Effects of a load on SOG and below grade

[DMWWEngr]

I have a question about loads on slab-on-grades and the effect if has on items below grade.

Here's the situtuation:

I have a 40,000 lbs load that is essentially uniformly disrutbed over 8'x15' (120sqft).....imagine a water tank. This is to be placed on a 6" slab. 10+ft below grade there is concrete pipe that we do not want to place any (very little) additonal load on.

My boss, a non-structural engineer, is certain that this load cannot be applied and the slab will have to be competely redesigned. I have gone thorugh the calculations and I see no reason that this 6" slab (with S&T steel only) cannot support this load. I also went on to run an approximate check of the additional load to the pipe and I arrived at a 0.56% increase in load (not much at all).

Does this sound reasonable?? My boss is certian it will not work and I'm getting it will not be a problem at all. Am I missing something??

The additon of this load will also apply a lateral pressure to the wall. The wall is setup like this...The load will be applied to the top slab.

Load
/----------------
/
Grad / Earth
---------/
/
Earth /
/

There is a slight amount of "retaining wall" and the wall then extends down and is a footing. My questin is, where would you check for flexure of the wall?? Would you only look at the small retaining portion since below that the wall would be restained by the earth on the other side?? Would you treat it as pinned between the top of the wall (where these is a slab) and the earth??

I'm just looking for a little guidance. Thanks in advance.

 

[ishvaaag]

I get 0.17 kgf/cm2 increase on load (about 1 m of earth surcharge), which is not much and might not cause much distress in a pipe with sound connections.

However 1 m of additional earth burden is 1/3 of a 10 feet depth, so I neither see a 33% load increase producing a 0.57% increase in pressure.

I don't understand well when you refer to the wall if you are talking about the lateral pressure in the pipe or in some real wall. If a real wall, check, for I have announced and then seen the fall of a house where a mere 50 cm tall band of earth was without retaining wall for over 6 months at the free air.

To ascertain the relative influence of your load I would go to use my Boussinesq freely downloadable Mathcad sheets, from which some assessment of both the vertical and horizontal pressures coming from the load I would derive. Still I muse the only 0.56% increase looks unlikely.

Other than that, I would have to delve in books for a more proper appraisal of your case and other may give better advice form his of her daily practice.

 

[DMWWEngr]

Thanks for the response. I think I explained it poorly...yet again! Second try...

Question 1:

The building is existing and has a floor elevation that is about 6' above the surronding ground. (It looks like the above picture, except the wall is straight The existing building was placed over the pipe and there is 10'+ of existing fill over the pipe already. We want to add a peice of equipment that wieghs 40,000lbs and is full bearing at 8'x15'. I get that the additional load of this 40,000 lbs will increase the load on the pipe (per sqft) by 0.57%. Does this sound reasonable??

Additionally, does it seem reasonable for a 6" slab-on-grade to support the peice of equpiment (neglecting all other paramters)?? Is there a way to check punching shear for this (very doubtful this will be anywhere close)??

Question 2:

If this peice of equipment is installed this will create an additional horizontal load on the existing wall (as shown above). My question is what is the length to consider for flexure of the wall above?? Is the length in flexure JUST the 6' "retaining wall" portion...treated as pinned?? Or is the entire length of the wall used for flexure analysis??

I hope this is easier to understand!!

 

[LPPE]

Draw your free body diagrams of your situation. That should tell you where your critical point is at your retaining wall. The resultant of your earth pressures where there is earth on both sides of the wall will be approximately equal in magnitude and location. Your worst case should be the unbalanced load condition at the top of the wall.
Also, I think your pipe would be fine 10' down from a 333psf loading. How would redesigning the slab reduce any loading on the pipe? Do you mean to redesign the slab to span between retaining walls?

 

[DMWWEngr]

pylko,

You got it!! He wants to design an "elevated" slab for this. I think he's crazy and he doesn't believe me that this will work...(since I'm fresh out of school and he has the PE)!! I want to get some insight from those with some experience before I tell him he's WASTING money. I figure many of you will have a "feel" for this type of loading and situation.....I don't have that yet

The worst part is this is being looked at to save money. Removing the slab and constructing an elevated slab (and somehow tying into the existing walls) would not be cost effective....let alone not needed based on my preliminary calculations.

Thanks for your comments. I'll work on drawing out a detailed FBD and wait to see what the others have to say!!

 

[ishvaaag]

Respect the wall I maybe would accept 1 to 2 or may be 1 to 1 in plan planes spreading form the load vertices defining what portion of wall would be taking the load. Once selected, I would place it at the distance and maybe would distribute all the load as if of a contiunuous load would be the case.

In any case I have also the option of directly determinig what the tota horizontal pressure in such a vertical plane a flexible rectangular load causes, by Boussnesq's equaiton integration of the stresses. A mix of both could give a more than safe enough solution not overly conservative.

Respect thickness, the thicker the better but if no punching only potential irrelevant to the behaviour cracks by this thing you will be avoiding.

