JStephen..yep...it's a special load. 70 tons is almost twice the legal limit in most states. But then, you don't see an Abrams being trailered down the interstate very often.
Analyzed a missile haul road a few years ago. Those missiles are heavy suckers, but the carriers have a bunch of axles and a bezillion tires on them! Lots of overlapping stresses.
Are you designing for the military load per AASHTO or are you envisioning hauling a tank or some other military vehicle over your bridge?
AASHTO Standard Specs 3.7.4 describes the military loading. In LRFD it's not called military loading - it's called Design Tandem, which is two 25kip axles spaced 4-feet apart Plus Lane Load.
If you're hauling a military vehicle that would be a permit vehicle. Your state DOT would have to give you some information then.
Actually, I believe that the original intent of the interstate highway system, instituted by Eisenhower in the early 50's, was to be able to move military vehicles internally in the event of a national emergency. So I do not think that the loads are out of the question here.
The only other limitation here besides the load would be the clearances needed too.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
Bridgebuster is correct here too as the 1963 California "Manual of Bridge Design Practice" shows just such a loading on page 3-A-1, referred to as the "Alternative Loading". It is usually represented to be the double axle situation for a semi trailer. It is also well to note that a safety factor is NOT included in any of these loads.
The only thing is that you would need a minimum of 7 axles to transport the load under the tank, which is exactly why a draggin waggin has so many axles. That loading would be substantially different than the "Alternative Loading".
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
I was thinking 70,000 lbs, not 70 tons, when I mentioned the trackhoe. Looking at the John Deere site, they have trackhoes that weigh 70 tons, but they're bigger than you normally see hauled around, too.
Mike...you are correct about the original intent of the Interstate Highway system; however, when the designs are done, they typically do not encompass such loads. When they are transported on the public roads, they are typically trailered on multi-axle, mult-wheeled trailers to spread the load.
Fortunately, a few trips of something very heavy will not usually hurt a pavement. After all, you could land a 767 on an interstate highway...you just can't do it a hundred times a day for 20 years!
