Scrapers and off road haul trucks on new bridge
Scrapers and off road haul trucks on new bridge
(OP)
I got a request from a contractor who wants to drive scrapers and off road haul trucks on a new PC beam bridge that I designed. My first response was to send me the model information and wheelbases for the equipment. I haven't received that information yet.
However, thinking about it some more, I seriously doubt there is enough capacity in the bridge. I looked up one of the scrapers I know is on the job, a CAT 651E. The loaded front axle weight is 126 kips and the loaded back axle weight is 112 kips. The axle spacing is 32.5 feet.
Anyone ever done this before? How did you distribute the wheel loads to the girder, because AASHTO distribution factors don't apply to something like this. Also I suspect the impact load is much different, though you might be able to control that with a speed limit on the bridge.
However, thinking about it some more, I seriously doubt there is enough capacity in the bridge. I looked up one of the scrapers I know is on the job, a CAT 651E. The loaded front axle weight is 126 kips and the loaded back axle weight is 112 kips. The axle spacing is 32.5 feet.
Anyone ever done this before? How did you distribute the wheel loads to the girder, because AASHTO distribution factors don't apply to something like this. Also I suspect the impact load is much different, though you might be able to control that with a speed limit on the bridge.
RE: Scrapers and off road haul trucks on new bridge
Figuring out how slow they'd have to go so that there's no impact could be tough. Per AASHTO, it's 5mph, but that's for highway trucks with typically 80psi in the tires. What it would be for a scraper with much larger tires at a much lower pressure is beyond my knowledge. I think one could confidently assume slowing them to 5mph would be adequate to get impact to a negligible level.
I wouldn't assume that the scraper would overload the bridge. After all, the HL93 truck load, including load factors, is 126 kips, and the design loading includes a lane load on top of that. If the load is distributed over more girders that a standard truck load, you may find that it's adequate. Much depends on the girder spacing relative to the wheel gauge distance (axle length) and the bridge span lengths.
Rod Smith, P.E.
RE: Scrapers and off road haul trucks on new bridge
I would use the lever rule to distribute like Hotrod mentioned. I kinda think your deck may control more so than the beams since you would have maybe 2x the normal wheel load.
RE: Scrapers and off road haul trucks on new bridge
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Scrapers and off road haul trucks on new bridge
Full-scale testing has shown that there is considerably more horizontal arching action within a concrete deck than traditional analysis techniques account for, provided that a few criteria are met (See the empirical deck design provisions in contrast to the traditional design in the AASHTO LRFD Art. 9.7). The results of the testing showed that concrete bridge decks essentially do not fail in flexure; when they fail (at around 5 times the traditionally-calculated capacity), it's actually a punching shear failure mode.
Computer analysis is the typical way to analyze the loads and capacity for bridge girders, but it requires the proper distribution factors to be input, so the correct loading to each girder is applied in the analysis. For a vehicle that has a non-standard gauge distance between the wheels at an axle, the distribution of load can be determined conservatively by hand with minimal effort (using the lever rule). If the girders are adequate for the loads using the simplified, conservative approach, there is no reason to expend the effort for a more rigorous analysis.
Rod Smith, P.E.
RE: Scrapers and off road haul trucks on new bridge
Use of a computer grillage or plate and beam analysis for bridge design is standard practice outside the USA and doesn't require the use of pre-determined distribution factors. For analysis of the distribution of non-standard loading with very high wheel loads it seems to me to be the easiest and most reliable way to go.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Scrapers and off road haul trucks on new bridge
All of the advice above seems sound in my experience. I've previously supplemented permanent bridge decks with timber mats (especially if you fabricate up timber mats with filch plates) to improve load distribution in a few cases. It's a lot of work with limited payoff, but sometimes is just enough to make something work.
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just call me Lo.
RE: Scrapers and off road haul trucks on new bridge
Rod Smith, P.E.
RE: Scrapers and off road haul trucks on new bridge
www.PeirceEngineering.com