westergaard report wheel loads on elevated slab

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westergaard report? wheel loads on elevated slab
i need to supply an axle load rating for a 40 year old concrete tunnel roof.  i am looking for guidelines for determining effective widths (one way bending assumed) for point loads and wheel loads for an elevated concrete slab.  i don't think the aashto code equation (4+.06s) really applies because my slab is about 1.75 feet thick (25 foot span), much thicker than bridge decks.  
after searching here i found references to this report be westergaard, which appears to be the basis for the aashto effective width equation.  but i can't find it anywhere and neither can the company librarian.  any suggestions on how to get it?
or suggestions on other methods to obtain a reasonable effective width?  the aashto equation would give me 5.5 feet, but i think given that is extremely conservative.  i have #6@12 to distribute the load out and i would think something like 8*d (12 feet) would be more reasonable.  i think a fea would give me more capacity than i want to use.  what i really want is something to stick in my calcs to verify my 8*d eff width.
any idea?
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i think you're stuck with aashto's distribution factor.  i've seen some other approaches to effective width, but none of them are based on slab thickness.  they are always some function of span length.  i believe the aashto distribution considers wheel spacing.  for 10 foot lanes, there's a line of wheels every 5 feet.
westergaard developed the distribution factors in the 1930's so it might not be easy to find the report; i tried google books, no luck.it's an empirical method but oddly enough it's still good enough even in this high-tech age.
anyway, here are a few thoughts...how much fill is on your tunnel roof? if you're over 2 feet follow the distribution prescribed for culverts. at 2" you would distribute one wheel over an area of ~26 square feet, assuming a 16k wheel.the deeper the fill the better with respect to live load.
also, look at the df  for precast concrete slab units. i'm not in the office and don't have access to an aashto. it might give you something worth using. a 1.75' thick slab would behave in a similar way to a p/c concrete superstrucutre.
bridgebuster, thanks for the precast conc slab idea.  i found in my pci hollow core slab manual a good explanation of effective resisting width.  
if anyone is interested, pci hollow core slab manual second ed., fig 3.2.2.  this is what i need and i think it applies for my case.  basicly in the center you use 1/2 span length for effective width and a narrower effective width at the supports.
still would like to review westergaard's paper though.
you might try the university of illinois for the westegaard report or your local public library should be able to locate it for you.
i thought that i had also seen a textbook by dunham that actually had one of westergaard's chart in it. unfortunately, i cannot re  
as bridgebuster suggested, if you have over two feet of soil over the tunnel the effect of the concentrated loads are substantially reduced. i believe aastho or local dot's may give some guidance.
the thickness of the slab will increase the effective width, but you may conservatively use the same effective width as for thinner slabs.
a single concentrated load may conservatively have an effective width on the order of 0.6 x span. the effective width, however, for one load will be reduced when 2 concentrated loads are closer than 0.6 x span of each other (which is normally the case). an easy way to then determine the effective width of one wheel is 0.3 x span plus 1/2 of the distance between wheel loads. that should get you close.
if you have a line of wheels such as a truck, this gets quite complicated by any other methods and i would have probably revert to using the aastho effective width.