Let's say you have a thin concrete slab, 3-4" thick. A thru-bolt will be used to pass through the slab to carry a load with tension and shear. The bolt will bear on a 3" square steel plate and the center of the bolt hole will be about 2.5" from one edge of the slab. Do you look at the thru-bolt as a concrete anchor per ACI 318 Ch. 17, or do you treat it as a mini column and check punching shear around the plate?
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Concrete Strength for Thru-Bolt Connection 1
- Thread starter DTS419
- Start date
Can you tell us a bit more about your situation?
1) Is this a flat plate slab or a slab on steel decking system?
2) What's the source of the load?
Given that small edge distance, I'd be hesitant to try to do anything too serious with such a connection. Drilling the hole for the through bolt may well initiate your side face blowout failure.
I tend to avoid through bolting in favor of some form of adhesive or mechanical anchorage precisely because there are more established design methods for those setups. That said, I do acknowledge that though bolting surely does produce the strongest connection in some situations.
In similar situations, it is quite common to use punching shear provisions. I've done it myself. One of the things that I dislike about that, however, is that the punching shear testing is mostly done on slabs with columns above and below the joint. I feel that rigid clamping action encourages a failure frustum that may not be particularly realistic for something like a through bolt connection. So, in this, I do favor some kind of anchorage check.
In your particular situation, I imagine that your shear load will tend to encourage a pryout failure mode as discussed in the anchorage provisions. I'm not sure that's something that punching shear will capture robustly.
Depending on your situation, another resource might be the SDI provisions that deal with concentrated loads on metal deck slab systems.
1) Is this a flat plate slab or a slab on steel decking system?
2) What's the source of the load?
Given that small edge distance, I'd be hesitant to try to do anything too serious with such a connection. Drilling the hole for the through bolt may well initiate your side face blowout failure.
I tend to avoid through bolting in favor of some form of adhesive or mechanical anchorage precisely because there are more established design methods for those setups. That said, I do acknowledge that though bolting surely does produce the strongest connection in some situations.
In similar situations, it is quite common to use punching shear provisions. I've done it myself. One of the things that I dislike about that, however, is that the punching shear testing is mostly done on slabs with columns above and below the joint. I feel that rigid clamping action encourages a failure frustum that may not be particularly realistic for something like a through bolt connection. So, in this, I do favor some kind of anchorage check.
In your particular situation, I imagine that your shear load will tend to encourage a pryout failure mode as discussed in the anchorage provisions. I'm not sure that's something that punching shear will capture robustly.
Depending on your situation, another resource might be the SDI provisions that deal with concentrated loads on metal deck slab systems.
- Thread starter
- #3
The calcs for treating it like an anchor per Ch. 17 give less capacity than punching shear in this case, even if I only consider bo to be 3 sides of the square plate (9") and d is 2". But, when you look at the Anc and Anco in Ch. 17, the failure cones are all pyramidal beginning at a point at the base of the anchor. In the case of a 3" plate, the using the typical failure cone and Anc seems to ignore any benefit of the plate in expanding the failure cone. So maybe another question is, if analyzing like an anchor per Ch. 17, should the failure cone and Anc be manually modified to assume that the cone begins at the edges of the plate rather than a discreet point at the end of the anchor?
JoshPlumSE
Structural
I remember seeing someone do this sort of thing so that they could bypass the normal anchorage calcs (which are really geared towards headed anchors embedded in concrete) and get more capacity out of the connection.
I can't remember the exact reference though. Sorry!
In concept, I believe it is valid to treat it as a mini-column with punching shear calcs. You'd have to check local bearing / crushing as well. I might also using a slightly thicker plate (more like a column base plate). Maybe grease the rod, or use a sleeve or something to prevent it from bonding to the concrete and acting like an anchor bolt.
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1
- #7
I might do the punching shear by default, but I think it's probably more defensible to do an anchorage check, but potentially projecting from the edges of the bearing plate if it's pretty stiff. But honestly, if it is close to failure I'd be worried about a bunch of other local issues as well so I'd be pretty conservative. If I was close to an issue I might drop a few bolts and a larger bearing plate across them, for instance. It's the sort of thing where, as long as it's a one off, it's better to spend a couple dozen or hundred bucks on install to be a bit stout than spend hours developing theory that's potentially a bit weak.
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