Checking & Spliting of wood beams 2

[randomizer]

Hello fellow SE's,

I have a situation where an old house has old 6x beams running the length of the center of the house, under the central bearing walls above. These beams have checking on the bottom (and top?), and long splits on the bottom. Yet, there are no checks on the sides.... perhaps it is a 6x6 turned sideways. I haven't verified the size yet.

Reading through Donald Breyer's book on wood design, I looked closely at the wording in the section that discusses ckecking and spliting (4th Edition: 4.13-4.14,4.24-4.25).

First, Breyer says checking relieves stresses: great, but you are still left with a reduced section & possible stress concentrations for future loads.

Here's the nitty gritty: On 4.14: "[Checking] causes a reduction in shear strength which is taken into account in the grading rules and tabulated design values"

Possible interpretations:
A) the NDS code tables assume all 6x's (plus) have checking and have already reduced the allowable stresses?

I believe that used to be the C-subH factor. But recently, it was found that the C-subH factor was redundant to another factor, and all the shear stress values in the 2001 NDS were increased by a factor of 2.

B) beams with a lot of checking are discarded because they do not meet the "grading rules" requirements.

Another quote (4.25) "Again, lumber grading rules provide limits on these types of defects."

So this could also be interpreted as options (A) or (B) but it seems to imply more of (B). This quote is what got me thinking about this in the first place.

In the NDS 2007, pages 17-18 seem to imply that wet-dry conditions (checking?) effects the long-term creep?
And there is the wet-dry factor C-subM described on pg. 28: this page seems to imply that a "wet" beam in a dry service conditions should have Cm applied to it...

Please let me know if you think I need to either reduce my net sections calculations for the checks & splits, or reduce my stresses by Cm or something else.

Thank you,
Michael C.

 

[Lion06]

I believe Cm only applies if the in-service moisture content > 19% (I don't think it has anything to do with checks). I don't know too much about older houses and the wood codes of the time, but the checks now are built into the NDS values. That is the point of the grading system, it takes these kinds of things into account, hence you have No.1, No.2 or better, etc...
Either way, if the section reduction is significant I would definitely reduce it.
Do the checks and splits occur along the entire length of the beam? If they are localized, perhaps you can provide a steel strap along the bottom of the beam, being sure to provide adequate fastening on both sides of the problem area.

 

[randomizer]

Cm is for in-service moisture content >19% for an "extended" period of time, where the environment is much drier. What is "extended?" I don't know. It could be 2 months, 6 months, 1 yr, 5yr, etc.
I have been told that larger beams are typically not kiln dried, so they would likely develop checks in-service, and to my knowledge that is mostly considered okay. I have not seen checks on the bottom of beams before, but can only assume that had more to do with the orientation of the grain, and perhaps the beam was only a 6x6.
Otherwise, I do understand that the current grading system revolves around this issue. The house was built in 1949, so perhaps I should try to find some references there.

But I was hoping to talk to someone with a good amount of work experience with this issue. The checks & splits are pretty long in this beam, and while I have seen a lot of this before, but I have not seen checks & splits as continuous, and not so big on a small beam. So, my question becomes, what limits have ya'll heard of in the past?

So, I didn't want to just speculate at answers, but prefer to hear if any experts have tried & true solutions.

 

[ctcray]

Seems to me that it's better to have the checking in a vertical plane because the weakest link for wood is shear in the horizontal plane. In your case, even if the checking was complete vertical split, you'd still have the same moment of inertia as the original beam.

If it were serious checking in the sides, i'd be more concerned.

 

[RARWOOD]

Normally checks and splits are not a probably in glulam or solid timber unless they occur in a region of a connection or in an area with a high shear load.

One of the important things in evaluating the condition wood is establishing if you have a check or a split. It is rare for a seasoning check to go all the way through a piece. If you can insert a card or thin ruler through the beam you probably have a split. With splits you need to determine if they were caused by an overstress of the member.

You can get checks on the sides, top and bottom of a piece of timber. Where they occur depends on how the piece was cut from the log and the width and thickness of the material.

You should contact both AITC and APA-EWS for more information. They both have excellent publications on evaluating checks and splits.