wheel load vs. retaining wall

huangyhg

wheel load vs. retaining wall
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!
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.
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.

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.
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!
you really need to pull your gdeotech into this equation.
mike mccann
mmc engineering
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
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.

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.
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
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