Point Loads Applied to Slab on Grade 5

[TTUengr51]

I'm currently working on a warehouse in which I have to specify the slab on grade thickness and reinforcing. I have found some good references (i.e. "Slab Thickness Design for Industrial Concrete Floors on Grade" and "Concrete Floors on Ground" by PCA) and they have helped somewhat.

However, after running through some of the design parameters, I feel like I'm getting a much thicker slab design than I originally anticipated. It also doesn't help that my project is being constructed adjacent to an existing 15 year old warehouse with a much thinner slab than I am calculating.

Point loads created by storage racks appear to be the controlling factor. Fork truck traffic is only requiring a 8" slab. A typical post for the rack storage has a max axial load of 15 kips distributed to the slab via a 6" square base plate (36 sq in contact area). Post spacing is 50" in both the X and Y directions. I have a subgrade modulus of 100 pci. Utilizing a concrete strength of 4000 psi and a factor of safety of 2, I'm coming up with a slab in the 11" to 12" thickness range.

Based upon my inexperience with this application and the fact that the adjacent warehouse only has a 7" thick slab, I am concerned that I've overlooked something that is making my design overly conservative. Any comments would be greatly appreciated...

 

[lkjh345]

Using your paramenters, I also get a slab 11" thick.

Based on some past experience with warehouse racks set on floor slabs, and given your loads and spacing, your thicknesses does not seem unreasonable.

You might consdier using 5,000 psi concrete. Gets it down to a 10" slab.

Also, may see if the Geotech engineer can suggest some soil work that can boost the subgrade modulus. This makes a huge difference, and may not be all that expsensive when done to a large enough area.

just my 2 cents.

 

[kslee1000]

I think punching shear is one of the controlling factor, have you checked it? (bo = 4*(6"+d)

 

[UcfSE]

Check the April 2008 STRUCTURE magazine. There is an article for this application: designing for concentrated loads for slabs on grade.

 

[TTUengr51]

lkjh345,
Thanks for taking a look. If you got the same answer, I feel much more confident in my approach.

kslee1000,
Yes, I am considering punching shear. Due to the thickness calculated from the flexural analysis, I'm sure it won't govern, but I will definitely account for it.

UcfSE,
Thanks for the article in STRUCTURE magazine. I actually had it laying on my desk to read in my free time. I just looked it over briefly, and it appears to be more related to column loading from a mezzanine platform. In the introduction of the article it states that it assumes column spacing is sufficiently large and does not account for closely spaced columns, such as my scenario.

The analysis this article included was surprising on how different the results were compared to what I've already done. If I considered my loading scenario, the article's calculation requires a depth of 4" to 6". Obviously, the closer spacing of the posts is probably the reason for the wide range of solutions.

 

[UcfSE]

The article describes a plate on an elastic foundation basically. Mezzanines and such are probably closer to what the author had in mind, like you mentioned. The closely spaced columns could probably be modeled as line loads rather than point loads, if you get really bored one afternoon. Good luck finding information on that topic. Perhaps the references used for the article will lead you somewhere.

 

[abusementpark]

http://www.eng-tips.com/viewthread.cfm?qid=213921&page=1

There is on-going discussion on this topic in another forum.

 

[spats]

TTUengr51: Your safety factor is way too high. A safety factor as low as 1.3 is justified. I don't have the reference handy... I'll follow up tomorrow.

 

[spats]

The document that I referenced in my previous reply is "Designing Floor Slabs on Grade", by Ringo & Anderson, Aberdeen Group Publication.

 

[TTUengr51]

Thanks spats,
The reference you cited prescribes a factor of safety of 1.7 instead of 2.0 as I founded in my previously quoted references.

Another note to point out, in discussions with my geotechnical engineer on the project, he has posed the option of increasing the subbase thickness from the standard 4" gravel to 12". By doing this, it will increase the subgrade modulus from 100 pci to 200 pci. This will reduce the slab thicknes by 2". This seems like a good idea, but I'm curious about the price difference between adding 8" of gravel versus the 2" concrete savings...

 

[KCRatnayake]

Spats,
I'm interested in buying this book "designing Floor Slabs on Grade". I'm in Sri Lanka. I tried Amazon and E-bay but with no success. Do you know any place where I can buy this online?

Clefcon

 

[nicam]

I never worry about what's been designed next door, especially if it's 15 years old.

 

[spats]

TTUengr51: The reference actually reccommends a safety factor of 1.3-1.7 for "important areas" where load magnitudes and loading frequency is less than for "critical areas", where they reccommend a safety factor of 1.7-2.0.

Section 1.4.2 further explains that 1.7 is consistent with concrete design load factors, but of course, that factor is now 1.6 in ACI 318. They also indicate 1.4 is acceptable when impact loads to not exist. I would think in the very least you would find 1.6 acceptable.

The other thing to consider is how your design post load was determined. Often owners will specify the highest stacking of the heaviest pallets, when reality is normally significantly less load. If you press them for how they figured the load, you'll probably find this to be the case. That is why I often see the precribed design load as being approximately 80% of max. If I remember correctly, this is normally referred to as a "use factor". Because of this I usually have no problem prescribing a 1.33 saftely factor on the worst case post load (0.75 x 9 ?f’c).

Sorry Clefcon, I can't find where anybody has this item in stock... not sure why. If I come across it I'll let you know.

 

[himat42]

TTUengr51

Based your case and discussion of other members, Please looked at Chapter 4 of "Ground bearing Concrete Slabs" By John Knapton published by Thomas Telford, 2003. This book used to available from the ASCE Books Store.
Another publication is by British Cement Association
"Concrete Industrial Ground floors-Aguide to their design and construction, BCA TR34, 2003

Another case there are few papers published.
Please looked at the following two papers
1. Elastic Analysis of ground floor slabs under multiple loads" By A.A.Abbas et al, ICE Structureas & Buildings, Vol.160, June 2007, pp. 151-164
2.Deflexion and Bending moments produced by a load distributed over a small area of an infinite circular slab on an elastic founadtion, By; W.E. Scriven et al, The Structural Engineer, October 1965, pp. 345-350.

Neitther of above publications have been cited in the Recently published paper in STRUCTURE magazine.

Good luck and Hope this will help you.