Cracking in Wood Joists 2

[Matt PE]

Called to look at a house yesterday.

Floor is sagging beneath a newly refinished bathroom (new tile, fixtures, etc. i.e. weight) to the point where the Owner noticed drywall cracks.

Looked at the joists in the crawl space. Many/ most joists in the area have a textbook horizontal shear crack. (Was unable to check joists for level in the tight crawlspace, but the floor above is out.)

Also noticed that blocking has failed in some bays and some joists have twisted.

Running numbers for 40 psf LL and 20 psf DL (tile/ fixtures), the 2x10 joists are overstressed in flexure, but well under allowable shear on this span. I think that's fairly typical with uniform loads on simply supported timber beams.

I'm hesitant to just write the cracks off as checking.

Regardless, I think the solution is to sister some new joists alongside the existing to stiffen the area and arrest future settlement.

That said, from an academic perspective, I'd be interested to hear your thoughts as to why what appear to be shear cracks were observed, but calculation suggests a bending failure and shear should be well under allowable. Are the shear cracks initiating at a defect with much lower shear resistance? Is the slight twist resulting in some minor biaxial bending and I'm seeing a flexural failure in the weak direction? Feels like I'm stretching with those explanations.

Thank you in advance.

 

[gte447f]

I think those cracks are due to drying shrinkage of the sawn lumber. Basically they are checks. In photo "20250626_162427.jpg" you can see the end grain of the lumber and you can see that the crack is in the radial direction (perpendicular to the growth rings) and perfectly aligned with the pith in the middle of the heartwood. This also explains why the crack is above the neutral axis of the joist, which is where it should have been if it were due to shear stress. This also explains the cupping (i.e. bowing of the joist) that is visible, which is due to more and more rapid shrinkage on the left face of the joist (as viewed in the photo) than on the right face, since wood shrinks more in the tangential direction than in the radial direction. This is why quarter sawn lumber is usually straighter and more dimensionally stable than flat sawn lumber for example.

 

[Matt PE]

I think those cracks are due to drying shrinkage of the sawn lumber. Basically they are checks. In photo "20250626_162427.jpg" you can see the end grain of the lumber and you can see that the crack is in the radial direction (perpendicular to the growth rings) and perfectly aligned with the pith in the middle of the heartwood. This also explains why the crack is above the neutral axis of the joist, which is where it should have been if it were due to shear stress. This also explains the cupping (i.e. bowing of the joist) that is visible, which is due to more and more rapid shrinkage on the left face of the joist (as viewed in the photo) than on the right face, since wood shrinks more in the tangential direction than in the radial direction. This is why quarter sawn lumber is usually straighter and more dimensionally stable than flat sawn lumber for example.

Thanks GTE,

Maybe it is checking. I appreciate the second set of eyes. I didn't want to immediately go there and write it off.

The joists calc out a little light for that span in bending and we have visible deflection/ serviceability issues at midspan as expected, so I still think I am going to recommend sistering some new joists together. Just wanted to make sure there wasn't something else going on. I'm not surprised that there is not yet any visible sign of a bending failure (other than the sag of course) but I was scratching my head a bit as to why I was seeing something that looked like a shear problem (when mathematically there should not be.)

 

[Matt PE]

Should have added that there is another area of the crawlspace where the joists are on a shorter span and none of them exhibit the end cracking.

That area is isolated however and definitely damper, so can't rule out drying shrinkage where I am seeing the cracked joists.

Both sets of joists are well below allowable shear stresses. The joists that are a couple feet shorter also work in flexure, although they are at about 90% utilization.

 

[gte447f]

@Matt PE Just to reiterate, look again at your photo that shows the end grain of the joist on the left side of the photo. The radial crack that is inclined at an angle relative to the x-axis of the cross section and is also perfectly aligned with the pith at the center of the heartwood is basically a textbook example of checking. Nevertheless, certainly if deflection has resulted is an unacceptable serviceability condition, then stiffening the floor by sistering the joists could be in order. Likewise, if there is a deflection problem and the joists also lack adequate bending strength by calculation, then strengthening the floor by sistering the joists could definitely be in order. Those two conditions can both exist without there also being a shear failure. In fact, in most typical residential floor joist configurations, I would expect limit states to occur in order of deflection, bending strength, and then shear strength. Basically, I am reiterating that I think those cracks are checking due to drying shrinkage, but that doesn't mean you don't have a deflection and/or bending strength issue to deal with. Sounds like you are on the right track to addressing the issues with the floor.

 

[Tomfh]

If they’re cracking and they don’t work easily I’d be sistering.

 

[Ron247]

I agree with sistering as a solution. Is there a grade stamp anywhere on the lumber? I remember some small local sawmills in the 70s and 80s that made lumber with no inspection, grading or seasoning. Some people wrongfully used it for houses although it was mostly intended to be used for sheds and barns. You may have one of those. It was generally flat sawn like you have and with advanced age, they tended to split similar to yours.

