Load Distribution of Wheel Loads on Waffle Slab

[MrEngineerUS]

Hi all,

I am looking at analyzing a precast waffle panel (8'x12') with 5 longitudinal ribs (spaced at 2') and four transverse ribs (spaced at about 4' o/c over the 12' span). The panel needs to be designed for drive-over wheel loads and I'm having a heck of a time finding a distribution model feel adequately models the situation.

The ribs (or joists) are tapered and average about 4" width and are 9.5" in height (equal to the panel thickness). The flanges of the T-beam sections (or panel "skin" as the precaster calls it) is 3.5" thick.

The geometry doesn't seem close enough to a bridge deck on stringers situation to utilize AASHTO distribution methods. However, when the panel deflects under equipment loads there will be distribution between ribs and so I have to come up with some acceptable rationale for distributing these wheel loads. The precaster is looking for me to sign off on these designs for a particular maximum equipment load so he can prequalify them for use in any project.

Suggestions are definitely welcome. Haven't built a finite model like this in a while but currently giving it a go using RAM. Any good resources or go-bys for building a model such as this would be a huuuge help but I'll still be uncomfortable with the results until I can put some hand calcs on it.

 

[KootK]

How will the panels be supported in service? All four sides? Just the two short sides? Just the two long sides? Will the panel edges be tied down to their supports or are they capable of lift off?

My first stop would be a punching shear check based on the 3.5" thickness and the assumed tire contact area.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jike]

If you model the waffle slab properly FEM, it should automatically give you the load distribution to the various ribs.

As KootK said, I would be much more concerned about punching shear thru the VERY THIN 3.5" slab.

 

[ukbridge]

Might be worth putting together a simple grillage to get a feel for the distribution of load. Check out Hambly Bridge Deck Analysis which gives a good summary of the topic.

 

[MrEngineerUS]

Sorry for the late reply here. Busy day!

So, the precast slab panel is to be pin supported along the 8' edges (i.e. the span will be roughly 12' long). They will not be capable of lift off.

I also thought the skin thickness of 3.5" was very thin and it was a major reason why I just can't buy into the AASHTO distribution model. Essentially, these panels are like a mini bridge deck loaded with standard truck loads. I just can't believe they would behave in a manner similar to a standard transportation bridge. I'm rambling... punching shear of the skin will definitely need checking.

Anyway, FEM seems the way to go but it's been so long since I've done this I won't be able to trust the results unless I can get some reasonable approximation procedure down to hand calc verify the analysis.

Thanks for the input so far!

 

[Tomfh]

FEM sounds the way to go for the ribs.

Re. punching, think too about what load area you use. I know of one case where someone jacked up a 4WD and the jack went thru the waffle pod slab.