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long shear failure of beam

long shear failure of beam

long shear failure of beam

Hi all,

I went to look at a home (for a separate reason from this beam) and noticed this beam with longitudinal cracking on both sides fairly consistent along the whole length (see pics). It spans approx 17' and is an 8x8.
To me this beam looks to have failed in longitudinal shear, looking for a second opinion on this.



RE: long shear failure of beam

Looks like standard wood checks to me. Did you run a string-line to check deflection? What is the relative humidity in that room/building? What kind of load is that beam carrying?


RE: long shear failure of beam

I did not run a string for deflection. The beam is in the basement and the home seems very dry rite now, it is heated with wood stove and pellets.. makes sense with the checking as the summer it will likely be very humid down there.

The beams trib width is approx 9' of D+L from the floor above, nothing out of the ordinary.

RE: long shear failure of beam

This is the beam I had come to look at....



RE: long shear failure of beam

Again standard drying wood checks. The upper beam is certainly not sagging from failure down onto the lower beam and has not sagged in recent history. Note the wiring run under the upper beam thru the cutout of the upper beam and on the lower beam. That wiring has been there a long time. Those beams were probably installed green and wet and they normally show severe checking due to drying with not loss of section or much loss in capacity. This information should be available in any green wood design source.

Check the members for deflection/sag with a stringline and check the section properties.


RE: long shear failure of beam

Calculate what the deflection should be for th 8" deep section. If the crack has created two 4" deep beams, the deflecion will be 4x what you calculated. If not, shrinkage cracking is what you see.

Mike McCann, PE, SE (WA)

RE: long shear failure of beam

These cracks likely are in the outer parts of the trees they came from, with the tree center somewhere inside what you see. Thus they likely are not connected from one side to the other. Outer parts of a tree tend to shrink radially more and with little if any near the center. I once had a log house and carefully checked the longitudinal cracks to see if they were connected from outside to the inside. Never found any draft or other questionable zone related to these many cracks and there were many due to fresh age of the logs used. In short,likely no structural problem at the subject site. From the photos it seems several beams were made from logs with outsides shaved to provide more "lumber-like" appearance.

RE: long shear failure of beam

Interesting related article from FPL: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: long shear failure of beam

Looking at the photos, it appears that some checks occurred prior to painting and some have occurred since. This implies that perhaps they are getting worse (as happens with moisture content going up and down). Do as others have noted with regard to deflection. Mark the ends of current observable checks and monitor for a couple of seasons to check growth rate. If they are growing, then you might end up with the condition that Mike described.

RE: long shear failure of beam

Such shear cracks are not unusual with wood beams. I have seen many of them over the years and to this date these beams are still structurally sound.

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