Existing Lumber Safety Factor

[gramy13]

A contractor has had several samples of existing wood floor joists tested by a wood testing company. I just received the test results which include ultimate averages for the following:

Shear parallel to the Grain (Fv) - 1,342 psi
Modulus of Rupture (Fb) - 10,629 psi
Modulus of Elasticity - 1,649,255 psi
Moisture Content - 7%
Specific Gravity - 0.46

Does anybody know what factor of safety I should apply to the Fv & Fb ultimate values to get allowable values comparable to what NDS publishes? My gut is telling me that dividing the values above by 4 sounds about right? Also, I don't believe that I need to apply a safety factor to the E, MC or SG.

Thanks!

 

[Signious]

If it is possible in your area I would suggest using LFRD instead of ASD being that you have accurate design data.

 

[Splitrings]

Take a look at ASTM D245.

 

[manstrom]

You should be able to pick values from the NDS based on E and SG since these are actual values and shouldn't have a safety factor. I'm sure there is some statistical work to be done. I think the NDS uses the 5th percentile (not sure).

I don't think you want to develop values for this specific lumber since the sample size is limited. I would just say - ok this looks like (SPF, HF, DF....).

I have seen actual vs. allowable stresses before. The factor is somewhere around 4 or 5.

The shear values kill me. The books have 175 psi on the high end. The samples failed at 10x that.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[SlideRuleEra]

Gramy13 - In addition to the testing lab's results, you also need to know the wood species and its size. The values from the lab are consistent with southern yellow pine. ASSUMING this is correct, take a look at the allowable for Fv and Fb for new lumber:

http://www.southernpine.com/app/uploads/TABLE01_L1.pdf

For evaluating the existing joists, I suggest going with Fv and Fb values not to exceed those shown in the attached table. If the grade is not marked on the lumber, be conservative in estimating its grade.

If, instead, you are considering reusing the lumber on a different project, all allowable values will be TOTALLY based on your engineering judgment. Marked grades not longer apply since the lumber may have been damaged during removal.

I agree with your opinion on the values of E, MC, and SG.

The safety factors for Fv and Fb are so large (in the range of 8 to 10, or so) because of the failure mode for wood. It does not break until long after the test loads have deformed it so much that it is structurally worthless.

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[MacGruber22]

Also, I don't believe that I need to apply a safety factor to the E, MC or SG.

Don't forget about C_L factor as it is a function of E_min unless you are providing bracing to set C_L = 1.0

I think it is likely you are going to have to make engineering judgement. I am wondering why they couldn't have determined the species during their testing, and they should have also visually-graded the pieces in addition to the testing you did get - it is very easy to do and is very important for validating your engineering judgement. Maybe they did?

If it is possible in your area I would suggest using LFRD instead of ASD being that you have accurate design data.

Careful, careful, careful. Structural reliability depends on the amount of certainty in your entire system. If the OP has 4 joists tested out of 100 to be used, you no longer have "accurate design data". Also, you have to be certain that the sample tested is from the same lot as the rest.

Regardless of your procedures, you should consider:
1. The size of the sample tested verses the lot, and whether you know this wood came from the same source, etc.
2. Setting upper bounds for yourself based on well-known typical structural lumber in use today.
3. What do you REALLY need, loading-wise? There is no point in chasing a precise number if your loads only require, say, SYP #1.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Tmoose]

Then there is the quality of the joists. They may not all be the same.

http://www.nelma.org/wp-content/uploads/2013/08/Section_5.pdf

 

[racookpe1978]

7% moisture?

Be sure to allow drying (shrink) margins if clamping/bolting together.

 

[gwynn]

Mamstrom,

You have to remember that shear values for timber are based on the timber being split at the support. Also,it depends on the test set up, though I assume there are methods in testing standards to compensate for that.