5" Slab Max Capacity for a Warehouse 1

[Claudia_C]

I work for a pipeline company in the La Porte, TX area and currently we are trying to change the use of an existing 5" slab on grade to be used as the floor for a "warehouse" that was previous office space.

The warehouse will now hold rows of shelving racks. The racks we intend to use can hold up to 9000lbs. The rack manufacture has told us that the 5in concrete slab will not hold the racks with max loading. They gave us two options. Option one: place footings under each rack post. Option two space our the shelving racks.

We suggested increasing the the plates where each post will sit from a 6INX6IN plate to a 12inx12into disperse the loading but they said this would not be enough.

Racks are 8ft wide by 4ft deep.

Could we place 2in thick x 8ft wide x 4ft deep steel plates under the entire row of racks to disperse the load and avoid having to place footings under each post? For example place a long continuous steel plate under the row of racks to help disperse the loading into the slab?

 

[JAE]

It isn't totally the slab thickness that matters but the stiffness of the soil or granular material below it.

If you had a subgrade with infinite stiffness the 5" would be more than adequate.
If you had loose muck below it an 8" slab might not work - or even 10".

We had a similar project a few years ago and had a local geotechnical engineer take some borings.
After analysis they provided us with a subgrade coefficient to use in slab design calculations (see ACI slab resources for this).

We've typically gotten the racking to work in the past way before the proposed forklifts. We've had to ask the client to iterate down in forklift size, wheelbase, etc. to find one that worked.

 

[dik]

With normal soils, 5" is a little light for all warehouse slabs other than those that are very lightly loaded. Rack loading can be interesting; the load from one leg can increase/decrease the loading on the adjacent leg depending on the slab/soil stiffness and the spacing of the legs.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[1503-44]

The rack mfgr is out of place telling you what concrete thickness to use. As JAE mentioned, it's controlled by soil below and secondary by slab reinforcement. Mfgr doesn't know either.

Your load figures to 300 lbs/ft2
Even bad soil alone should hold 1000 psf.
Most likely is that the probable clay below is good for 2-3000. Probably 3 to 4000, if it hasn't gotten wet during the last 25yrs.

If the condition of the slab is good, i.e. no cracks from expansive gumbo clay prevalent in the area, go ahead and load it up. The clay there is typically expansive, but very stiff.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[JAE]

I agree that rack loads, and forklift wheel loads, might be way below the soil's maximum allowed bearing capacity.

But keep in mind that the slab thickness derivation isn't based on soil strength - but rather soil stiffness - the modulus of subgrade rigidity (k)
Two different things.

The geotechnical engineer can investigate what you actually have there in terms of stiffness.

 

[1503-44]

If it isn't wet, it's nearly concrete. The water table may be the key to that. La Porte has areas where it is high. Your geotech will know where it is problematic.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EdStainless]

We had 6" of good concrete over very good base in our high rack area.
The limit was the forklift loads.
We couldn't use our larger trucks.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Claudia_C]

Thank you all for the great input!
Sounds like my next step is to get a boring sample and find more information on the subgrade to determine if the racks can be placed?

I would like to avoid placing a footing under each rack post/leg because doing this would kill out budget and schedule.

 

[LittleInch]

How many posts in your 8 x 4 rack?

Your issue for such a thin slab with no mention of rebar or mesh is localised soft spots which crack and subside.

Or build some good foundations at say 8 ft separation and span across with a steel or concrete beam?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Claudia_C]

Good morning littleinch
Excuse the late response. The rack has 4 posts. One at each corner of the rack. The first shelf of the rack is at about 4ft high.

I believe the existing foundation is a post tension slab. I believe there is rebar in it.

We plan on placing these racks right next to each other, say a few inches apart to maximize our storage space. Each row of racks will be spaced by about 8ft or so.

The rack manufactured suggested we build foundations but we do not have the time to do this or the budget.

 

[1503-44]

If you add a full foundation, you will have to bust up the slab to do it.
Load it up.
If the slab breaks, build your foundation. Nothing lost. It was always going to break.
If it doesn't break, you're home free.
But first discuss that "slab test method" with the boss.
Like, "You know, I was thinking we could save time and money ... What is your thoughts on actually trying that out?"

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Claudia_C]

Thank you! I'll suggest testing out the slab first to make sure we exhaust our options.

 

[LittleInch]

So you're going to need two 6 x 6 plates next to each other loaded with at least 2,250 lbs on each.

But also consider your shock loading form however you're going to load things onto these racks. So maybe double that.

So in the area of basically two truck tyres you're going to load up 4,500 lbs on each one just over two tons (2000kg)

Or just get park a trailer and get it loaded up on the front rests? or drive a suitably loaded truck and trailer round and see if it breaks

Or a forklift?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Claudia_C]

To be clear, adding the plates under the racks could help with the load distribution?

 

[1503-44]

It's probably not the total load that will be dropped on the rack at once.
I'm imagining many smaller items that will be pushed into the racks one at a time, so I don't see huge x2 drop shock loads.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Maybe not, but as a static test it needs something extra to simulate this loading over years.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Claudia, have you found a local geotech?
I'm convinced, and I have done a lot of work in the Houston area, if you do not have a high water table, this may not be any problem at all.
Invite him to lunch. See what he's got to say.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Claudia_C]

1503-44 we have a meeting this week with the rack manufacturer and the Geotechnical eng. Hopefully we can get some answers. I'll post an update.

 

[1503-44]

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[chicopee]

Dunnage or spread footings under the legs of the racks could be a viable solution. Check out loading from hydraulic and lattice boom cranes which operate on all types of soil found in construction sites. If skeptical do a test load.