Safe Reinforcing Methods for Existing Glulam Beams 1

[IlliniEngineer]

I am working on a project that involves reinforcing a 5 1/8 x 30 glulam beam due to additional of heavy mechanical units on the roof.
The existing glulam beam is simply supported between two steel posts.

My preliminary thought is to through bolt C channels on the sides of the glulam near the bottom of the beam. I am concerned with through bolting the existing glulam and fear this will weaken the beam if the bolts are placed in between the laminations.

Another reinforcing option would be to install a steel plate at the bottom face of the beam and lag bolt into the underside. Would this method weaken the glulam if the lag screws are 4" long and penetrate the (2) bottom lamination layers?

Any guidance would be much appreciated here. Thank You

 

[dik]

Can you 'plate' the sides using 3/16" material and glulam rivets? I've done that as well as jacking the structure to remove loading and securing steel BAR stock to the bottom flange with adhesive and some 'nominal' glulam rivets. The tricky part is finding documentation for the strength of the adhesive. I've used LePage's PL Premium... done about 20 years back and still working. I was concerned about creep of the adhesive. If adding channels, can you put a strap under the beam connecting the channels on each side to better transfer the load?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[IlliniEngineer]

I can do anything, but is drilling into the side of a glulam beam safe or will it weaken the beam?

 

[dik]

It depends on how much load you have to transfer... bolts and lag screws are not an optimal method of transferring load in bearing. You may want to check with a glulam suppier about drilling holes. If most are needed at mid span and near the centroid the holes will not affect the flexural capacity much and shear may not be affected... depends on what the SFD looks like.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Deker]

I wouldn't be concerned about drilling between the laminations, but I am wary of drilling the tension lams if you're pushing the beam hard. I've seen GLB failures initiate at holes drilled in the tension lams. If going with the Cx retrofit, push the channel up against the bottom of the purlins and make sure you address how the shear will get out of the Cx and into the support.

I'd avoid the steel plate retrofit as it will be difficult to guarantee that fastener slip / deformation won't neuter the composite action you need to make it work. External post-tensioning can be an effective means of strengthening if you just need to support a single heavy unit.

 

[wrantler]

I like to use FRP or GFRP for strengthening. You may consider this...quakewrap makes a product but there are many out there.

Link

 

[Agent666]

You have to remember in these situations that the load already in the beam is locked in. So you need to look at it really in terms of if you add new plates or similar they will only take any additional load, and you still need to share the additional load in the timber based on the stiffness of side plates. You're not going to make it in any shape or form "composite" because of the slip involved in the bolts. Bolting a plate to the underside will almost do nothing as a result.

If you have access to both sides, bolt a channel on each side as you've noted and work out the load sharing between these and work out an estimate of the load carried already, then proportion the additional load to get to the ultimate factored load between the three elements and make sure you check the timber stresses and steel design under the resultant loading.

Searching for the many discussions here on eng-tips on flitch beams would be useful to outline the concepts involved. It's been discussed many times over here, people seem to think bolting something to the side of a beam somehow makes things composite, when really you're actually setting up a vertical load-sharing arrangement only.

I would drill holes all day long in a member without any concerns provided you observe edge distances, min spacing requirements, and observe any transfer of vertical loads perp to grain that might cause splitting perp to grain.

If we cannot drill holes in timber or really had concerns about this, then how would we ever connect anything in wood? I wouldn't sweat it, just follow the guidance around locating these holes in your local code just like you would in a greenfields design

Holes for small-diameter dowel-type connectors generally won't affect the ultimate bending capacity, as the maximum stress is only reached at the extreme fibres of a timber beam not where the bolt is if observing edge distance requirements. There is a marginal change in elastic modulus if you were to consider the bolt hole, but nothing to worry about. Timber is designed elastically (i.e. uses the elastic modulus), so the holes have virtually no impact.

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