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dengebre (Structural) (OP)
24 Jun 09 13:55
Our firm designs buildings (not bridges) so please forgive what may be a basic question to the bridge design community. We have a retaining wall that will be subject to a wheel load directly adjacent to the wall. The heaviest wheel load will be a double tire (53 kips per tire x 2 tires = 106 kips; 4 tires and 212 kips per axle) with a tire contact area of 13.4" x 33.6" per tire (900 sq. in. for a tire pair). What is the design strip width for the retaining wall? Using the smallest dimension of the tire contact area seems overly conservative. Is the wall design strip width defined by a 45 degree line from each side of the contact perimeter? This may already be addressed in AASHTO but we do not know where to look. Thanks!
Lion06 (Structural)
24 Jun 09 14:48
I would look at Bowles.  There is some good information in there.  I would use the actual width - the lateral pressures from a local surcharge are often pretty small.
ishvaaag (Structural)
24 Jun 09 15:05
I have one old AASHTO 15th ed. 1992 ASD code. There in Section 3.20 says

When highway traffic can come within a horizontal distance from the top of the structure equal to one-half of its height, the pressure shall have added to it a live load surcharge pressure equal to not less than 2 feet of earth.

5.5.2 Earth pressure and surcharge loadings (on walls)

too long to copy, repeats the same with the precision on that the resultant of such pressure will be at midheight of the backwall (logical, since a uniform pressure at the surface).

Select hence the mandatory code in your area and see if this or similar thing applies.



 
dengebre (Structural) (OP)
24 Jun 09 15:30
The surcharge will be a point load of 106 kips adjacent to the retaining wall. The smallest dimension of the wheel contact area is 13.4". Using that dimension as the design strip width will result in a surcharge lateral pressure of 8.5 klf/ft on the retaining wall (assuming Ka = 0.5). That just seems a bit overly conservative.
ishvaaag (Structural)
24 Jun 09 17:30
What I meant copying the AASHTO code is that it is for walls and that it seems to forfeit actually the effect of wheel loads as a point (or on small area) loads. This they wouldn't be doing without the experience of it being safe enough, for otherwise we can make the usual assumptions from simplified figures or Boussinesq and find a local pressure maybe not as tolerant as AASHTO. But if they allow this, it must work. Or so it should!
msquared48 (Structural)
24 Jun 09 17:49
You really need to pull your Gdeotech into this equation.

Mike McCann
MMC Engineering

Helpful Member!  mudflaps (Structural)
24 Jun 09 18:25
I'm haveing some real issues with the axle loads on this one. Just what kind of vehicle do you have in your neck of the woods?

Old CA SE
MaddEngineer (Structural)
24 Jun 09 18:25
Those wheel loads are very large, (i.e., not HS20-44 loading) what kind of truck/equipment is this.  What is the spacing between the axles and between tire lines. Is it a mobile crane or other type of heavy construction equipment.

Is this load during construction or is it a permanent service consideration.  Will there be a concrete slab in place to distribute the wheel loads.  How close will the wheel loads be to the structure.

You have to check using as concentrated loads using boussinesq distribution in vertical and horizontal axis.  USS Sheet Pile Manual and California Trenching and Shoring Manual address analysis of surcharges like these.  NAVFAC DM-7.2 also has design nomographs, (see attached pdf)

I would verify the loads given (they are pretty large) and the tire and axle geometry.  You may be able to distribute the load as a rectangular load if there is a stiff enough slab in place and simplify your analysis.  If this is a during construction condition I would see if you could install some cribbing beams and spreaders that would throw the load on top of your wall and on a bearing pad a distance away to minimize surcharge effects.

 
Ron (Structural)
24 Jun 09 19:08
The vertical load "is what it is", meaning it is the wheel load applied to the contact area.  The lateral load on the wall will depend on soil type and the corresponding phi angle.  Check with your geotech for that info.

