stress Combination: Wednesday's Word(s) Of The Week

To design structural members, engineers need to know the material properties and shapes of the parts they are using. Most materials come in a variety of strengths. There are grades of steel, compressive strengths of concrete mixes, and for glulam we have stress combinations.

Stress combination refers to the specific composition of lumber in a glulam beam and the associated material properties of the finished glulam member. Specific stress combinations are designated with a Combination Symbol. A table of industry standard Combination Symbols is available in Table 5A Expanded and Table 5B of the NDS.

Because the Combination Symbols listed in the NDS are industry standards, the allowable stresses and other properties can be used for design regardless of who produces the glulam.

These tables in the NDS are actually copies of tables in ANSI 117 "Standard Specification for Structural Glued Laminated Timber of Softwood Species." ANSI 117 also includes the lay-up requirements for each combination - where and how much of different wood grades are required in the finished beam.

We will have more information on what the different Stress Combinations mean in upcoming weeks.

*Wednesday's Word Of The Week is a feature on Unalam's Wood Times Blog. Each Wednesday our structural engineer, Rik Vandermeulen, will discuss a new term associated with glulam manufacturing. He will do this until we run out of words. If there is a timber or glulam term that you have heard of and want to know more about, let us know in the comments.