Uplift on Slab-On-Grade

[pbc825]

Here's a doozie.

If I place an anchor bolt in a slab-on-grade concrete floor (let's call it somewhere between 4" and 6") with unknown reinforcing steel layout (often the reinforcing steel is laying flat on the ground in SOG's), would you assume the defining capacity is the concrete cone that breaks out from tension, or would it be defined by another mechanism. Perhaps one involving the weight of concrete that could stay together from cohesion (i.e. in lieu of reinforcing steel)? Using the 6" SOG as an example with an anchor embedded to 3.5", the projected area of cone pull-out is ~30in2 (45 degree angle) and the associated ultimate pull-out would be something in the 10 to 15 kip range (verified per an anchor bolt manufacturer). But, I think the concrete would break up well before this force was realized. I'm trying to think of unreinforced bending mechanisms that might take over, but I can't help but think of floors with lots of wide cracks through them making this arrangement invalid.

The reason I ask it that I'm designing a crane column footing for a client who is interested in a "heavy" steel plate resting on top of a slab-on-grade. The crane is a free-standing unit with four posts and two crane runways. These configurations are often generated by crane manufacturing companies who are interested only in light-weight/low-cost structures and have little regard for the big picture. When we are involved from the get-go for new cranes in old buildings, we design crane structures with bracing and portal frames such that lateral load is not resisted by moment at the baseplate level (unless the building is green-field).

Any thoughts would be helpful. Even more helpful would be some guidance on standardized slab-on-grade uplift engineering recommendations.

Thank you in advance.

 

[WARose]

[blue](pbc825)[/blue]

If I place an anchor bolt in a slab-on-grade concrete floor (let's call it somewhere between 4" and 6") with unknown reinforcing steel layout (often the reinforcing steel is laying flat on the ground in SOG's), would you assume the defining capacity is the concrete cone that breaks out from tension, or would it be defined by another mechanism. Perhaps one involving the weight of concrete that could stay together from cohesion (i.e. in lieu of reinforcing steel)?

To me it would be 2-3 mechanisms (at least I've always checked it that way):

1. Getting the anchor load in there as per Appendix D of ACI 318.

2. The shear strength of the slab (see #3).

3. The flexural strength of the slab. Basically what I have always done (in the situation you are describing) is figure how much slab it will take to hold down the uplift force......then consider that portion as a plate (in two-way bending) with simply supported edges [being the only support] and your uplift load as a point load at the center. (If it's out in the middle of the slab; not near a joint.) Then check moments (and shears) as per code capacities.

The problem with that may be that you may not have any reinforcing steel in the upper half/portion of your slab to resist this. That makes it plain concrete.....and for a uplift situation, I don't think that is advisable (even if it works on paper). It's one thing if a slab on grade cracks/fails from a down load.....but a uplift case? Not good.

A alternative might be to cut out a portion of the slab and place a footing.

 

[JAE]

I don't have an answer for you on the anchor bolt capacity question - but per ACI 318 - section 15.7, min. depth of footing to rebar (d) is 6".
So a slab of 4 to 6 inches thick, with unknown reinforcement cannot serve as a "footing" for a column.

Technically speaking, it could be argued that ACI 318 doesn't deal with slabs-on-grade so perhaps a point load on a slab would fall under similar design guidance for PCA slab techniques such as for shelving post loads.

With the heavy steel plate you could argue that your "footing" is the plate, not the concrete.

Are you looking at post-installed anchors here? Sometimes there is a minimum concrete thickness for these as well.


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[pbc825]

I appreciate the advice JAE and WARose. I agree completely with cutting the floor and placing a proper RC footing. I highly recommend this in the majority of instances. However, I face two challenges here. 1) This particular client is a short-term tenant and the steel plate may (??) prove to be lesser cost if you consider the client is a steel fabricator (and not a concrete contractor). 2) There are some "yahoos" out there who arrive on site from a crane manufacturer crank in some anchor bolts and leave. These particular folks have visited this client in the past and the client has "lots" of cranes with steel base plates anchored directly to the SOG.

With unreinforced concrete one could at least conceptualize using the modulus of rupture for limited bending resistance, but I agree this is not wise as the concrete could crack over the high moment areas for really any reason at all (shrinkage, a new anchor bolt hole, et cetera).

The minimum thickness of concrete is typically embedment depth +2" I suspect this is to prevent spalling on the back side during installation. I hear you on this topic though JAE.

The vertical loads are minimal. Considering all loads plus impact, the vertical component is no more than 6 kip. With a giant baseplate, I'm not concerned with vertical load on the SOG.

If the plate weighs enough, there is merit to the consideration of the steel plate acting as a concrete foundation would in this situation. For this particular combination, I'm estimating a 72"x72"x4" plate weighing nearly 6,000 lb to be suitable. *sigh with head-shaking*

 

[MotorCity]

I cannot imagine that the slab would have enough strength to "uplift". It would probably just crack into smaller pieces and you would get some sort of a pull out cone. If you have doubts as to where or what the slab reinforcing is, you should assume its unreinforced. Also, you will want to deduct some minimum amount from the nominal slab thickness in order to account for any high spots in the subbase.

 

[jayrod12]

I agree with MotorCity's last statement.

I have one industrial client that I do a bunch of work for, their existing drawings for the building indicate a 6" slab-on-grade. Everytime we cut into it for one reason or another it varies anywhere between 3" and 12" thick.

The biggest problem is their main lift supplier keeps telling them that they can put a 2-3 ton crane on a 6" slab-on-grade no problem. Then they come to us to sign off on it and we make them put in a proper footing and everyone cries foul.