I know one that also sold plywood that was referred to as "Blows". We had an owner buy some one time. They were supplying the material. They did not understand it was slang for "Below Grade". No stamp. Out of square and the plys had already started separating. Fun, fun, fun to work with.

 

[lexpatrie]

I'm going to guess you're not making this mistake, but I'll ask anyway, are you considering the shear check at the end support or in the middle of the span? Mid-span the shear is "zero" so there should not be a horizontal shear crack forming there. If the cracks extend to the bearing, or appear to originate at the bearing, possibly historic high loads at the ends started the failure and it's progressing when it goes over the allowable shear, creating slow-growth crack propagation that is shear.

If you're going to reinforce the floor, so long as the new joists are stable and the existing don't contaminate them with fungal, the new joists could probably be designed for the full weight of the floor, which is likely what you're doing versus trying to pro-rate load distribution between the existing and the new.



Last, depending on the floor installation, some of this tile can get quite heavy (Look for some Frank Woeste articles on that).

@Ron247 - There's a bad moon on the rise.......

 

[Ron247]

@Ron247 - There's a bad moon on the rise.......

I would rather argue they deceptively changed to those words 50 years after releasing the song, than truly admit I was wrong all those years.

 

[Matt PE]

@Matt PE Just to reiterate, look again at your photo that shows the end grain of the joist on the left side of the photo. The radial crack that is inclined at an angle relative to the x-axis of the cross section and is also perfectly aligned with the pith at the center of the heartwood is basically a textbook example of checking. Nevertheless, certainly if deflection has resulted is an unacceptable serviceability condition, then stiffening the floor by sistering the joists could be in order. Likewise, if there is a deflection problem and the joists also lack adequate bending strength by calculation, then strengthening the floor by sistering the joists could definitely be in order. Those two conditions can both exist without there also being a shear failure. In fact, in most typical residential floor joist configurations, I would expect limit states to occur in order of deflection, bending strength, and then shear strength. Basically, I am reiterating that I think those cracks are checking due to drying shrinkage, but that doesn't mean you don't have a deflection and/or bending strength issue to deal with. Sounds like you are on the right track to addressing the issues with the floor.

GTE, I agree with everything you are saying.

I would not expect a shear failure unless there was a point load coming in somewhere.

It was just odd to me that I had those cracks on the end of every joist on the longer span (not all of them had that distinct pith....maybe a bad picture to share) but on none of the joists in the area of the shorter span.

The joists on the long span have a sag and theoretically are a little overstressed in bending whereas the ones on the shorter span (calc out OK in flexure) do not sag.

We are going to sister the joists. Mostly just looking for a sanity check to see if I was over-looking something structural.

Appreciate your help and second set of eyes.

 

[Matt PE]

I'm going to guess you're not making this mistake, but I'll ask anyway, are you considering the shear check at the end support or in the middle of the span? Mid-span the shear is "zero" so there should not be a horizontal shear crack forming there. If the cracks extend to the bearing, or appear to originate at the bearing, possibly historic high loads at the ends started the failure and it's progressing when it goes over the allowable shear, creating slow-growth crack propagation that is shear.

If you're going to reinforce the floor, so long as the new joists are stable and the existing don't contaminate them with fungal, the new joists could probably be designed for the full weight of the floor, which is likely what you're doing versus trying to pro-rate load distribution between the existing and the new.



Last, depending on the floor installation, some of this tile can get quite heavy (Look for some Frank Woeste articles on that).

@Ron247 - There's a bad moon on the rise.......

Correct, shear check at d/2 away from bearing.

The cracks are in the ends of the joists, over the bearing, where you might expect high shear, propagating maybe 12"-18" inward. However, calculation shows shear in this region should only be about 30% allowable. Looking for some validation could that be drying? Consensus seems to be yes.

 

[Matt PE]

I agree with sistering as a solution. Is there a grade stamp anywhere on the lumber? I remember some small local sawmills in the 70s and 80s that made lumber with no inspection, grading or seasoning. Some people wrongfully used it for houses although it was mostly intended to be used for sheds and barns. You may have one of those. It was generally flat sawn like you have and with advanced age, they tended to split similar to yours.

I know one that also sold plywood that was referred to as "Blows". We had an owner buy some one time. They were supplying the material. They did not understand it was slang for "Below Grade". No stamp. Out of square and the plys had already started separating. Fun, fun, fun to work with.

Thanks Ron, that's about the right era for construction of this house. I don't recall seeing any grade stamps. I'll have to look back through my photos to see if I picked one up in the background. Good to know.

I think the answer is sistering.

Still trying to figure out exactly what the question was though. More of an academic exercise at this point.

 

[Ron247]

By chance, did you probe the wood looking for signs of wood destroying fungus? Advanced WDF is easier to spot than mild to medium degradation. I would probe a few joists looking for indications of it. That loss of strength due to WDF can cause any sag to become worse. I have had several that visibly looked ok but probing proved they were deteriorated internally.

I did not see any pinholes that looked like powder post beetles in your pics so I ruled that out.