Slab on Grade (PCA vs. Westergaard)
Slab on Grade (PCA vs. Westergaard)
(OP)
I'm currently designing a slab on grade for heavy industrial wheel loads. I've been reviewing ACI 360 and found that it gives the Westergaard equations and also presents the PCA load tables in the design examples. Are these two methods to be used in conjunction with each other, or can I use the Westergaard equations independently of the PCA design tables? The PCA tables must be entered knowing the wheel spacing, but the Westergaard equations are independent of wheel spacing. Why would the Westergaard equations imply that the wheel spacing does not impact flexural stresses? I'm finding that in my case the Westergaard equation (for interior slab loading) leads to thinner slabs than the PCA charts do.
Is there anyone out there who does a lot of slab design that can help me reconcile these two methods?
Is there anyone out there who does a lot of slab design that can help me reconcile these two methods?






RE: Slab on Grade (PCA vs. Westergaard)
I'm not an expert on the subject but I'll give you my understanding...
My understanding is that the Westergaard solutions don't consider wheel spacing because they only include the effects of a single point load. In examples I've seen, the design stress has been increased by a set percentage to account for a second wheel (see for example, the slab-on-grade spreadsheet found here: http://www.calculatoredge.com/index.htm). In that sheet, they say that for a 3 to 4 foot wheel spacing, the second wheel will cause a 15% to 20% increase in stress - although I don't know what the basis is for that, so I wouldn't be inclined to take it at face value without more research.
Based on this, unless you're designing for a heavily loaded unicycle (and stranger things have happened), I would go with the PCA charts. Note also that the Westergaard equations are based on wheel load, while the PCA are based on axle load.
Hopefully this helps!
RE: Slab on Grade (PCA vs. Westergaard)
RE: Slab on Grade (PCA vs. Westergaard)
You could use these to investigate the effect of multiple loads at various spacings. Results thus obtained could be used to fine-tune whatever your tables and design codes are telling you.
RE: Slab on Grade (PCA vs. Westergaard)