Tire loading on a one-way slab.
Tire loading on a one-way slab.
(OP)
I have a situation where I'm being asked to design a one-way [removable] slab that is going over a [open] concrete trench [packed with cable trays]. The traffic will conform to HS20-44.
Although I'm not the best guy with AASHTO, I think I've come up with a pretty good design......except for one thing: my slab's thickness is outrageous (19"). My controlling case for this is: the tire approaches the support (16k unfactored load). I'm only spreading the load out [i.e. the effective width of my section: "bw", to use in the shear formula] by the assumed width of the tire (8"; one reference I have says the assumed width of the tire is 1" per ton....which seems ultra conservative considering all the dulys out there [double sets]) plus the depth of the slab (d/2 on each side). That may seem ultra conservative, but most of the methods I see for spreading load out apply to moment (not shear as far as I can tell). Any ideas?
Thanks in advance.






RE: Tire loading on a one-way slab.
But I thought I'd check with the experts, one had an example that looked pertinent to your queston:
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http://w
Dear Tire Doctor,
I have a truck with a GVWR of 66,000 lbs with a wheelbase of 302" and a tandem rear axle spacing of 60". The rear tires are Bridgestone M711 11R22.5 G. What is the length of the contact patch of the tires?
Best regards, Leanne
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Dear Leannne,
Thank you for contacting Bridgestone for help with your question.
If we assume the drive tandems are being run at the maximum legal (U.S.) weight of 34,000 lb., we would have a load of 4,250 lb. per tire (34,000/8). Let us also assume an inflation pressure of 95 PSI.
Using the T&RA 'Pavement Contact Pressure Guide' formula, we get a gross contact area of 58.4 square inches. Since the tread width of the 11R22.5 M711 is 8.1 inches, we divide 58.4 by 8.1 and arrive at the footprint length of 7.21 inches.
We hope this has been of some use in answering your question.
Best regards, Tire Doctor
RE: Tire loading on a one-way slab.
Look in AASHTO as they have formulae for wheel loads on concrete bridge decks. There is an equation that sets the value of E which is the width of the slab that resists the concentrated load.
Using only the wheel width for bw is extremely conservative and also simply wrong. When your slab deflects downward under the load, there is more than the wheel width of slab that moves down. By Hooke's Law, if it moves, it has stress. It would be impossible for only an 8" width of concrete to move down while the adjacent concrete stays put.
RE: Tire loading on a one-way slab.
thread507-223271: Point Load on One-Way Slab
RE: Tire loading on a one-way slab.
RE: Tire loading on a one-way slab.
RE: Tire loading on a one-way slab.
One thing I did find was section 3.30 in AASHTO [17th ed.]that lets me use a wider width of tire than I've been using.
Thanks to everyone for their input thus far.
RE: Tire loading on a one-way slab.
RE: Tire loading on a one-way slab.
JAE has quoted some applicable stuff from AASHTO.....and it looks good; but I'm trying to be sure it is applicable to shear. The failure plane I assumed (width of tire+d=bw) may be way too conservative by this approach.
Again: (so I am clear) it's a one-way [removable] slab spanning 5', that is about 17' wide. I determined the critical case [for shear] is when one of the tires gets close to the support (say within "d").
RE: Tire loading on a one-way slab.
RE: Tire loading on a one-way slab.
Be careful how you are mixing codes. If you are using ACI for the design and using AASHTO LRFD load distribution widths then you may be unconservative. AASHTO also requires transverse "distribution reinforcing" to spread the load out. AASHTO also has greater load factors (i.e. Strength I: Impact factor=33%, LL factor = 1.75).
RE: Tire loading on a one-way slab.
As such, AASHTO (17th Ed.) does not require a check for shear. And many bridge slabs have been designed without any shear check.
I cannot find this provision in AASHTO's newer LRFD design manual.
RE: Tire loading on a one-way slab.
I think I've got a conservative design.....just waiting for the client to balk at it.
RE: Tire loading on a one-way slab.
RE: Tire loading on a one-way slab.
cap4000, the dimensions given are 5 feet x 17 feet x 19 inches thick (must have been a typo). It appears that you are saying that this is not a bridge.... I would assume this is because of some of the dimensions however, are bridge girders not spaced closely together?
RE: Tire loading on a one-way slab.
RE: Tire loading on a one-way slab.
I believe section 3.24 addresses one-way slabs of any length. And as miecz states above, any slab designed in accordance with that section shall be adequate for bond and shear. If the slab was designed in accordance with that section then there shouldn't be any concern. If you are concerned, then increase the thickness of your slab a little bit.
RE: Tire loading on a one-way slab.
RE: Tire loading on a one-way slab.
Based on the above, 19" thickness is way overdesigned.
Details of the reiforcement, cover, e.t.c. could be found here:
https:/
Shear is not an issue with this type of slab, but due to the fact that's moveble, reiforce the edges and provide strong transverse rebars in the end zones.
RE: Tire loading on a one-way slab.