dump truck axle loading - while dumping

huangyhg

dump truck axle loading - while dumping
a project i am currently working on has an area where dump trucks back into the building and dumps aggregate into a bin and hopper system.  the entire system is elevated and i need to design the support beams for the truck.  i am familiar with aashto criteria and impact loading, but this is all for the truck while it is moving.  i am designing for hs-20 and 32 kip axle load.
once the trucks stops over the bin and begins to dump its load, common sense tells me the centroid of the load moves further back toward the rear axles and the axle force will increase.  once the material begins to slide out of the truck, the load will come back down.  how can i quantify the maximum load without knowing the exact model of truck that will be delivering the material?  
lastly, what spacing should i use on the beams that will pick up the wheels of the truck.  i am planning to use a stocky w12 with up to an 18" wide to plate with stiffeners to support the wheels.  i am assuming that if i can pick up one wheel on each side of the tandem i will be ok.  the overhead doors are only 10 ft wide, so if the truck is not inline with the window, it won't be able to make it inside the door.  the architect involved is researching the beam locations, but i would like to be able to verify this dimension on my own.  the hopper has a grizzly system that the manufacturer tells me can support the weight of the truck, but i am not too confident that it is actually engineered.  if the wheel does not align with the beam, the grizzley should be able to transfer the load back to the steel beams i will specify.
thank you in advance for all your help.
a couple of things to keep in mind- some of the dump trailers raise the front axle which transfers all the rear weight onto the back axle when dumping.  and depending on the locality and patrols, loaded weights can run somewhat over the legal limits.
you first need to research some of the load rating trucks in the local or for that state or county.  it is unlikely that you actually recreate the dump truck loading configuration.  the aashto vehicles are not acutal vehicles but are design vehicles and as such should represent the maximum loading you may see.  however a quick check of the operating and inventory vehicles checked for load rating by the county or state will confirm this.  
if the hs20 doesn't agree with you most interstates these days are designing for hs20 modified or the hs25 loading.  so the axle load is 40 kips not 32 (25% increase hence the 25).
don't forget that your elevated ramp has to be designed for the entire truck and not just when dumping.  as a result you could have several different truck placements for maximum loading.  that is, what creates the maximum positive moment in the span may not create the maximum reaction at the support or hopper.
as for the beam spacing - if you have a structural slab to transfer the vehicle load, the beam spacing won't have to exactly match the wheel lines of the dump truck.   
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
you must certainly design for everything that qshake said, but to give a direct answer to your question:
i'd find out what the front axle load is on your particular hs loading.  for the case when the truck dumps, i'd take the front axle load and add it to the rear axle load, essentially balancing the entire truck and its cargo on the rear axle(s).  this is the most it could be, neglecting dynamic effects.