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I have got a design problem where I need the ultimate shear stress of Southern Pine to estimate shear capacity of a pile made of Southern Pine. I went through several codes and manuals, and could only find the allowable stress. If you know any reference where I could get this information, I really appreciate it.
You might try FPL’s Wood Handbook:
It is an excellent reference and probably the best data as can be found. However, you have to be very careful how that data is used. In Chapter 4 you will see information that indicates “Modulus of Rupture” is the computed maximum stress (in bending). That is for small, clear (no knots) samples that are loaded slowly for a short time and under laboratory conditions. Typically, the Mod. of Rupture will be 8 to 10 times the allowable stress, depending on wood species and moisture level.
For shear, a good value for ultimate strength is even more elusive. Probably the best available is “Shear Strength Parallel to Grain”. Again, this is based on small-scale tests under optimum conditions.
The values (allowable stress has a 1.6 to 2.0 safety factor) are very good, but are most likely based on the elastic limit, not the ultimate strength.
IMHO (and that is all, just an opinion), when EXCEEDING the elastic limit for a “real” field application my best guess at “ultimate strength” is 4 to 6 times allowable bending stress and 4 to 6 times allowable shear stress.
Of course, exceeding the elastic limit means that the members may be “worthless” for structural purposes after these loads have been applied (assuming they survive at all).
A final note, your problem is even more hard to pin down since “Southern Pine” can be any one of a number of species with differing properties.