Three Hinged Glulam Arch - Wood Rot

[KootK]

I'm investigating a building consisting of 18 120' glulam arches spaced at 20' o/c. Unfortunately, the last six feet of the arches protrude through the building envelope and have, over the last 40 years, rotted significantly.

My question is this: will it be absolutely necessary to remove the rotted wood? I suspect so but want to be sure as the need to remove the rotten wood will complicate things considerably.

Is there any way that the rotten wood could be treated so as to neutralize it and keep the fungi from colonizing the competent wood beyond the rot? If I could somehow guarantee a low moisture environment (<= 19%), would that obfuscate the need to remove the rotten wood?

Thanks for your help.

 

[Ron]

Assuming the arches are structurally competent even with the section loss, you might consider epoxy injection to fill the rot voids. They should be cleaned as much as practicable, then injected with epoxy.

 

[msquared48]

If this is a three hinged arch, then moment capacity should not be a problem, only the shear capacity. You still need to see what the minimum section left is for the code applied loads of today. Check out the soft spots with a knife and figure it out for each arch.

Mike McCann
MMC Engineering

 

[BAretired]

Mike,

That is an interesting conclusion. Why should moment capacity not be a problem?

BA

 

[msquared48]

If it is a true three-hinged arch, then there is no moment at the base by definition, only vertical and horizontal reactions.

I am assuming here that the reaction points and crown connection are pinned. Maybe this is otherwise OP?

Mike McCann
MMC Engineering

 

[BAretired]

If a three hinged arch is loaded with a point load half way between two hinges, moment is definitely a factor...and could be the determining factor.

BA

 

[msquared48]

I fully agree, but the OP stated that the rot is in the last three feet of the member, where the moment declines rapidly, very rapidly, and, when compared to shear considerations, is more likely than not, not critical.

I did not mean to imply that the arch saw no moment between the hinges, but that the moment went to zero at the hinges. I'm sorry if my statement was not clear. The arch has a rather large span - 120 feet as stated by the OP. The maximum moments will be in the vicinity of the 30 and 90 foot points of the arch depending on the loading conditions. At the start of the rot, the moment should be 10% or less of the maximum.

All this structure is is two simple beams linked at a crown by a pin. Nothing more. The simplest arch.

Mike McCann
MMC Engineering

 

[BAretired]

Yes, Mike the structure is about as simple as you could expect to find anywhere. But it is hard to come up with a definitive answer to the rotten wood problem without knowing the exact geometry and loading of the three hinged arch.

BA

 

[apsix]

And fundamental to an arch, axial capacity must also be checked.

 

[KootK]

Yes, the arches are truely pins at the bearings and shouldn't generate any serious moments there. However, my pimary question here is whether or not a repair can be executed that does not involve removing the already rotted wood. Anybody have any thoughts on that?

 

[InDepth]

Thornton Tomasetti performed a project (ASCE conference Long Beach - paper?) in Texas for a training center for the Dallas Cowboys (?) where they essentially, surveyed using destructive and non-destructive testing the extent of rot. Note that even though the wood looks good, there may be bacteria or mold embedded in the wood beyond the zone. I believe bacterial analysis was performed.

Then for the repairs, they shored, then they cut section of the laminates and glued them back using gorilla glue (check the shear stress). They also have finger splicing between lams. For higher stressed areas (moment areas) they used some steel plates.

You should look into AITC documents.

http://www.aitc-glulam.org/

Some other repairs

http://www.spsconstruction.com/glulam.html

Also, Call AWC

http://www.awc.org/index.html.

I remember one of the directors performing an exact same repair. In this case they actually cut the end of the member, placed a new "bucket" steel bracket and pin and then provide a gap so water wouldn't acummulate at the end (sourec of issue ----don't let it happen again). I beleive they also installed anti-mold rods in the end. There may be a diagram in the AWC standards.