Slide 28 of 67
- Effect of Lumber Species on Strength
- Prescription Standard for retrofit work: Use 4-inch nail spacing and 8 penny nails (top row)
- If using douglas fir–larch, you will get much stronger shear resistance than with hem fir or redwood.
- For an equivalent allowable shear in Hem-Fir, you would need to increase the nail spacing to 3-inch spacing
- For an equivalent allowable shear in Redwood Open Grain lumber, you would need to increase nail size to 10 penny.
- Some Information on Redwood Species
- Two types of redwood species for strength considerations: open grain and close grain.
- Because lumber in a covered structure is not always visible or accessible, verification of the grain pattern in existing buildings may be difficult or unreliable.
- For this reason, structural designers commonly use the 35% reduction for the less dense open grain.
- When verification of the denser close grain is possible, shear wall strength values may use an 18% reduction like hem fir.
- Older buildings are more likely to have close grain from old growth trees.
- We know that the lumber species is important……so how do we determine the existing species?
DETERMINE EXISTING LUMBER SPECIES
- Structural designers in California normally assume that existing lumber is douglas fir-larch.
- You should become familiar with what lumber is assumed by structural designers in your state
- This is very important to remember, because when retrofit work uncovers existing redwood or hem fir lumber, contractors and inspectors should notify the structural designer so that additional shear wall strength can be provided.
Otherwise, hem fir framing will provide only 82 percent and the redwood will provide only 65 percent of the shear wall strength intended.
- Keep in mind that prescriptive standards for retrofit work generally do not consider the effect of different lumber species, but they should.
- Although not required in such standards, additional sheathing fasteners are recommended for the softer lumber species.
- See the shaded portions in the table - Effect of Lumber Species on Strength.
- When termite or fungus damage requires the repair or replacement or existing framing members, contractors should use douglas fir-larch lumber to replace all studs, blocking, sill and top plates.
- Douglas fir-larch should be used for both pressure treated and non-pressure treated lumber.
- Denser lumber always means better fastener strength and, as a result, stronger shear walls.