Wood cracking in a recent structure

[Jeepitou]

Hello everyone,

I recently did an inspection of a glued laminated wood structure which had a lot of cracking in the columns. The beams seemed to have little to no crack.

I think it's principally due to shrinkage of the wood, since I could unscrew bolt from the wood by hand. The problem is that some crack are really deep, they are through the whole thickness of the column.

I have some difficulty thinking that only the drying of the wood could do such damage to it, unless they put it in a pool just before installation.

Any of you seen anything similar? Or have any others causes in mind that could have such behavior?

Thanks

 

[racookpe1978]

So, reading through the above, the vertical cracking is caused by the two different loads on the two different sides of the single wide glulam vertical: The right (taller, outside) side is permanently loaded with building and roof dead loads, plus environmental live loads, and the left (inside, shorter) part of the member is loaded with the static crane rail dead loads plus the moving (start/stop) and downward crane lives loads. So each side of the current vertical is trying move against the restraints of its mating half every time the crane is moved or loaded and unloaded, or the outside environment changes (heated, cooled, snowed or rained or whatever.)

Together, both are tearing the vertical apart.

To fix, would not each column have to be supported (braced up) then the member removed and replaced with either a new steel vertical, or (better yet) two separate steel or wood verticals. Once the new member are in place, the temporary verticals are relaxed, moved sideways, then jacked back up again. Assuming the new verticals are physically able to move as each is separately loaded, then there would be no vertical tearing stresses.

I suppose theoretically, you could cut the current member in two, but the tearing is irregular already, and the torn segments would kill the vertical capacity of whichever side was most damaged.

 

[HouseBoy]

I'm not sure the existing columns need to be replaced.
Slenderness might be a factor but what if they can be adequately braced for slenderness?
Even still, maybe only one half of the existing column might be replaced -or- what if the "separation" can be made complete and the two existing wood column can each handle their share of the loading (one part of the crane and the other for the building)?

Still think drying shrinkage could be a factor also.

 

[msquared48]

In a glulam, drying shrinkage should not be a problem.

Mike McCann, PE, SE (WA)

 

[jayrod12]

Agreed with Mike, in a proper glulam column they should already be at kiln-dried state prior to the lamination process. These types of members are supposed to be as dimensionally stable as engineered lumber products. Therefore I do not believe it would be drying shrinkage but rather something worse. That is why my comment above was to engage the glulam supplier immediately.

 

[HouseBoy]

"In a glulam, drying shrinkage should not be a problem."
I agree that we don't usually find it with glulams but two things come to mind:
1. Except maybe when the faces are so well restrained by the face plates to which they are so massively connected.
2. We don't know the MC of the wood at the time of installation.

 

[hokie66]

Glulam or not, shrinkage is an issue here, and I agree with HouseBoy. These are not just little bits of wood glued together. The column is 300 x 600, and it looks like each lamination is a 300 wide (12") piece of sawn timber. The mere fact that Jeepitou could unscrew the bolts by hand means that the wood has shrunk. Because the bolts and plates restrained that shrinkage, it cracked. The stress riser at the step didn't help, and neither did the loading from the bridge crane...if the crane has been used.

 

[jayrod12]

My point is, shrinkage should not be an issue. Therefore contact the supplier as there appears to be something wrong with either the manufacturing, or the performance of the columns.

 

[molibden]

I agree with hokie and HouseBoy. Here in Central Europe glulam is made of max. 260 mm wide lamina, everything above that is block glued.
This can easily be restraint drying shrinkage and loading (to me it looks like moment resistant connection at the base).

I think this can be solved with fully threaded screws.

 

[hokie66]

jayrod12, add "or the design", because this is a silly use of the material.

 

[dhengr]

Jeepitou:
The crack in the second photo looks like a pretty std. checking crack, down the middle of the wide face of a ply. I would look into how moist this material was when the cols. were glued up. I’ll bet they were not well dried, and uniformly dried. I can’t really make out the thickness of the plys, it appears they may be different under the crane rail beam than those which continue on up past the stl. beam, they look thicker than normal 2x plys. Actually, for that size of GluLam col. the bolted connections seem very light, small in dia. and few in number, bolts or structural screws. Someone has to explain how that structural system was intended to work, and how it is actually working. It looks like those GluLam cols. are taking high lateral load moments about/above the conc. base structure. The timber rivet pls. on the 12" face would have been put there with some moment fixity in mind. They would induce high loadings on the outer plys, as these loads try to distribute into the rest of the col. I wonder which Arch. designed those GluLam cols., with little timber engineering knowledge.

