Ultimate Capacity of Southern Pine 4

[URSWayne]

Friends:

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, I could only find the allowable stress. If you know any reference where I could get this information, I really appreciate you.

Thanks,

N.S.

 

[JAE]

Fairly difficult to find as wood varies so much that any ultimate stress you might "get" would not necessarily match the actual wood you are investigating.

I tried to contact various NDS code writing officials some years ago and only got 1.6 to 2.0 as the "safety factor" built into the NDS values.

 

[wannabeSE]

You might try FPL's Wood Handbook

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

. Their strength properties appear to be at the breaking point rather than a value used for design. If ultimate strength properties are needed for strength (LRFD) design, the NDS has formulas that convert the reference design values for use with strength design.

 

[SlideRuleEra]

URSWayne - The book that wannabeEIT recommended 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 that JAE has offered (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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[hokie66]

Excellent advice from SlideRule as usual. 6 is always the number I think of for wood breaking/allowable. But I would steer clear of thinking of "ultimate shear stress" as something quantifiable in wood.

 

[Lion06]

I was doing some research several months ago for a condo complex that had deficient joists. I found a resource that gave the (supposed) "actual" safety factors for different wood species and grades.

I'll see if I can dig it up next week.

It may not be accurate, though, because I don't remember any of the numbers approaching 4-6 except for the lowest grade lumbers (were around 3).

 

[URSWayne]

Hello all:

Thank you so much for the responses. But, I have another question related to the same topic.

If I want to find the ultimate horizontal shear capacity of pile (force acting horizontal),can I use the ultimate shear strength (parallel to grain). is shear strength in any direction the same?

Please give me your thoughts.

Thank in advance

N.S.

 

[hokie66]

Wood strengths are different depending on the direction of the stress.

 

[dhengr]

But, the fact is that the shear parallel to the grain is the weak link. And, we know from Theory of Elasticity that the two orthogonal shear stresses at any point are essentially equal. Therefore, a significant horiz. load on the pile top will not cleave it off, but it will split the pile longitudinally rendering it useless. The shear strength is not the same perpendicular to the grain as it is parallel to the grain. But, at the mid cross section the shear stress is max., and is the same in both orthogonal directions, thus the member will fail about the weakest plane, parallel to the grain.