Cracking in Sawn Lumber Beam

[ATSE]

For an existing 6x12 sawn lumber beam (circa 1985) that spans 20ft, there is longitudinal cracking at midheight, about 5 ft from end.
Roof beam of utility building (masonry walls).
The cracking is not abnormal - irregular and oriented mostly horizontal along the grain, about 1/8" tall and 10" long. Not completely thru the thickness, but almost. It appears like shrinkage cracking.
About 3 beams show this type of cracking.
I think it is technically "checking" (crack perpendicular to rings).
No excessive deflection or twisting.
My old 1997 NDS Supplement gives CH (shear stress factors) for splits and shakes, but CH is never less than unity.
Does the NDS or any other code limit the amount of this type of cracking? The owner is nervous and even did a very silly looking repair (which seems unnecessary) so I don't want to say "don't worry about it" without some backing.

 

[racookpe1978]

If the "silly-looking repair" involved drilling holes through the beam, or (worse) nailing into the split wood near the crack (thus increasing the "splitting" and prying forces already there, then you must anticipate more permanent repairs.

Through bolts as clamping forces pushing the wood back together, with shear loads taken up by the through bolts and glue/epoxy to the reinforcements on both sides of the beam, will be a good start.

What loads are present at what location along the beam?

 

[hokie66]

As described, with the check at quarter span and there are no serviceability issues, I wouldn't worry about it.

 

[msquared48]

Agree.



Mike McCann
MMC Engineering

 

[Lion06]

I'd be interested in when someone would worry about this. It seems like a place where a shear failur would initiate.

 

[Ron]

Code doesn't necessarily limit. Code is based on grading values from certified grading agency. Grading rules set the criteria for such anomalies.

As others have noted, I wouldn't worry about it. If you want to repair, epoxy and spiking will work. Epoxy injection will work if longer sections develop.

 

[BAretired]

For the peace of mind of the owner, why not blow out the dust and fill the crack with epoxy?

BA

 

[shobroco]

The latest versions of the Canadian timber code have changed methods & criteria but they used to assume one full-length split in beam and stringer sized sawn timbers for strength calculations. The current methods are equally conservative, so it would take some pretty drastic defects to put a timber beyond the conditions assumed during design. Sawn timber splits. Concrete cracks. The world is not perfect.

 

[woodman88]

Per the WWPA Western Lumber Grading Rules Section 220 - Shakes, Checks and Splits "In all grades of Beams and Stringers, theses characteristics are measured only in the middle half of the width. Restrictions on checks apply for a distance from the ends equal to three times the width of the wide face..."
Which for a 6x12 would be the center 6" of the vertical face for a distance of 3' from each end.
If the beam is not WWPA graded, you should check the critical of the grading rules that apply to it.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Lion06]

The difference between concrete and wood is that concrete has the steel designed to take the required tensile forces. Wood has no such mechanism when it splits.

 

[ATSE]

Thanks for the responses.
Can anyone recommend an epoxy specifically for wood (manuf and product)? Not sure if this is warranted, and I am leaning toward the "it's okay as is" position, but this will probably make the owner happy.

 

[woodman88]

Lion06
Shakes and Checks are a separation of the fiber bond (and decrease the resistance to shear) in wood. The fibers act like the steel reinforcing in a concrete beam. For this reason the shakes and checks are restricted in areas of high shear stress in bending members. In members subjected to only tension or compression they have little effect on the strength of the member.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Lion06]

Garth-
I understand that, that's why I'm wondering two things.

1) When WOULD you be concerned about a check or shake or split near the ends?

2) Why is the allowble shear stress in dimension lumber typically unchanged between the grades?

 

[JoshPlumSE]

The CH factor was the "checking" factor with the old NDS (1997 and earlier). Essentially, the base allowable shear stress assumed a certain degree of checking would occur over the life of the structure.

Sometime around the 2001 NDS, the allowable shear stresses were doubled. This was due, I believe, to an error with the interpretation of old test results not to a change in design philosophy. But, when they made this change the code writers did away with the checking factor.... The reason for this was never explained that I know of. It is still a theoretically valid factor to use to increas the allowable shear stresses. Though you would be going out on a bit of a limb since it is no longer officially in the code.

 

[woodman88]

ATSE,
I would not use epoxy or glue to seal the checks. I have used self tapping wood screws (Simpson SDS25600) and thru-bolts as repair options.
Lion06,
1- I would be concerned if the check, shake or split exceeded 80% of the code allowable. In the past, when it exceeded the 80% I have done repairs to keep them from increasing. I one case (as the client did not want to do the repair on a architectural interior beam) I noted that the client could delay the repair (with yearly inspections) if the checks did not increase in depth.
2- The shear stress is basically a function of the specific gravity of the wood. The higher the specific gravity the more material exists within a certain volume. The more material there is the better bonding between the materials you have.
JoshPlum,
They did not “...did away with the checking factor...” What they did was basically to apply the checking factor of two across each species and said there was no need to take a reduction for checks. This is why the shear values almost double for each species in the NDS 2001. Using this factor with the new shear values is not allowed.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JoshPlumSE]

Woodman -

Actually, that was the whole point of my post. The checking factor is still a valid concept even if it isn't in the current version of the code. Look up the reasons for the adjustments to the allowable shear values. It was based on an over-conservativeness in the ASTM testing procedure.

Though the values for shear stress are double what they used to be, they could theoretically be doubled again for a beam which does not have any significant amount of checking.

 

[woodman88]

JoshPlum-
The reason for the 2001 change in shear values was that the procedures used to establish the allowable shear stresses were reduced by two separate factors for consideration of splits, checks and shakes. One reduced the base values for splits, checks and shakes. I have not found what this reduction was. But lets say it was to reduced the value by 30%, which means the non-reduced base values (for checks, splits and shakes) would be only 1.4 times the current values. You would apply a factor up to two to the current values giving a maximum value of 2 times the current value. I think that even if I knew what the reduction of the base values were, I would still not use the shear factor.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JoshPlumSE]

Woodman -

I understand you better now. Yes, the concept of the CH factor may still be valid....an un-checked or un-cracked beam would theoretically have a higher shear value. But, you don't have any code guidance about whether an increase of 2.0 for an uncracked section would still be considered valid or whether the change in the ASTM would now result in a lesser increase in the allowable.

I agree with that reasoning.

FWIW: I attended a lecture on the new wood codes back in 2002 or 2003. The professor giving the lecture was from up in Washington and made it sound like he had been very involved in the code process and that wood engineering was his focus of specialty. Unfortunately, I don't recall his name. He brought up the issue about the increase in shear stresses and implied that the removal of the CH did not have any real basis in theory and was merely a reaction by the engineers on the committe that felt uncomfortable with engineers using quadruple the allowable shear stresses that we used to use.

That's absolute hearsay, so it probably doesn't mean much. But, that was the basis for my response. Though your argument about whether or not a 2.0 factor is still valid or whether it needs to be a lesser value is an excellent and valid point.