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Shrinkage in deep glu-lam to steel connection

Shrinkage in deep glu-lam to steel connection

Shrinkage in deep glu-lam to steel connection

Hello all! Long time reader, first time poster.

I'm looking at a connection in which a deep (36") glulam timber tees into a W beam, with steel deck and slab over both. By my math, this timber will shrink by roughly 5/8" from its as-manufactured moisture content of ~15% to in-service MC of ~8% (conditioned interior space in a institutional building). Having T.O. timber drop 5/8" below T.O. steel seems to me like a serviceability problem with the slab in the corridor above. I'm trying to come up with some options for the architect here; do any of you know of a way to mitigate this?

I'm looking at some of the fancier timber hanger products out there and wondering if maybe there's a way to suspend the timber from a point other than the bottom, but so far that seems like a red herring.


RE: Shrinkage in deep glu-lam to steel connection

What does your framing system look like? If the gluelams are acting as joists and the steel beam is the girder, you could set the initial glulam elevations at least 5/8" above the top of steel beam.

I think you want to stick with a hanger connection that supports the glulam from the bottom. I've seen knife-plate type connections with a bunch of bolts fastening the glulam, but my understanding is that creates restraint to shrinkage across the depth of the beam, which could cause splitting.

RE: Shrinkage in deep glu-lam to steel connection

Adding to what bones said, I'd also stick with a 'saddle' type bearing connection. But consider shrinkage when you place your bolts.. You don't want to create a situation where your bolt is carrying your member alone and the beam has shrunk away from the saddle's bearing point.

RE: Shrinkage in deep glu-lam to steel connection

Use elongated bolt holes in steel plates.

RE: Shrinkage in deep glu-lam to steel connection

Agreed, I'm generally using saddle-type hangers and paying attention to slotting any bolt holes that aren't right near the bearing surface to avoid creating cross-grain tension stresses with shrinkage. AITC 104 (Typical Construction Details and Details to Avoid) is a nice resource for this stuff. With smaller timbers, I know there are reinforcing tricks using specialty hangers and fully threaded screws to do something like a centroid hanger rather than a saddle, but I don't think I can pull any of that off here given the shrinkage strains.

Bones, you might be on to something with setting the timbers high, if the finishes can accommodate the motion as the floor settles down to ~level. Seems like it's that or switching to clad steel beams.

RE: Shrinkage in deep glu-lam to steel connection

I was also under the impression that Glu-lam beams were signficantly drier than in-service conditions before glued and pressed. If you talk to a local glulam supplier, I don't believe the shrinkage you're expecting will come to light. I wouldn't be surprised if it's considered dimensionally stable or if anything a slight swell.

RE: Shrinkage in deep glu-lam to steel connection

Timber beam can be manufactured with moisture of 12% or less. Then you have installation when it could shrink or swell. Then you have finished building and drop in moisture to let's say 8% so it will shrink or stay the same as installed. What I'm trying to say it's not certain it will shrink so much. Even it could swell. I must say I never check shrinkage in such medium sized beams in enclosed buildings. But I demand 12% moisture.

RE: Shrinkage in deep glu-lam to steel connection

molibden, if I'm understanding you correctly, you spec your glulams to be dried to 12% or lower? It makes sense that the mfg could put the lamstock in the kiln longer and be laying up at 12% instead of 15% MC...

RE: Shrinkage in deep glu-lam to steel connection

I work in Europe where it is pretty standard for engineered timber to have 12% MC. If work is done in humid conditions, timber can swell quite a bit. But it shrinks back when dry condition kicks in... So basically you need to allow some movement. I don't think your case is critical.

RE: Shrinkage in deep glu-lam to steel connection

Across the pond, we're used to seeing 15-17% MC for glulams, which is typically not a problem because everything moves more or less equally. My concern here is the intersection of timber and steel and what that sudden discontinuity will do to the floor above (and some other components like railings).

If this was a European project, the whole building would probably be timber and I wouldn't have this problem!

I'll get with the project team and see if they can design movement into the floor above; that would solve everything.

RE: Shrinkage in deep glu-lam to steel connection

Sounds like maybe you need:
1) An adjustable saddle, or
2) The glulam in the space ahead of time, so it has time to acclimate to the ambient humidity, or
3) An elastomeric cushion under, or on top of, the glulam.

RE: Shrinkage in deep glu-lam to steel connection

HotRod, I like the idea of an adjustable saddle. Building manager could advance jackscrews a bit seasonally for the first few years of occupancy until the timbers come to equilibrium. I'll propose that as an alternative to making them deal with the full deformation in the slab.

RE: Shrinkage in deep glu-lam to steel connection

This is not quite the same but in the ballpark. I'd also have done it a bit differently with a pair of stiffeners in the beam and the rest of the hanger stuff terminating at the bottom flange.

If you can upset the glulam 1" relative to the beam, you should be able to sleep easy with respect to dimensional stability. In my experience, it's a rare owner that would actually get the adjusting done on an adjustable hanger.

RE: Shrinkage in deep glu-lam to steel connection

"...it's a rare owner that would actually get the adjusting done on an adjustable hanger."

