Warehouse Slab on Grade Supporting Rack Post Loads 5

[ajk1]

Anyone really experienced in design of slab on grade supporting rack post loads? If so, can you help with the following:

Is it more economical to use reinforced or unreinforced slab on grade?

Any good reference papers specifically addressing this?

If you use reinforced slab, how far apart do you place the constucution joints?
Do you use intermediate saw cuts?
What might be the typical reinforcement used?
Do you use SAFE to model it
In an unreinforced slab, where do you place the sawcuts - do you do one down the middle of the aisle where stresses are amximum?
Do you thicken the slab at edge conditions?

The post loads are about 18,000 pounds unfactored, the modulus of subgrade reaction is 100 pci. I was thinking of 30 MPa concrete (4350 psi)with a specified 28 day shrinkage not exceeding 0.035% as per CSA A23.1.

Very preliminary calculations indicate a 10" thick unreinforced slab would work. I suppose that an 8" slab with rebar with equivalent resisting moment to the plain concrete might also work.

 

[a2mfk]

Just went through a lot of research on this for a project, and the most common method is to size the slab as unreinforced to carry forklift and rack loads. Then you size the reinforcement for shrinkage stresses. There are many methods and papers on this subject, the PCA and Army Corps have similar charts and methods..

If you use reinforced slab, how far apart do you place the constucution joints?
>> This is up to the contractor, but I would suggest using diamond dowels to reinforce the joints. Especially if you have forklift traffic.

http://www.pna-inc.com/products/diamond_dowel/

Do you use intermediate saw cuts?
>> Absolutely, space depends on thickness, reinforcement, column location, geometry of the slab

What might be the typical reinforcement used?
>>Depends on what method you use and slab thickness, see below

Do you use SAFE to model it
>>I use a couple of different methods and compare the results, the WRI article below is a good place to start

In an unreinforced slab, where do you place the sawcuts - do you do one down the middle of the aisle where stresses are maximum?
>>If you can avoid aisles and rack posts, that is always nice


Do you thicken the slab at edge conditions?
>>Depends on where it is, forklift traffic, other loading conditions


I like the somewhat conservative confirmed capacity procedure for designing the reinforcement.

http://www.wirereinforcementinstitute.org/pages/pubs/pdf/TF 705-R-03.pdf

Good general article I just found:

http://www.structuremag.org/article.aspx?articleID=422

Mathew Stuart's online class is a good primer:

http://www.pdhonline.org/courses/s132/s132.htm

Older free ACI 360 which has many of the different methods and charts:

http://civilwares.free.fr/ACI/MCP04/360r_92.pdf

This is a newer, alternative method... I have not heard too much from others on this method, but it seems like you get much more liberal and likely accurate/practical designs:

http://www.structuremag.org/article.aspx?articleID=613

 

[nutte]

This is a newer, alternative method... I have not heard too much from others on this method, but it seems like you get much more liberal and likely accurate/practical designs:

http://www.structuremag.org/article.aspx?articleID=613

I came across this article a while back. it gives significantly higher load-carrying values than the three methods in ACI 360. The drawback is it applies only to concentrated loads applied at the center of the slab, not near an edge. Does anybody know if there is a comparable procedure for concentrated loads located near a slab edge?

 

[Ron]

Agree w/ a2mfk. Joints are an issue, but generally less so than we anticipate...but then, usually the actual loads are less than anticipated as well.

Design the slab for the forklift traffic and the rest essentially takes care of itself...for instance for larger racks and higher racks, you need a larger forklift, so you design for higher loads.

The static concentrated load at the rack legs usually results in a thickness controlled by shear. Agree again with a2...use plate dowels at joints and this won't be a big deal...but again, you're designing for the forklift!

 

[dik]

I've often found that for an elastic solution, with large rack loads, in addition to the shear, the flexural strength also plays a significant role. With an elastic solution, the spacing of rack loads can either increase the flexural stresses or diminish them, depending on the spacing...

Dik

 

[ajk1]

a2mfk, Ron and Dik - thanks for taking the time to reply. You have given me a lot to study here, particularly a2mfk. Thanks again.

 

[hokie66]

With heavy racking loads, make sure you involve a good geotechnical engineer in the design. If the stack loads are high enough, settlement in the stack areas can result in a floor with ridges and valleys. Not a pretty sight.

 

[hokie66]

Another thread, with differing opinions, about warehouse floors. My experience is different from some, in that I think joints are most often the problem. Jointless aisles are now common, poured in long strips. Jointless floors are less common, but coming.

thread507-264176

 

[dik]

Until my latest computer upgrade... lost my 5-1/4" disk capability, I used a PCA airport paving program to establish the elastic stresses. Based on early work by Packard (?). The 5-1/4" disk is copy protected (made a 3-1/2" image that is non-functional). I haven't had a need to upgrade it, and, will do that if necessary. I found it to be a very good program... from racks to 747's...

Dik

 

[Ron]

dik...still have my old elastic layer programs as well. The one that Packard did was based on ELSYM5 and Chev-PC, both of them 5 layer elastic layer analysis programs.

The State of Washington DOT has a series of programs that are free and easy to download. One of them, EverSTRESS, is an elastic layer analysis program based on the same algorithms as the older programs. They are pretty good, but a little quirky on the input data. Once you get the hang of it, not bad though. Check them out.

If you would like, I'll send you a copy of ELSYM5. It is a DOS based, public domain program, but it is very quirky on the monitor output...you have to make sure you can emulate ansi.sys in the command.com file in the root directory of the C: drive. That gets harder and harder with each successive iteration of Windows.

