Design of Slabs on Grade for Warehouses 3

[ironmon]

Design of Slabs on Grade

We are in the process of designing a large warehouse with both Forklift traffic and heavy post loads from pallet storage systems.

We have a initial thickness design of the concrete slab following the methods outlined in PCA publication “Slab Thickness Design for Industrial Concrete Floors on Grade” as well as WRI Charts as used in Ringo/Anderson’s Book “Designing Floor Slabs on Grade”. These elastic methods give slab thicknesses in the range of 10 + inches.

Peter Higgins cited an article by Shentu “Load-Carrying Capacity for Concrete Slabs on Grade” (ASCE Journal of Structural Engineers 1997), that results in thinner slabs (Less then 8 inches), however I have NOT been able to find any reference to the usage of this method in design practice. The current ACI Report on “Design of Slabs-on-Ground” ACI 360R-06 (2006 publication) does NOT reference this ultimate, non-elastic method.

My question is, there anyone using a method other then the conventional PCA or WRI methods to size Slabs-on-Grade for Storage Rack Post Loads, and if so what is their basis and background.

Advise and guidance is welcomed on this matter.

 

[JAE]

We don't use that ultimate method cited...always have relied on the Ringo book.

Also - did you account for granular subbase and its upgrading effect on your k value? This makes a big difference in thickness.

 

[oneintheeye]

If you want to go thinner, concrete society report 34 has guidance. 150-200mm. It can be downloaded from IHTS if you have access. Think I have posted a copy on here previously. Do a search.

 

[civilperson]

Punching shear for steel wheel forklifts and post loads may control the thickness.

 

[hokie66]

The thinner you get, the more potential problems you have with the joints, which is where warehouse slabs nearly always break down. Thinner slabs and less joints can be achieved by post-tensioning.

 

[msucog]

a big warehouse might be a good try to try cement stabilization to dramatically improve the subgrade conditions. it's a big open area so the pulverizer can make long straight passes. depending on the subgrade gradation and plasticity, you might see some bigtime gain from perform such a stabilization. i do suggest using a reputable company though since "cheap" is often "cheap" when it comes to these sort of things (that's my experience anyway). you could probably check it out on paper before doing anything that costs real money by seeing what kind of improvement you can get and need for it to be cost effective. in other words, if you can cut the concrete thickness down by say 3", how much soil cement (real soil cement...not cement stabilization) does that buy you?

 

[oneintheeye]

would agree that improving the ground is a sure way to improve capacity if using westegradd (sic) or winkler spring thickness design. Look up thread on joints in ground slabs, document i mentioned earlier is in there. sorry don't know how to link.

 

[ironmon]

For interested parties, there is a Article in Structure Magazine that addresses this issue.

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

 

[abusementpark]

Ironman, that is a great article. Thanks a lot.

It looks to me like amount of steel reinforcement is not a big consideration in these designs??

 

[dik]

civilengineer... you have to be careful about looking at punching shear... often the governing stress is due to flexural tension caused by the point loading and the spacing of the point loads if within a radius of influence. This is where the modulus of subgrade reaction has a real impact...

Dik

 

[haynewp]

I am surprised by how high the values are given in that article.

 

[hokie66]

The article in Structure Magazine really only addresses the thickness design of the slab based on the leg loading from the racks. I have investigated a lot of failed warehouse floors, and have never actually seen a punching shear failure in one. The two most common modes of failure in my experience have been 1)settlement of the storage area relative to the aisle, and 2)joint failure due to the forklift wheels, especially with steel and hard rubber wheels. The first is a subgrade issue, and the second is a joint design problem, neither having much to do with slab thickness. For external storage slabs the most common failure I have encountered has been pumping at the joints.

 

[msucog]

from my experience with external storage areas, i also see the pumping at joints quite often. it's usually traced down to water going through the joints and sitting there until a fork lifts or heavily loaded tractor trailor drives over the area repeatedly. it eventually causes the materials around that joint to move around and deteriorate until the pavement begins to crack and fail and then the problem is amplified as more and more water gets to the subgrade. this problem is not seen nearly as often where the concrete pavements are maintained and joints caulked.

and i never understood just how bad forklifts were until i did a few designs. hard wheel forklifts are about as bad as it gets, so any slight irregularly will be pounded by the wheels day in and day out...especially at joints.

discuss the issue with your project geotech. some soils here suck as subgrade materials while some do fairly well if you can find them on the jobsite. so the geology may have an effect on the k value. the differences between wheel loads and storage rack support will also change things a little.

 

[abusementpark]

hokie66,

For the joint design, is the primary concern to ensure that the dowels are spaced close enough to transfer the load? Are there other concerns?

 

[hokie66]

The capacity of the dowels is part of the problem. Yes, you need a sufficient number and size of dowels to transfer the load without deflection across the joint. But the dowels can also contribute to the problem by causing restraint cracking perpendicular to the joint, particularly near the corners of panels. For that reason, in Australia we tend to use square bar dowels with a compressible material each side. Diamond shaped plate dowels are also gaining in popularity. Look up Danley Construction Products.

Deterioration of the joints due to lack of support for the edges is also a big problem when you have hard forklift wheels. There are a lot of ideas of how to solve this problem. In the past, a lot were armoured with angles, but that was tedious and did not always work. There are proprietary systems now which do a good job. The one which I have had experience with is by Lesa Systems. This company also supplies the dowel types I mentioned above. And they also specialise in repair systems for failed joints. Joint protection is not cheap, but "pay me now or pay me later".

This is from an Australian perspective, but I think you would have similar products in the US.

 

[LIGWY]

I can not find a copy of the Ringo/Anderson’s Book “Designing Floor Slabs on Grade” anywhere. Does anyone have any suggestions? I am starting to design more floor slabs.

 

[ironmon]

It is put out by the
Aberdeen Group
426 S Westgate St
Addison, Illinois 60101

Phone 630-543-0870 (from an old publication)