glulam repair 3

[bjb]

I have some glulam beams that have suffered checking as a result of severe fluctuations in moisture content over 30 years. I have evaluated the checks based on AITC Technical Note 18, and have determined that some of the checks have decreased the shear capacity of the beams. Does anyone have any thoughts on how to repair these checks? I read in some other AITC literature that repair of checks by epoxy injection is not the best solution when your goal is to fix the shear capacity. My thought is to put some thru-bolts in the beam with top and bottom plates. I would design the bolts based on transering the shear flow, similar in theory to bolting cap plates to a steel beam. I might also inject some epoxy anyway, as some insurance.

 

[Trussdoc]

I would look into the following:

Calculate the actual shear on the beams and see if it exceeds the reduced allowables. Shear may not be the critical factor.

Check to see if the reduced cross-sectional area resulting from the through bolts doesnt become critical...the beam width will now be reduced by the diameter of the bolt hole.

If shear is critical, look into lag-bolting some thin steel plates onto the sides of the lam. This may be easier to access than trying to get at the top of the beam.

 

[boo1]

"Top and bottom plates", are usually used to increase bending capacity. If you are concered with shear, add doublers on both sides. You may find they are easier to install. Epoxies are weak in shear and design strenghts difficult to quantify.

Cheers

 

[bjb]

The stress that must be transfered is the horizontal shear stress, and that ability is degraded by the checking. The formula that describes this shear is (VQ/Ib). Therefore, the fasteners have to be designed for the shear flow, which is VQ/I.

 

[boo1]

BOCA Report 96-6 & SBCCI Report 9625A:
Horizontal Shear Stress, Fv, shall be modified by Shear Factor, CH;

0.69<=CH=KLx[850/(b x L)]0.2<=1.0
where:

KL=loading coefficient (1.0 for uniformly distributed),

b=width of beam, in.,

L=span of beam, in.,

APA has an article Evaluation of Check Size in Glued Laminated Timber Beams:

http://www.apawood.org/pdfs/managed/EWS-R465E.pdf