 

[dik]

Thanks, Ron... be appreciated.

Dik

 

[a2mfk]

hokie- can you expound a bit? Lack of joints is a problem because it leads to cracking? Or joints are a problem because of forklift wheel damage?

Also in your long strip scenario with no control joints, do you let it crack and then epoxy fill the cracks? Or try to provide longitudinal reinforcement to keep the cracks tight.

 

[hokie66]

a2mfk,
The joints are damaged by forklift wheels, and in some cases this is aggravated by curling. In external slabs subject to rainfall, joints also are where you get pumping of the subbase.

You can't stop cracking, but you can reinforce to keep the cracks tight. Filling is only required if for some reason a crack opens wider than expected.

 

[a2mfk]

BTW Hokie, was not being argumentative, just picking your brain. I have done very few warehouse slabs, but we have one right now which is where I got all that research info from.

So you are saying there is a movement in warehouse slab design with heavy forklift traffic to limit the joints by placing in long strips, or if possible to coordinate the rack placement with the joints so the joints are not exposed to wheel traffic? I have not heard of this before, any links to articles?

Funny how long concrete slabs have been around, and they seem so simple, and we are still tweaking how we design and build them...

I posted yet another driveway where the use of control joints has paid off, I could not find a crack... Of course, as you contend Hokie, this is not a good approach with warehouse slabs and would be pretty expensive.

 

[a2mfk]

Ooops, here is the crack-free driveway...

 

[hokie66]

a2mfk,
A lot of the information available to date is somewhat proprietary, and some of it is just advertising. Jointless floor construction has not yet come of age as a "code" way of doing floors, but a lot of big industrial jobs are going that way. Most involve steel fibres. One UK company's website attached.

http://twinplan-t-med.com/downloads/Twintec-Industrial-Flooring-Brochure.pdf

 

[ajk1]

1. What concrete strength do you usually specify for the warehouse floors with rack storage?

2. Do you use a dry shake floor hardener and if so, any particular type, product and rate of coverage?

2. What size base plates are used on the posts? Does the rack manufacturer determine that? Or should we as the structural engineers for the floor tell the rack manufacturer the size of base plate required so that the slab works for shear?

The warehouse is about 100,000 sq.feet and the racks are 4 levels of shelves, about 20 feet high, back-to-back racks.

 

[Ron]

1. 4000 psi or 30 MPa

2. No. Not a fan of dry shake hardeners. Use good quality concrete and make them put in what is specified without wetting it down. Flatness and finish are important.

3. Base plates are usually standard sizes on racks. Keep in mind they are sized by the manufacturers for anchorage and not bearing. They are usually the same size as the rack column, with a short tab extending inboard of the racking system to keep them out of the aisles and to provide unobstructed storage on the floor between the rack columns. They are anchored against lateral movement from loading/unloading and from overturning when loading/unloading onto upper shelves.

 

[ajk1]

Thanks Ron. That is helpful. Any comment on the following would also be helpful:

1. I am thinking of specifying the concrete sterngth at 90 days rather than at 30 days. Example 30 MPa at 90 days.

2. I would provide reinforcement as crack width control, based on frictional drag, using friction coefficient of 1.5 or maybe 2.0.

3. We normally pour the slab in 3 m (10 ft) widths and do transverse sawcuts at 40 times the slab thickness, using early entry saw. We use 40 mm (1.5") coarse aggregate and specify the concrete mix to be designed to achieve maximum bulk density. Also require Modulus of Rupture precosntruction tests.

4. Do you have any internet reference site that I could find the manufacturer's base plate size for a 9000 pound rack post load?

5. Is there any maximum distance between construction joints that you recommend?

6. Is there any reference for the diamond plate joint shear transfer dowels - how to specify them; their spacing, etc.

7. If the racks are 4 shelves high, with a post load of 40 kN (9 kips) and 80 kN for a pair of adjacent posts, would it be necessary to also check the slab for the forklift truck wheel loading?

 

[dik]

1
You can spec any strength you want, as long as you are comfortable with what you get. If you have experience with 30 MPa at 90 days, and it will work properly, then spec it, else, spec 25 MPa at 28 days... it may be the same. I don't buy the longer time period, except for high fly-ash concrete where there is a slower strength gain. I sometimes look at the extended period of time for problems encountered during construction.
2
YOu can calculate the friction drag for rebar. It provides a bit of a quantified approach to providing rebar. Proper concrete and proper consolidation, sawcutting, curing, etc. will go a long way. I recently did a printing press slab that had 0.8% rebar because of the L/W and no joints permitted for the length.
3
Just be careful about the L/W ratios.
4
You can check it for confined bearing and will end up with a small pad. Check the small pad for punching shear and don't go too tight. Check the slab for flexural stress and provide a suitable safety factor. Check PCA Airport Slab publication for permissable tensile strength, in a pinch. Also use ACI guidelines. Check concrete tensile strength, not compressive, too. Beam tests or splitting cylinders.
5
Control joints should be 15' to 25' depending on the slab thickness. Critical is sawcutting time and curing.
6
I've not used diamond plates, but they should work well. Someone else can fill in the blanks... Smooth round dowels can be supplied in a 'rack' that can readily placed and they offer less perimeter for cracking.
7
Forklift loading can often be critical, depending on the front wheel load and tire construction and frequency of travel. Joints with forklift are often critical and I think I've done more floor investigations for damage caused by channelisation of forklift traffic and crossing joints than all others combined. Use a proper 'hard' or suitable joint filler to confine the concrete edge at the joint.

Others can expand or clarify the above.

Dik