Mounting unistrut to 8" hollow core precast slab?

[StructureMan44]

We need to mount unistrut to the underside of 8" hollow core precast slabs. The slabs are existing and the cores are unfilled. There is a topping slab that fills the gaps between the slabs.

Can post-installed anchors be installed into the infill concrete between the slabs? I'm worried about hitting reinforcement or the 1.5" thick area below the hollow areas. Is there any published guidance for installing anchors between precast slabs?

 

[TehMightyEngineer]

I'd say the real question is how much load are you putting on the unistrut and what do you need to connect. I imagine you could fit a small screw anchor with a length less than the typical clear cover to the reinforcement and the hollow areas.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[structSU10]

look into drop in anchors such as these:

https://www.us.hilti.com/medias/sys...chnical_information_ASSET_DOC_LOC_1543430.pdf

 

[KootK]

Are we not able to generally rely on the prestressing to fall between voids in hollow core?

At the side joints you'd be dealing with cold joints and a narrow grout thickness. You'd want to go deep and, if it's a load that matters, consider some in situ testing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Yeah, just like the diagram in StructSU10's attachment.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Hilti also has an allowable load for HY70 adhesive anchors into hollow-core... Capacity is low, but it might be enough for what you need.

 

[StructureMan44]

Thank you all.

TehMightyEngineer: We have 2 200lbf loads at the 1/3 points of a 4' long section of P1000. Although once this in place we expect other to attach to the unistrut for convenience.

StructSU10: I was unaware of HDI-P sleeves, what kind of anchors can fit in the sleeves?

jayrod12: It looks like the HY70 will also work for this application

 

[Teguci]

P1000, L = 48" (say span = 36"), P = 1.1 K at 16" and at 32". You will have close to 3/8" of deflection before the strut fails due to exceeding it's moment capacity by a factor of 2.5. Did you mean P1001?

There are code approved, internally threaded inserts that can be installed into a 2" deep hole. X-ray the slab for tendons in any case (don't you dare nick a tendon).

 

[Ron]

Tapcons at 12" o.c. are normal for such applications. Don't like to put expansion or epoxy anchors into hollow core.

 

[StructureMan44]

Teguci: I meant P1000 but maybe my math is wrong? Using (PL^3)/(28*E*I) I get a deflection of 0.033in with 2 200lbf loads at the 1/3 points and using E=29000ksi and I=0.815in^4 from:

http://www.unistrut.us/index.php?WP=cat_detail&P=P1000

 

[Ingenuity]

I get a deflection of 0.033in with 2 200lbf loads at the 1/3 points and using E=29000ksi and I=0.815in^4

I think you have a transpose error. The I of a P1000 is 0.185 NOT 0.815.

And when you say 2 200lbf, you mean to 2 @ 200 lbf, not 2200 lbf, right?

 

[StructureMan44]

Thank you Teguci and Ingenuity I did make an error: with the correct I the deflection is 0.147in

If the concrete (or grout?) between the hollow core slab is allowed to reach at least 7 days of cure time, for what reasons is adding anchors there an issue?