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Why do bridges have weight limits?

  1. Mar 28, 2004 #1
    You've probably seen pictures of bridges carrying rush-hour traffic, so obviously the bridges are designed to bear enormous weight. Yet, they have weight limits on trucks. Imagine - a truck hauling only maybe 1/50 of the total mass of the rush-hour traffic is supposed to be dangerous to the bridge.

    How come?
  2. jcsd
  3. Mar 28, 2004 #2
    1.) What if 60 of them decided to cross at the same time?
    2.) When there are 120 cars on the bridge, they are not all at the same point at the same time. They are spread over a distance. Think Bed of Nails.
  4. Mar 28, 2004 #3
    yeah i also remmber there is a formula that state force appiled on an object also depends on the area applied.

    think if 3 baby sit on a sofa it sank less at each section but compare to one adult. that totally squash the sofa at one section into ....

    and that is simmilar to the truck, as it go over the briedge it can cause damage to the road surface.
  5. Mar 28, 2004 #4


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    Because it is not just the weight of the truck, but depending on the bridge design, the shear strain that it causes, or possibly the bending of the bridge (another type of strain), which induces a stress. It is usually the stress, not the force directly, that causes the material and structural failures.

    I am not a civil engineer, however. This is stuff I picked up from my autocad, general engineering, and engineering ethics classes.
  6. Mar 28, 2004 #5
    A heavy vehicle can cause a localized failure in one structural member without bringing down the whole bridge. Further, there is the issue of the stress on the pavement itself directly beneath the truck's tires. Also, the truck may not always be traveling in a straight path, which can induce a transverse stress on the bridge. The exposed structural members above the road surface, and the strength of the gaurd rails that protect them must be taken into account as well. I hope this helps. -Mike
    Last edited by a moderator: Mar 28, 2004
  7. Mar 28, 2004 #6
    I would have thought that the tires on a typical truck - there are so many like 16 - spread the weight over a big area. However, apparently accidents like impact with guard rails have to be taken into consideration as well as bridge loading, as M.D. Sewell points out.
  8. Mar 28, 2004 #7


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    Not sure how much of a factor this is, but most bridges large enough to accomodate several trucks at once are constructed in descreet sections. Each section is only large enough for a single truck to occupy at one time, so the weight o fan individual truck is the deciding factor for how much load the bridge section must support, another truck of equal weight sitting on the next section over would be of little consequence.

    Come to think of it, I don't recall ever seeing a weight limit posted on large, long bridges like the one suggested by the original post (wherein a single ruck is only 1/50 of the total weight on the bridge). Can anyone think of a specific example of such a bridge having a posted weight-limit? Not saying I've never seen one, just that I don't recall seeing one; but I freely admit that my forgetter works far better than my rememberer.
  9. Mar 28, 2004 #8
    Maybe my rememberer worked worse than my forgetter. I don't recall seeing signs for major bridges either. I am now regretting that I started this topic, but I will keep it open in any case a civil engineer jumps on this someday.
  10. Mar 28, 2004 #9
    Typically there are guidelines to how much weight per inch of tire is permitted on the road in general and not limited to bridges alone. Obviously, the bridge that is not posted has the same restrictions that the road itself does. Although many bridges WILL have stricter rules. In my state, I believe the typical 18 wheeler is limited to 80,000 pounds. That is 40 tons. They are allowed 34,000 on the set of tandem/duals (a total of 8 tires) for the trailer, 34,000 on the set of tandem/duals on the tractor and 12,000 on the front tires on the tractor.

    Some short bridges are actually shorter than the tractor/trailer, but they STILL usually rate them for X number of pounds per rig. I don't know why since a semi that is technically overloading the bridge will put less weight on it because the wheels are so spread out than for the rating of a short straight truck that is technically NOT overloading the bridge with all wheels on it at once.
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