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Post Installed Concrete Anchors with Existing Shallow Conc. Slab

Post Installed Concrete Anchors with Existing Shallow Conc. Slab

(OP)
Hello all,

I have a grand stand bleacher project with an existing 3" concrete slab, the details are showing post installed anchors to secure the angle frame bleacher.
I am running Hilti Profis software and it seems like I can't get a structural anchor to work with less than 3.25" slab. Any ideas or tips?

Thank you,
Jon

RE: Post Installed Concrete Anchors with Existing Shallow Conc. Slab

Perhaps structural anchors are not required. If there is a possibility of the bleachers overturning or uplift, then I see no way around anchors. But if you are simply just trying to resist sliding of the bleachers, you may want to consider if there is really a need to prevent it. Obviously if there are just a few rows of bleachers it would be less of an impact than if these are larger bleachers. Just an avenue to consider.

Alternatively if you have a slab on grade, could you cast concrete piers on the slab or use drilled piers through the slab to provide additional anchor depth and cover?

RE: Post Installed Concrete Anchors with Existing Shallow Conc. Slab

(OP)
JGard: thanks for the quick response. The client would prefer not to install through bolts and does not want them exposed.

RE: Post Installed Concrete Anchors with Existing Shallow Conc. Slab

(OP)
MotorCity: thanks for the response. The bleacher is being subjected to overturning and uplift. The slab is not on grade. I am trying to relocate the anchors to a back CIP wall, we will see if I have any luck with the client.

I appreciate the comments.

RE: Post Installed Concrete Anchors with Existing Shallow Conc. Slab

is it a slab on metal deck? Hilti catalogues have info for expansion bolts into a thin slab on metal deck.

RE: Post Installed Concrete Anchors with Existing Shallow Conc. Slab

A (nominal) 3 inch thick slab with a calculation needing 3.25 inch long Hilti's?
1) Ignore the minor difference, accept as-is.
2) Add a few more holes and Hilti's. (One or two per bleacher frame or row.) More
threaded rods = less force per rod needed = 3 inch becomes an acceptable engagement depth.
3) Drill each all the way through (many places the nominal concrete will be deep enough to be acceptable. Use stainless rods and glue (epoxy) them in to withstand corrosion from below slab water.
4) Drill all the way though and use pull-back (through wall) type Hilti's. You will get better capacity per bolt.
5) SLightly increase the diameter on each rod. You will increase the pull-out force on each because there is greater circumference for the epoxy to grip, so your 3 inch to 3.25 length difference becomes less important.

RE: Post Installed Concrete Anchors with Existing Shallow Conc. Slab

One potential problem is blowing-out the concrete at the bottom of the slab while hammer drilling. An expansion anchor with 1-1/2" effective embedment probably requires a 2" deep hole so the anchor can be set. It might work if the slab is consistently 3" min. I suggest you call some anchor manufactures to get there recommendations.

RE: Post Installed Concrete Anchors with Existing Shallow Conc. Slab

You might try looking at some "drop-in" style flush anchors. The 1/2"-diameter "Hilti" anchors require only a 2"-deep hole. You might want to core the holes though, as opposed to hammer-drilling them, to prevent backside blowout of the underneath of the slab.
Dave

Thaidavid

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