Tire loading on a one-way slab.

[3doorsdwn]

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.

 

[CarlB]

The maximum allowable highway load is usually around 20,000 lbs per axle, and 600 lbs per inch width of tire. Doesn't a HS-20 use 32,000 lbs per tandem axle (16k per axle), this would put 8000 lbs on dual tires. To figure approximate tire contact area, divide load by tire pressure. Typical tire pressures are 100-120 psi.

But I thought I'd check with the experts, one had an example that looked pertinent to your queston:

------------------------------

http://www.bridgestonetrucktires.com/us_eng/answers/doctor_specs.asp

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
- - - - - -
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

 

[JAE]

3doorsdwn,

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.

 

[JAE]

Check out this thread - it has the AASHTO formula in it:

thread507-223271

 

[3doorsdwn]

Thanks all. JAE, I wasn't just using 8". It was: 8"+d (the assumed failure plane as you approach the support; I don't know if I was too clear on that). Thanks for the link and other info.....I'll see what I can come up with.

 

[SteelPE]

I have only slightly touched the AASHTO code. Are you designing your slab in accordance with section 3.24 (Distribution of Loads and Design of Concrete Slabs)? If not, what section are you using?

 

[3doorsdwn]

JAE, I see the formulas you are citing from AASHTO. But when you look at the sections they are under, it strikes me as being more applicable to moment than shear......what do you think? The reason I ask is: I just have a hard time picturing a tire (say 8-10" wide) having a effective length/engaging 48" of a thin (say 8") slab [this is in the case where the tire is at the distance "d" from the support ("d" being the depth of the 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.

 

[cap4000]

Westergaard's formula for the wheel load just off the support in shear is approx 45% of the total load. 16k x 0.45 = 7.2k design shear. What did you come up with?

 

[3doorsdwn]

cap4000, my problem wasn't figuring load......my approach was figuring effective width of my one-way slab. [I.e. what bw to use in the shear strength formula: phi*Vc=phi*2*(f'c^0.5)*bw*d] So it's sort of a problem of assuming what length of slab resists this force (or another way to put it: what's the length of the failure plane)?

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").

 

[cap4000]

From Timoshenko's plate theory for a point load not a tire b=0.326L. Therefore @ 5ft b=19.5 inches. I have seen a finite element analysis use double that for an H25 load condition. Based on this you are somewhere between 20 inches and 40 inches as a max. Using 24 inches should be fine.

 

[larsacious]

My State's Bridge Manual provides an exception that allows the designer to disregard shear when designing a concrete slab bridge. We only need to design the slab for flexural considerations.

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).

 

[miecz]

AASHTO's 17th Edition, Article 3.24.4 "Shear and Bond" reads: Slabs designed for bending moment in accordance with Article 3.24.3 shall be considered satisfactory in bond and shear.

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.

 

[3doorsdwn]

miecz, I saw that section too (i.e. 3.24.4).....but me being ultra conservative: I disregarded it. I don't mess with shear.

I think I've got a conservative design.....just waiting for the client to balk at it.

 

[cap4000]

This is not a bridge layout. Slab is noted as 5ft x 17 inches x 19 inches thick?? This is for a cable tray which is more of a thick plate. As such shear must be checked.

 

[SteelPE]

In my message above, I was eluding to the same paragraph that miecz pointed to. I would think that if you were designing to the requirements of section 3.24 then you would be OK. During repair, when I drive over them, I don't ever recall any bridge decks being 19" thick.

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?

 

[3doorsdwn]

SteelPE, there are no girders [steel or otherwise]. Any traffic must be supported by a single [removeable] one-way slab that spans 5' and is 17' wide.

 

[SteelPE]

3doorsdwn,

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.

 

[miecz]

You need to determine which Code governs your design. Most DOTs have gone to the LRFD code. If you are designing to the AASHTO Standard Specification (17th Ed.), then 3.24 negates a shear check. But then, you need to meet all of the Standard Specification, i.e., minimum concrete strength, transverse reinforcement, etc. Also, your layout must look like a typical bridge deck. A 5 foot stringer spacing is a wee bit narrow, but not out of bounds.

 

[wiktor]

5' span is a typical stringers' spacing in a multistinger bridge. Older decks were 7.5" thick, new ones are 8.5" thick. This deck is designed to support HS-25.
Based on the above, 19" thickness is way overdesigned.
Details of the reiforcement, cover, e.t.c. could be found here:

https://www.nysdot.gov/main/busines...detail-sheets-usc/ss-superstructure-slabs-usc

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.

 

[Splitrings]

The 17th edition is to be used with existing structures. The 5th edition LRFD should be used for all new structures. Section 4.6.2.3 specifies the width of a slab type bridge (which I think is what 3doorsdwn is analyzing) to be used for both shear and moment calculations. The shear capacity concrete is covered in Section 5.8.