Base Plate & Existing Slab On Grade Connection of Square Tube Post for Handrail 1

[dursunlutfu]

Hi Folks,

I have a 1-1/2"x1-1/2"x3/16" square HSS tube post for handrail. I have a couple questions and maybe some of you guys have already encountered with a similar condition. The problem that I am having is with the base plate because I need to anchor the base plate to the existing slab on grade which is only 4". I am using the simpson anchorage software to design the anchor bolt and set xp epoxy however as per the code I need min 6" thickness. Do any of experienced engineers know a reference to justify 5/8" or 3/4" dia a.b embedded in set-xp epoxy probably max 3". The uplift/withdrawal that I am having at each bolt is 1040 lbs. According to Simpson Set-XP design information which is in lieu with ACI 318-14,
min conc thickness = emb. depth + (5) drill bit hole
Unfortunately,even if I use 3/8" dia. threaded rod with 1/2" drill bit hole and with min depth which is 2-3/8", the min. concrete thickness required is 4.875 in.

I would appreciate if anyone could direct me to a reference ..

Thank you so much for your time!

 

[hokie66]

I think your main problem would be with the strength of the concrete, not the anchorage thereto.

 

[dursunlutfu]

@hokie66 I am not sure why you think the epoxy will not fail before concrete in uplift/withdrawal. That is interesting to me.

 

[GC_Hopi]

Not knowing any better I will throw this out there. Depending on the number of posts why not core the slab and pour back a small concrete foundation with an embed to weld the post to. Trying to epoxy into a 4 inch slab will lead to field issues, ie blowing out the back of it if they drill too deep or not drilling deep enough . I doubt the slab has a uniform thickness.

 

[wannabeSE]

Try Dewalt (Powers) Pure110+ or Hilti RE500 V3 or another manufacture. Simpson minimum thickness requirements are stricter. You should be able to get a 1/2” anchor to work. The biggest concern with thin slabs is breaking out the bottom with the hammer drill

 

[BridgeSmith]

hokie66 is correct; properly mixed adhesive placed in properly cleaned and prepared holes, is stronger than the concrete and the failure will be a cone failure of the concrete.

The requirement for 5 X the hole diameter below the hole is to ensure that the drill (typically a hammer drill) does not break out the bottom and allow the adhesive to go out the bottom.

I would suggest looking into a mechanical anchorage, such as a wedge anchor (Hilti Kwikbolt, etc.) According to the Hilti Load Tables, depending on the concrete strength, a 1/4" anchor with 1-3/4" embedment may be adequate. The 3/8" with 2-3/8" embedment should be more than adequate.

 

[dursunlutfu]

thank you all! I will use Hilti Kwik Bolt 3 which is totally sufficient.

 

[jayrod12]

Personally, I hate using mechanical anchorage in an exterior condition (Not sure if that is the case here or not), I picture moisture getting into the hole around the anchor, freezing, and breaking the concrete etc.

 

[hokie66]

I don't think this is a practical or satisfactory way to fix a post to a thin slab. I would core a hole through the slab, embed the post perhaps a foot, and grout around it. Galvanized posts at a minimum.

 

[XR250]

Most 2 post automotive lifts are installed on 4" slabs. Dunno how they get those anchors to work.

 

[BridgeSmith]

XR250, I suspect for the automotive lifts that some combination of the following mitigating conditions are true:

1) The base plate is bolted through the slab to an anchor plate underneath.
2) The slab is thickened in the areas around the posts.
3) The base plate is relatively large, allowing the anchors to be spread apart.
4) The lift is configured as a moment resisting frame, which requires only shear capacity from the anchors.

 

[jayrod12]

HotRod10

In my own experience:

1) definitely not, post-installed expansion anchors
2) Also no, slab is usually existing long before someone thought of a lift
3) Surprisingly no, just the standard 6" bigger than the post size
4) In the one direction possibly, definitely not in the other, I've always had to design around a moment.

 

[BridgeSmith]

jayrod, you must be talking about a system with 4 posts, correct? In that configuration, the only moments would be due to the guy working on it, cranking on a breaker bar. With the axial/punching shear load also spread amongst the 4 posts, I can see that working adequately. I was thinking the systems I've seen with just 2 posts connected by a strut.

 

[dursunlutfu]

Then should we not trust the values that are listed on HILTI's technical submittal sheet ? According to HILTI my loads are totally fine for both bolt and concrete considering the minimum concrete depth required for the listed values. Also I am not sure why I should be concerned about the slab failure in terms of the 200 lbs point load which will not be a frequent loading condition and it is not a dead load. I thought the 200 lbs point load requirement was designed for emergency situation like a fireman pulling the handrail etc. which will be experienced may be once or twice in its life time. I might be thinking wrong though, and I would be happy to get the right angle on that ..

