Q:

What is a span table used for in the lumber industry?

A:

Span tables are used to determine the dimensions and wood class necessary for load-bearing wooden parts, such as beams, studs, joists and rafters, in a construction. Span tables are published by lumber producers so that their customers can check what loads a given dimension and class of lumber can support without either breaking or bending too much.

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The capacity of a wooden construction element depends on many factors, such as the species of wood, the grade, the dimensions and the distance between support points it has to span. Span tables provide values for both strength and stiffness for the all the different sizes and types of lumber sold by a producer.

To use a span table it is necessary to first calculate the stress on each building element, both the dead load (from the building) and live load (from its occupancy and use, such as parking a car in a garage). The stiffness needed varies between different parts of a building. For example, a ceiling with plaster needs relatively high stiffness, as it cannot bend much without causing cracks. Stiffness needed is only calculated against the live load. Standard loads for different parts of a house are defined in the local building code. An example of span tables, in this case for southern pine lumber, can be found at SouthernPine.com.

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