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IFRs (Petroleum) (OP)
13 Jul 05 20:55
I have two pieces of 2x6 Douglas Fir #2 or better, each about 8 feet long. The ends of both pieces are smoothly drilled with a single 9/16" hole to accept a long 1/2" carriage bolt. The bolts go through the 6" way, they is 2 1/2" in from the end of the boards. The two boards are joined with metal plates on their short sides, using the bolts. My question is: what bearing strength or stress should I use when calculating the allowable tension (or compressive) load in this assembly? I guess while I'm asking questions, what would the tensile and compressive allowable stress be for the boards?
Tmoose (Mechanical)
14 Jul 05 12:39
I am not clear on if you building a beam (lateral loading)or a column (axial loading).

http://www.bdg.nus.edu.sg/labs/structures/images/beam2.jpg
SacreBleu (Structural)
14 Jul 05 13:19
Bearing stress perpendicular to grain allowable for Doug-Fir is generally about 625 psi, with some factors to modify that. Duration of load factor is not allowed however. Check out WWPA.org website for more info.
SlideRuleEra (Structural)
14 Jul 05 18:06
Assuming that your members are beams, American Institute of Timber Construction (AITC) Technical Note 19 address your  question directly. Here is a link to this free .pdf document
https://www.aitc-glulam.org/shopcart/Pdf/aitc_tn_19_%20may_2002.pdf
Discussion starts near the bottom of page 2.

From the "Western Wood Product Use Manual", 2001 Edition, the properties of No. 2 Douglas Fir fall with in the following ranges:

Single Member Fb: 850 to 900 psi
Fv: 180 psi (This value has been corrected per 2001 NDS)
Tension Parallel to Grain: 525 to 575 psi
Compression Perpendicular to Grain: 520 to 620 psi
Compression Parallel to Grain: 1350 psi
E: 1.2 x 10^6 to 1.6 x 10^6 psi

www.SlideRuleEra.net

IFRs (Petroleum) (OP)
19 Jul 05 22:47
Folks -
Thanks for your help.  To make sure I am communicating properly, think of this as a splice in the bottom chord of a truss.  The lumber is6" vertical, 2" horizontal (nominal).  It is a wierd connection - the metal plates are on the 2" sides of the 2x6 (top and bottom) and the bolt is about 7" long, through the end of the 2x6's.  The smooth holes are 2 1/2" from the ends of the 2x5's.  I can check the metal plates for bearing and tension, and the net section of the 2x6 for tension, and the bolt for shear.  I am a little queasy about the 2x6 in bearing.  The AITC document is for glu-lam beams, which this is not.  The WWPA document does not have allowable or ultimate bearing stresses.  The Forest Product Laboratory 1999 Wood Handbook Chapter 7 Fastenings is confusing because it clearly references better research post 1991 but then goes on to discuss allowable loads based on prior work.  For instance, it says "Prior to 1991, bearing stress was based on test results at the proportional
limit; since 1991, bearing stress is based on test results at a yield limit state, which is defined as the 5% diameter offset on the load–deformation curve (similar to Fig. 7–3)."  However, the following paragraphs, discussion and data are all for "bearing stress at proportional limit load" which was in use prior to 1991.  It never discusses yield limit state determined bearing stresses.

Any help on bearing stress, or proper edge distsnces?
SlideRuleEra (Structural)
20 Jul 05 11:11
IFR - You certainly have a strange situation, I still can't picture the details. Anyway, a few thoughts...

The allowable values for designing with wood are set well below its maximum strength. This is because of the huge variability in how trees grow & and the non-linear performance of (heavily) loaded wooden members. Even the maximum strength, Modulus of Rupture (measured in psi), is a generalized number. For example, here are some values for seasoned Douglas Fir:

Maximum Allowable Bending Stress: 900 psi (from reference document in my previous post)
Modulus of Rupture: 12,700 psi (from this link http://cee.wpi.edu/CE3026/Lab%20Data/wood_table.html )

This logic applies to all the engineering properties of wood.

For the issues on bearing, perhaps this reference, "Wood Structural Design Data" will provide more insight. Look at Page 40, in "Chapter 2A - Wood Beams". Here is a link to the free .pdf download
http://www.awc.org/Standards/wsdd.html

As for how to analyse your situation, I would start with the simplest assumption, i.e.
The 2x6 has a full length, uniform crossection of 5 1/2" x 15/16" (1 1/2" true dimension - 9/16" hole diameter)

See what the numbers tell you to determine if you need to "dig deeper".

Designing new structures with wood is well defined by NDS requirements. But figuring out what is acceptable and what is not for an existing application is still as much "art" as it is "science".

Best Wishes

www.SlideRuleEra.net

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