Bending and axial tension
Members subjected to combined bending and axial tension should be proportioned to satisfy the interaction equations
Roof slope to prevent ponding
Roof beams should have a continuous upward slope equivalent to 1/ 4 in/ft (20.8 mm/m) between a drain and the high point of a roof, in addition to minimum recommended camber to avoid ponding.
Fasteners for wood
Nails and Spikes
The allowable withdrawal load per inch (25.4 mm) of penetration of a common nail or spike driven into side grain
(perpendicular to fibers) of seasoned wood, or unseasoned wood that remains wet, is
Size and Volume Factors
For visually graded dimension lumber, design values Fb, Ft, and Fc for all species and species combinations, except southern pine, should be multiplied by the appropriate size factor Cf, given in reference data to account for the effects of member size. This factor and the factors used to develop size-specific values for southern pine are based on the adjustment equation given in American Society for Testing and Materials (ASTM) D1990. This equation, based on in- grade test data, accounts for differences in Fb, Ft, and Fc related to width and in Fb, Ft, related to length (test span). For visually graded timbers [5*5 in (127*127 mm) or larger], when the depth d of a stringer beam, post, or timber exceeds 12 in (304.8 mm), the design value for bending should be adjusted by the size factor
Radial Stresses and Curvature Factor
The radial stress induced by a bending moment in a member of constant cross section may be computed from
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ADJUSTMENT FACTORS FOR DESIGN VALUES
Design values obtained by the methods described earlier should be multiplied by adjustment factors based on conditions of use, geometry, and stability. The adjustments are cumulative, unless specifically indicated in the following:
The adjusted design value Fb‘ for extreme-fiber bending is given by
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Compression on oblique plane
Consider that a timber member sustains a compressive force with an action line that makes an oblique angle with the grain.
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Recommendations
RECOMMENDATIONS OF THE
FOREST PRODUCTS LABORATORY
The Wood Handbook gives advice on the design of solid wood columns. (Wood Handbook, USDA Forest Products Laboratory, Madison, Wisc, 1999.)
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Combined bending and axial load
Members under combined bending and axial load should be so proportioned that the quantity
Pa/P +Ma /M <>
Columns
The allowable unit stress on timber columns consisting of a single piece of lumber or a group of pieces glued together to form a single member is
Beams
The extreme fiber stress in bending for a rectangular timber beam is
f=6M / bh2
=M/S
Bearing
Grading of lumber
Stress-grade lumber consists of three classifications:
1. Beams and Stringers. Lumber of rectangular cross section, 5 in (127 mm) or more thick and 8 in (203 mm) or more wide, graded with respect to its strength in bending when loaded on the narrow face.
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