I'd have to agree with that. I didn't realize when I made the suggestion that the timeframe for the shrinkage to occur was in years. I was anticipating it would be more like a few months, where the adjustments could be made near the end of the construction.

The typical solution, I think, would be to have the slab rest on the secondary beams (the glulams?) only, at least initially. Assuming the slab spans between glulams at locations away from the steel girder, why could it not do the same at the steel girder location. If for some reason the steel and glulam both need to support the slab, you are in kind of a pickle. In that case, some kind of compressible sheet between the top flange of the steel girder and the slab might do the trick. As the glulam shrinks, it gradually transfers some of the load to steel girder, as the material compresses and/or disintegrates.

RE: Shrinkage in deep glu-lam to steel connection

A preformed expansion joint filler board for concrete could work as a temporary spacer, although it would likely have to be an inch or more thick, so that the load is still distributed to both members after shrinkage of the glulam. A compressed joint material might also work for this purpose. You'd probably have to work with a technical rep. from the manufacturer to get the right thickness to provide the proper support for the slab.

RE: Shrinkage in deep glu-lam to steel connection

HotRod, thank you for diving in deeper here! I am looking at exactly that pickle, in which the steel and glulam both support the slab. I'm not familiar with compressed joints, is that something like https://wbacorp.com/products/bridge-highway/joint-... ?

RE: Shrinkage in deep glu-lam to steel connection

Can the manufacturer make the glulams using lumber with a moisture content closer to 8%?

RE: Shrinkage in deep glu-lam to steel connection

bones- I imagine one could get lamstock dried more, and that's an option on the table. The challenges there are keeping it dry until the building is finished and the cost of that special treatment. I'm holding out hope that the project team can come up with some shrinkage-tolerant details.

RE: Shrinkage in deep glu-lam to steel connection

EmilC, I was thinking specifically of the FS Seal material from WABO or the Emseal the BOR or BEJS material for an elastic material. For something that would just squeeze down or just disintegrate over time, there's corrugated cardboard, polystyrene (styrofoam), or expansion joint filler board. I recommend working with a supplier to get the right thickness and width to provide a pressure distribution that meets your design assumptions.

RE: Shrinkage in deep glu-lam to steel connection

We obviously can't stress this enough Call a local supplier to determine their expected shrinkage.

Wood shrinkage is such voodoo in a sense. Sometimes it comes to light, sometimes it won't. You're spending significantly more time thinking about this than I would have. Why not have the glulam connection in the upper bit of glulam to minimum the shrinkage seen? Around here we can get away with detailing the intent of the glulam to steel connections but can pass the true design to the glulam supplier and steel connection designer to coordinate.

RE: Shrinkage in deep glu-lam to steel connection

jayrod, you make a good point- glulam mfg tells me that 12% MC is average out of their doors (and we should be able to keep it that way), which is better than I had in my head. We typically assume wetter stock, but this project won't be sitting around in anybody's yard for long. Unfortunately, I am the glulam sub and detailer of the steel-to-glulam connections on this project, so I am coordinating!

Re connecting higher on the member- some of the nice European hangers (Knapp, etc.) seem like they restrain the timber about its centroid rather than from the bottom like a regular saddle hanger, but I don't have anything to back that assumption up.

I think I've got a good quiver of options here, thank you all for adding to it! Time for a RFI to the architect.

RE: Shrinkage in deep glu-lam to steel connection

It's been a long time since timber design class, but generally how deep you have to go on the beam to support it would be dictated by how much of the cross section is required to resist the applied shear, wouldn't it?

RE: Shrinkage in deep glu-lam to steel connection

HR10, correct. Although glulam is a tricky animal since its lamination laid flat, I haven't looked to deeply into their shear failure modes from knife plate connections, but I assume the plies splitting apart would be a concern.

RE: Shrinkage in deep glu-lam to steel connection

My understanding is that supporting timbers from points other than the bottom is a tricky art... NDS has rules for notching at supports, but glulams are only permitted to be notched to a tenth of the depth at most in the NDS. There are tricks like using fully-threaded screws as reinforcing, and techniques for calculating the additional stresses on those fasteners from resisting shrinkage (My-Ti-Con has some whitepapers). With the lower shrinkage estimate, I could look at that more... it didn't seem plausible with my initial ~5/8" shrinkage estimate to be using a connection that would be restraining shrinkage with the same fasteners that are holding up the beam.

The Wake Tech pedestrian bridge collapse a couple years ago was caused by notching glulams at supports without regard to splitting, it's a brittle failure mode and definitely something to watch out for. My understanding is that the member can still fail even though the x-section at the notch is sufficient to resist the shear, if the member isn't able to develop the tensile stress across the grain required to concentrate the shear in the upper portion of the member.

RE: Shrinkage in deep glu-lam to steel connection

My local airport has a gorgeous glulam-framed roof, with steel framing at the perimeter. I was standing in the security line last week (staring at the ceiling as engineers tend to do) and noticed that there were in fact ~1/4" gaps between the roof and timber beams extending about 3-4 ft from the connection to the steel beam. In this case, the serviceability of the roof was probably not adversely affected, but it does show the shrinkage gap phenomenon does happen.

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