Slab on Grade Capacity (Wheel Loader) 1

[Simba13]

Hey everyone,

I'm working on a capacity check for a slab on grade (an existing boat ramp to be specific). The client basically just wants to check that he can run a wheel loader on it to clear off silt build up. I'm not running a model, just sticking to hand calcs, I figure that there are two general things I need to check, flexural stress (Westergaard) and punching shear. I've muddled through using some assumptions for subgrade modulus to get my interior flexural stress to be around 140 psi (I'll leave corner and edge out of this post for simplicity). Am I correct in thinking that I am comparing this to the modulus of rupture (perhaps with a FS = 2) on the capacity side? I've never done this particular calc before so a sanity check would be much appreciated.

Thanks,

I'm just a humble EIT so be gentle

 

[phamENG]

Take a look at this document: Link

If you know the thickness and reinforcing, you can probably use the design curves there to back out an allowable capacity/size of forklift.

Question, though: why are they trying to use a forklift to move silt on a boat ramp? That just seems like a bad idea. If you're talking a standard fork truck (like you'd find in a warehouse), they have a lot of usage limitations on slopes. And depending on the tire material, a slippery slope could be a really bad idea. And how are they moving the silt? Did somebody come up with a loader attachment for the forks? There's specialty dredging equipment made just for the de-silting of boat ramps for a reason. It's faster and safer to use the right equipment.

 

[Simba13]

Sorry that is the second time I've done that today. By forklift i meant wheel loader.

 

[phamENG]

Ah. Makes more sense.

 

[JLNJ]

The main driver for slab on grade strength is the quality of the subgrade. I don't know anything about boat ramps other than that they experience a lot of water events. If you have a really weak spot, the typical slab-on-grade design methodologies won't apply.

 

[SlideRuleEra]

Simba13 - Slope of a boat ramp will alter the loader's weight distribution to the tires compared to loader manufacturer's specs. This will be especially true with a load in the bucket. Check to see if this factor is significant.

http://www.SlideRuleEra.net

 

[Simba13]

Ah, yea, that makes sense SlideRuleEra, I hadn't thought about the slope changing the distribution of load. Thanks.

 

[Lomarandil]

I always assume that wheel loaders can have 85%+ of the load in the front wheels, even without slope

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just call me Lo.

 

[Simba13]

That looks a bit closer to 100% on the front wheels . Taking this one step further, I guess it would make sense that I would also have to consider not just most of the load in the front two tires, but also more load on one front tire if it is sideways on the ramp (like when turning around for example), I'm not a contractor but I'm guessing they won't just drive straight down and then back straight out.

 

[SlideRuleEra]

...more load on one front tire if it is sideways on the ramp...

Excellent insight! I was making this sketch when you posted:

Also, depending on how the ramp was constructed, quality of the concrete for the lower part of the ramp (where the silt will be) may not be very good. Use conservative concrete properties for the analysis.

http://www.SlideRuleEra.net

 

[EdStainless]

We always used a tractor with a grader blade on the back. We would back down the ramp, take a bite, and drag it up the ramp, and then use the front end loader in the parking lot. We didn't want to be loading nose down.
It was a bit more of a mess but avoided the load issues. And if you are nose down and loaded, on a sloped wet surface, and you unload your rear tires backing uphill can be a real challenge.

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

 

[Simba13]

SlideRuleEra, yeah, they've said 4000 psi but who knows. This isn't a big project so I'm just going to have to make some assumptions as far as materials go. Your sketch definitely makes sense, now I just have to mull over how to actually calculate this worst case load, 3d problems have always been kind of tricky for me.

EdStainless, that's a really good thought, I might call and see if that's something they'll consider, it would certainly simplify my calculation.

Thanks for the help both of you

 

[WARose]

Am I correct in thinking that I am comparing this to the modulus of rupture (perhaps with a FS = 2) on the capacity side?

Yes, that is typically what I have seen. (I.e. the [flexural] capacity being= the modulus of rupture/FS.)

(I'll leave corner and edge out of this post for simplicity).

A word to the wise: for a slab on grade.....those conditions can (and typically do) control the design.

 

[Simba13]

Yea, WARose, I have it calculated, I just didn't want to throw a bunch of numbers at everyone, and you are correct, they control over the interior stress.

I have a follow up question though, about the loaded contact radius of a tire, the tires I am working with vary from 17.5-25 12PR L2 to 20.5-25 XHA tires, this is for a CASE wheel loader. And So I get the loaded radius off the manufacturer site, and I get around 26", ok, well then I calculate it just to be safe using sqrt(P/(pressure*pi)) and I get 6.7" (P=12000 lb and pressure=85 psi), this drastically changes the stress, I pass with the manufacturer radius, but fail with the calculated one. How can they be so different? I realize they will vary based on pressure and wheel load but I actually don't see a way to get 26" in that equation using realistic pressures and wheel load.

Thanks,

 

[CarlB]

I think the "loaded radius" dimension given is not related to the contact area.

https://tireguides.com/TireTips/TireDocument/9

 

[SlideRuleEra]

So I get the loaded radius off the manufacturer site, and I get around 26"...
...then I calculate... 6.7"...
How can they be so different?

Because the tire manufacturer went to great lengths to design tires to maximize properties needed for harsh, off-road use... including contact area.

A friend and colleague of mine is the retired lead engineer for Michelin's military tire development. They are precision-designed for specific applications.

Keep in mind that the contact area loading is being applied on a sloping surface. This may not be significant and can be neglected, but you should not just ignore it without comment (making it look like you didn't know or forgot).

http://www.SlideRuleEra.net