Oversized hole considerations for anchoring overhead conduit supports - Unistrut P2073A Post Base and Hilti HDA-PR M10 undercut anchors

[Ronengo]

Hi all, I am working on some designs for overhead conduit supports in a seismic region (SDC D). The contractor is proposing to use Unistrut frames to support the conduit. The anchorage they are proposing is Unistrut P2073A Post Bases and Hilti HDA-PR M10 undercut anchors.

My concern: Per the Unistrut catalog, the post bases come with 3/4" bolt holes. The diameter of the M10 undercut anchors is about 3/8". Therefore, the holes are even beyond "oversized" for the anchors per RCSC Table 3.1.

The post base anchorage is subject to sustained tension and shear during seismic events. Am I correct in my understanding that using this combination of anchors and hole size is not acceptable, even if we use a large enough washer?

Modelling the post base in Profis, the only HDA anchor size that will work with the edge distance/anchor spacing dimensions of the post base is the proposed M10. The Unistrut Ohio site states that the post base has 3/4" mounting holes for 5/8" anchors, but this is perplexing as it doesn't seem any of Hilti's offerings that I'm used to specifying would work with the bolt dimensions of the post base.

It seems like I need to push for either welding plate washers, or using custom post base plates? Would slip critical connections also be an option here?

Any thoughts are welcome, thanks in advance.

 

[HDStructural]

I have not seen slip critical connections used with undercut anchors. That doesn't seem like a good idea. Can you use an adhesive anchor instead of undercut? They might have better edge distance requirements.

I don't really see an issue using an oversized washer with the 3/8" bolt if that is your best option. Make sure the washer is thick enough.

 

[ANE91]

I understand that you’re stuck between a rock and a hard place. Anchor too small : oversized for baseplate holes. Anchor too big : edge distances and spacings become a problem.

I’ve installed undercuts; the MPIIs are very specific. They’re kinda the cream of the crop when it comes to mechanical anchorage. Seismic bracing, sustained tension, etc. are certainly where you see them. My last install was for retrofitting/bracing a tall parapet in CA, years ago.

Whether welded washers are “allowed” is up to both Hilti and Unistrut. It’s trickier if the posts are galvanized. Other than using larger anchors at minimum embedment, this is likely your best option.

Alternatively, consider using Hilti’s equivalent of Unistrut, instead. It’s called MI or MT or some such. I’m sure they have the section properties you need with smaller baseplate holes.

 

[Ronengo]

Thanks both.

@HDStructural I'd love to be able to make a judgement call that forces are low and slip is fine because of expansion joints in the conduit, but RCSC 3.3.2.1 states "For snug-tightened or pretensioned joints subject to shear or combined shear and tension, oversized holes are not permitted." Without diving into the reference they have (Allan and Fisher (1968)) I hesitate to wave my hand and call it good. But very curious if someone has run into this and been able to justify it.

@ANE91 We're a little too far along to switch strut products. Switching anchors would be more palatable but again I'm struggling to find mechanical anchors that are big enough yet short enough. While it may calc out, we would prefer to stay away from adhesive anchors for the overhead application.

I might have to just push forward with welded plate washers, and do a hand check on the shear as if the 1/4" post base plate thickness is an offset. Still open to hearing how others have dealt with this, I imagine these post bases are very common.

 

[bones206]

I don't really see an issue using an oversized washer with the 3/8" bolt if that is your best option. Make sure the washer is thick enough.

I tend to agree with this. I can’t visualize base plate slippage of ~1/8” being a major concern in an earthquake. If you can show that 2 out of the 4 anchors can take the shear force, then I think you would be covered.

 

[driftLimiter]

I can’t visualize base plate slippage of ~1/8” being a major concern

I think this is the key thing for the OP. You need shear resistance, but can you tolerate some movement at the support in the design event.

If the shear is only for EQ that makes sense, if its for some other types of loading maybe not.