 

[msquared48]

You know, until now, I have never seen a check in a glulam, beam or column...

Mike McCann, PE, SE (WA)

 

[dhengr]

MikeMc:
That’s probably because the outer couple plys are almost always of a superior grade of material, less likely to check. And, they are not likely loaded the way that member might have been loaded, with the stl. bm. rocking about the two web stiffeners, and loading only one edge of that outer ply, as the beam deflects.

 

[chicopee]

Wow, we had over a dozens of overhead cranes rated as low as 10 tons all the way to 50+ tons in our heavy structural steel plant and we never used wood columns. Shear madness. I would replace all these glulam columns with steel columns. Bite the bullet instead of getting involved in liability and compensation cases.

 

[RFreund]

Are those bolts at the bottom of the column attached to a single steel plate? If so are they farther than 5" apart? If so the NDS code (used in N.A.) does not allow that. Because of shrinkage restraint.

EIT

http://www.HowToEngineer.com

 

[Sawbux]

I have seen lots of checks in glulam. Typically they do not extend clear through the member and are not a concern. AITC has a publication related to that.
These appear to be cracks originating from the restraint of the base connection as noted by others above. If these are cantilever columns, the cantilever beam action is impaired since there the horizontal shear (vertical in this case) in the beam (column) is gone.

First check the moisture content in the beams. If it's less than 10% or so, shrinkage has probably stabilized.
Check humidity in room, if it is a conditioned space. There are some relationships between atmospheric humidity and the equilibrium MC of wood, you will need to look up.

You can repair with epoxy injected into the cracks plus steel dowels across the cracks to restore shear capacity.

If moisture content is above 15%, I suspect the glulam supplier did not meet standard for MC in the lams before lay-up. Not sure what Eurocodes call for, so you will need to verify.

 

[FoxSE14]

I think the culprit is the crane loads, plain and simple. I really doubt shrinkage is playing the major role here. If ASCE 7: Sect 4.9 is anything to hang our hat on - there are gravity, longitudinal to rail, and perp to rail forces that are taken into consideration. It appears the longitudinal and lateral crane wheel forces are causing all kinds of movement, the most concerning is the cracking due to tension perpendicular to grain.

Have you ever seen a bridge crane fly down a bay and slammed to a stop? No question those columns are moving around within their base assembly -- wrong type of material for this application.

Not knowing anything else besides the pictures given above -- I propose fixing by adding supplemental steel posts alongside the wood columns, sized to handle all bending and axial loads from the crane rail. Weld new posts to crane rail beam above, and try make use of the existing piers/foundation below by doweling (if possible).

 

[chicopee]

Have you determined the compressive load when the crane trolley rides over these columns. What I see from the pictures is that the rail beams are loading two separate bearing plates on top of the glulam columns.

 

[hokie66]

We don't really know much about this crane in terms of its capacity and service conditions. Most of the things which go wrong with crane runways have been found due to in service conditions, i.e. trial and error. I think this one adds to the list...don't support crane runways on stepped wood column, or wood columns in general.

 

[Sawbux]

I've seen a lot of cranes supported on wood, most in the PNW where good D fir 12x12's were plentiful. Cranes were 15 - 20 ton capacity, up to 75 ft. span and going 600 FPM! Most built from 1917- 50's.

Typical detail was to have separate columns for the building and craneway. They were joined with spacer blocks, if required to get clearance to the end trucks to the building columns.

A lot had external batter braces on the outside of the building for lateral loads, but some had cantilever columns. Usually those were in buildings with multiple bays with adjacent craneways. Columns were trussed with diagonal bracing on the faces of the columns and through bolted. Anchor bolts were attached with side brackets to the outermost columns for base fixity.

I think in this case, if they had used separate columns and bolted them together with shear plates, it would have worked better.

 

[ThePioneer]

Why not use glued laminated timber for columns supporting crane beams? My factory has them in a very similar situation as this case. And they have been doing their job properly since 1940! Not only are the columns of glued laminated timber construction, the support beams for the steel rails and the crane beams themselves are glued laminated timber. All doing great for over 75 years of everyday service in a manufacturing plant!