Since the stresses are moderate, effects on the pipe, other than "change", always an imprevisible companyon (invariably in Sapin, when there is a ruin with fatal casualties the construction was being structurally affected, lately mostly "rehabilitations" or betterments", 2 cases this week in Spain), look should be tolerable.

Yet, again, I see not (without making calculations) how a 33% increment in the load may cause only a 0.56% maximum increment in the Load of 0.57% at so low depth. A 20% or so or even more sounds more likely (except you use design equations that have embedded coverage for some non-extant but acknowledged burden of earth of about 14 feet).

 

[DMWWEngr]

Thank you ishvaaag,

I found the error in my logic. I knew that the previously posted 0.56% was too low, hence the reason for asking the question. After making the necessary changes I now get an increase in load on the pipe of 18%. Much more reasonable!!

Also could you go into a little more depth on this statement....

".....if no punching only potential irrelevant to the behaviour cracks by this thing you will be avoiding."

Thanks for your help!!

 

[ishvaaag]

I meant by that sentence that if the load is really overally uniform, not resting on just say 3 or 4 pads, punching on the ample perimeter is unlikely to control. Only a check on the flexural effects of the new load may discover something of interest, it might be that the slab on the ground cracks on having so slight reinforcement out of flexure, yet it remains unlikely if uniform (imagine 1m of earth, everyone has seen big mounds of say salt on slabs and no cracks at sight).

Cracked or uncracked to the soil is more or less the same at some depth, this completes the comment.

 

[Ron]

DMWWEng...the increased load on the pipe is not a big deal. Yes, your equipment and slab will increase the lateral load on the retaining wall IF the slab is not supported by the wall (floating slab); however, this is also not such a big deal. This can be easily computed with the soil properties and accomodated in the design.

I would not try to create a slab/beam system to span the walls. This is an expense that isn't necessary.

The 6-inch thick slab on grade can support significantly more than the load you are placing on it. As an example, a truck wheel load of 4500 lb. would create a load of just over 100 psi (contact pressure)on the slab (your load is just over 2 psi), while the 4500 lb load would be transferred through the slab (though with a lot of attenuation) and into the soil. Shear on the slab due to the load would be very small.

 

[dlew]

DMWWEngr,

Some thoughts....

1) The new 40,000# distributed over the 8' by 15' area give a pressure immmediately underneath the slab of 333 psf.
On top of the concrete pipe (ten feet below the slab) the additional soil pressure is approximately 0.25 times 333 = 83 psf. I am getting the additional pressure from a "bulb of pressure graph", based on Westergaard's formula and published in the book "Soil Mechanics" by Sowers and Sowers. Since the graph is for square footings, I approximated the equivalent square footing as 11' by 11'.
The existing soil pressure on top of the concrete pipe is, say, 90#/cf times 10' = 900 psf.
Thus the additional load on the pipe is 70/900 = 9.3%.
A load increase on the pipe of 9.3%, could justify to investigate the pipe stresses. There is an engineering procedure to calculate the stresses on buried pipes. They depend on the load above grade, the depth of burial, and on how was the pipe placed (trench condition or on fill). This procedure could be found in drainage and piping handbooks.

2) If the additional equipment is not located over the "active pressure wedge" of the fill behind the wall, its weight does not affect the retaining wall. For a 6-ft high wall, locating the equipment foundation at least 4' away from the inside face of the wall would keep additional loads off the wall.

3) To investigate the existing slab, you need to have an idea of the underlying soil. The design of slabs on grade is based on the modulus of vertical subgrade reaction of the soil. It can be obtained from soil lab tests, but generally you could get a typical value from "Foundation Analysis and Design" by Joseph Bowles.
The existing, under-reinforced slab could get some cracks under the new load, unless the subgrade is well compacted and have a high subgrade modulus. The critical bending stresses would occur around the periphery of the loaded area. Shear on the slab should not be a problem. I think it would be a good idea to cut the existing slab, and pour a new reinforced concrete mat under the equipment, with an isolation joint between the new concrete and the existing slab. A good guide to design slabs on grade is a brochure from the Portland Cement Association "Slab Thickness Design for Industrial Concrete Floors on Grade", by Robert Packard

Hope this could help
AEF

 

[DMWWEngr]

I would like to thank everyone for there responses. I now feel much more confident discussing my position with my boss. I appreciate everyones input and I will do a few more checks before approaching my boss.

In the plans this building was "supposed" to be constructed on sand fill. Since this is not known for sure I was leaning toward cutting the old slab out, compacting the soil as necessary, and pouring a new slab that is isolated from the existing slab. As dlew also suggested. I will look into obtaining the PCA reference that dlew suggested.

In regards to the pipe: This load will not be placed directly over the pipe. In reality this load should be placed far enough away from the pipe that no increase in load should occur. I wanted to check out the "worst case", if the load was directly over the pipe. When the exact position of the load is determined I will check the actual increase in load on the pipe.

Thanks again everyone!!