You will have lateral loads anywhere from about 0.15 x(vertical load) to 0.50 x (vertical load), depending on the soil conditions.
BlastResistant (Structural)
24 Jun 09 19:42
Hi Dengebre,

I have been a bridge engineer for fifteen years and have just a few comments/questions.

1. Bridges (and other structures subjected to vehicle loading such as retaining walls, culverts, etc.) are designed per the AASHTO LRFD code. This code (reference chapter 3) requires the design of such structures to withstand loadings from "HS-20" truck types. For design purposes, these trucks have a total weight of 72 kips, or 8 kips for the front axle and 32 kips each for the two rear axles. Are you sure of your wheel loads? That is one hell of a big vehicle you are designing for. Such a truck would need a special permit just to be on the road in the state I live in. Such a truck could collapse a typical freeway bridge or elevated interchange. Is this a special military vehicle you are designing for on a military facility? Such a truck is nearly 7 times what a freeway bridge is designed for and would never receive a special permit to pass on a typical freeway structure.

2. AASHTO requires 250 psf surcharge (approx. 2 foot soil surcharge) for loadings where highway traffic is close to a retaining wall.

3. Tire contact area, whether one or two wheels, is taken to be 20 inches wide by 10 inches in length.

Good Luck
msquared48 (Structural)
24 Jun 09 19:56
I agree with Blast Resistant here...

An M60 tank is about 57 tons and the M1A1 65 tons.  But these are tracked vehicles.  A 10 yard concrete mixer is only 35 to 40 tons.  

You are describing a single wheel load of over 25 tons.  Is this a giant Euclid earthmover?

Mike McCann
MMC Engineering

BlastResistant (Structural)
24 Jun 09 19:56
I forgot to mention, if you have a copy of the AASHTO code, look in chapter 3 for loading criteria and chapter 10 for retaining wall design. If your loads are correct, I would do a Boussinesq analysis.

Good Luck
Ron (Structural)
24 Jun 09 20:57
I have designed roadway sections for such loads...missile haul roads, log stackers,etc.  They are not unheard of, but not "run of the mill" either.

Verify the loads.  If they are accurate, apply them as noted.  This is not a bridge, nor a roadway.  It is a static surcharge on a retaining wall.  Treat it as such.
BridgeGirlie (Geotechnical)
25 Jun 09 3:44
I agree with Blast Resistant's second post. If you verify that those loads are correct, you should treat the wheel load as a static point load over the surface area of the point of contact (20"wide by 10" long per AASHTO)and do a Boussinesq analysis. If you have not done this in a while (probably since college for most people), it might be worth your while (and peace of mind) to hire a geotechnical engineer or specialized structural foundation engineer to do it for you.
yakpol (Structural)
25 Jun 09 5:53
Dengebre,
I attached pdf borrowed from USS sheet pile manual with diagrams and formulas for surcharge loads due to point loads and line loads. It provide all the neccessary information for your design. They are classic formulas from US Army geotechnical manual.
MaddEngineer (Structural)
25 Jun 09 7:40
Just a thought, it seems that those wheel loads may be metric 53kN = 11,900 lbs per tire; 11,900 x 4 = 47,600 per axle.  I have seen these axle loads for FDNY Pump Trucks.

The tire contact area also seems a bit off 13.4 x 33.6 centimeters maybe... 13.4cm = 5.3in, 33.6cm = 13.2in looks more accurate for a contact area of 5.3 x 13.2 = 70in2.
dengebre (Structural) (OP)
25 Jun 09 9:04
The wheel loads are for a container handler: Model TEC-950L by Taylor Machine Works. The drive axle is comprised of 4 tires with an axle load of 212 kips. The steering axle is comprised of 2 tires with an axle load of 15 kips. The contact area is 450 sq. in. per tire. Another load case is for a DOT Class V loading, but it is the container handler load that will govern.

Further searches of this site suggest that the answer may be in Boussinesq's method of soil stress analysis for a point load. It appears the stress distribution may be approximated by an inverted triangle. Alas, we would love greater involvement from the project's geotech but it has been difficult.
darkwing888 (Mining)
25 Jun 09 10:19

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