Load capacity of "typical" Slab on grade

[vger]

I'm an electrical engineer that's been given a supervisory role relating to a production change and associated construction.
We are getting a small hoist installed on a pre-engineered rig that can handle 4000lbs bags. It's a 4 post arrangement with 4"X4" feet that are to be lagged to the floor.

The engineering firm doing our construction work says 7" of reinforced concrete slab on grade will be more that sufficient. The new concrete floor replaces a previous slab that while slightly cracked had no issues.

So my questions are:
1. Is he right?
2. What are the engineering units for "strength" that I need to be concerned with? How are they defined and what typical values should I anticipate.
3. What testing should I expect to confirm that it is sufficient?

The firm is very reputable, but I would like to be more informed about the proposed slab as I don't know if they have done calculations or are just using a "rule of thumb" with a safety margin.

Thanks

 

[rday]

Why are you questioning the work of a firm you state as reputable?

If you would simply like to understand a little more of what is going on you should ask someone with all the information at their disposal. The firm doing the work.

 

[csd72]

It seems reasonable to me, 4000lb is not a lot of load for a ground slab.

It does depend 100% on the ground conditions though, in some very soft soils this may be inadequate, in some strong soild this may be overdesigned.

 

[vger]

Thanks.

I don't have test data regarding the ground underneath but, it's a reuse of existing floor space within the plant which has been here for many many decades. The previous flooring held up racking and supported more substantial loads.
Still, it's nice to have an informed outside perspective.

Thanks again.

 

[civilperson]

Check punching shear, using maximum load on one foot, (4000 pounds and one fourth the hoist weight). Use square 8" x 8" for d/2 around the perimeter of the foot, assuming steel is 3" above subgrade. If shear is too great for the capabilities of the slab, then increase the size of the foot. If this concrete is 3000 psi, then 28000 pounds would be the Vc, (shear strength of concrete), for foot in the middle of a slab.

 

[msucog]

if you're worried about it and want a little more safety factor, ask the engineer to thicken the slab where the posts will be located. you could drop an extra thousand bucks on concrete and significantly thicken the post areas. let's say 8cy concrete / 4 posts = 7'x7'x1' extra concrete.

either way, i'd suggest you rely on the engineer to do their job. and if you would like a more-refined design, get a geotech involved to check out the subgrade/foundation conditions.

p.s. i've had old industrial slabs that were being torn out to add footings for some machinery. under the slabs we found old fill, tires, etc. in other words, be cautious.

good luck

 

[cvg]

why not ask the designer to submit sealed structural calculations? If they can't provide, then they probably didn't do any.

For testing, you should check temperature, slump, air content, take cylinders and break at 7- and 28-days and check compressive strength. Also get the delivery tickets for the concrete.

Since subgrade strength is an essential component of the slab, a geotech could inspect the subgrade before you place the slab.