 

[jayrod12]

No I most definitely mean 2 post systems. The moment I see on that system would be due to the load being off-centre. Happens all the time.

Also I am not sure why I should be concerned about the slab failure in terms of the 200 lbs point load which will not be a frequent loading condition and it is not a dead load

This is a concerning statement. You must trace your loads all the way to a termination point(i.e. the ground). If the applied loading cannot be adequately resisted by the slab, then you have a failure of the system. If this happens, and you don't have the proof that the slab should have been fine (i.e. calculations showing the slab should be adequate) then you could be in big trouble.

 

[dursunlutfu]

jayrod12 I am a little confused on that and I am open to discuss it. The values that are listed on HILTI technical sheet are not sufficient for us to design our members ? For example, if you check the simpson set-xp values, they also have reference calculations indicating how they evaluate the listed values. Their calculations are based on ACI-318 which includes the concrete breakout capacity for shear and tension loads, and the pryout failure. I am not sure why I should be worried about the structural elements that are structurally justified on their technical sheets.

 

[jayrod12]

Their values are certainly sufficient to ensure the load makes it into the concrete, assuming you've used their technical sheets correctly. I know it's likely peanuts, but where does the load go once you've put it into the slab. I'm just playing devil's advocate anyways and making sure you've adequately thought about the problem all the way to the end.

 

[Ron]

One addition to HotRod10's comment. Failure of adhesive anchors is more likely to be from bond failure of the adhesive to concrete interface. This type of failure is typically a result of poor installation (cleaning) practices, though not always. This does not produce the preferred cone breakout failure in the concrete. Even when anchor holes are cleaned properly and the anchors are fully bonded, the resulting cone breakout occurs near the top of the anchor, not at the bottom as would be experienced with a wedge anchor, so the failure mode is progressive from a stress accumulation from the bond interface as it moves up the anchor during loading to the point where cone failure occurs. The cone failure that does occur has been shown to be somewhat irrelevant to the capacity of the anchor.

 

[BridgeSmith]

"This type of failure is typically a result of poor installation (cleaning) practices, though not always."

Generally, there are 2 ways we see the adhesive anchors fail - holes not cleaned, (concrete dust coating the sides acting as a bond breaker), or incomplete mixing of the 2 components, resulting in an epoxy that doesn't cure (harden). I've walked up and pulled out a #7 rebar dowel with my hand 3 days after it was epoxied in, because they didn't mix the 2 parts together well and much of it never hardened.

"...the resulting cone breakout occurs near the top of the anchor..."

It would have to be stressed nearly to the yield point of the bar or bolt, I would think, for that type of failure. A larger diameter anchor should prevent that.

Regardless, adhesive anchors are a great tool where they're applicable and properly installed. For the situation under consideration, it doesn't appear an adhesive anchor will work, due to the bottom of the hole being too close to the bottom of the slab. I presume that cover requirement is to prevent the drill bit from punching through (or weakening) the bottom of the slab, which would allow the adhesive to be pushed out the bottom of the hole when the anchor is installed.

To address the potential rusting issue brought up earlier, numerous suppliers have wedge anchors available in stainless steel. A quick web search turned up a supplier with Simpson Strong-Tie 3/8" stainless wedge anchors in various lengths, all for under $2.00 each.

 

[Yves De Lathouwer]

If you have a 4 inch concrete slab, although it's forbidden by the norms, a very efficience way to use them is to use concrete screw that are getting close to the edge. Of course you won't get the whole capacity of the anchors but you will get followinf of our experience very close to it.
It's also pretty easy to use. And contratily to the wedge anchor that are starting to be effective in a much lower depth than the one you install first (also because you pull the anchor during his installation and because the connections are located above the extremity of the anchor).
Chemical anchor is also problematic with very thin concrete slab because if you drill through, your chemical will spread, and then you need to use a special net with your chemical.

So, at the end, concrete screw shoul be the best option. In Israel we use a lot the Powers De Walt wedge bolt (find ESR approval of it here

https://www.adit.org.il/wp-content/uploads/2018/06/BT-American-Agreement-ESR-2526.pdf)

but the Hilti Hus 3 should give similar results.

Note, that another option, which is also not included in the norms, but works, is to use the net used for chemicals in hollow block, through the concrete slab if it's empty on the other side. Then your chemical will produce some kind of mushroom at the opposite side of the concrete slab and increase the concrete cone failure resistance. That's obviously a solution not included in norms and not fit to all the situation. I didn't manage to add a picture but you could find one on this page to understand what I spoke about

https://www.adit.org.il/wp-content/uploads/2018/07/עיגון-כימי-בבלוקים-אנגלית-1.pdf

Regards
Yves De Lathouwer
Head of Engineering Department Adit Ltd

http://www.adit.org.il