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Load moving horizontally across bridge, speed affect loading

  1. Jun 5, 2015 #1
    a load moves horizontally across a flat bridge, eg a train, does the time the load is on the bridge affect the vertical force on the bridge.

    put another way; can a bridge support a load greater than it's maximum load it can support for a stationary object if the load is only on the bridge for a very short time.

    I ask this because as a kid we had bridge building contests with balsa wood, we determined the best designs by adding a stationary weight until the bridge collapsed. when that same weight was sent across the bridge at speed on a cart the bridge seemed to hold up even when the load increased.

    flat bridge means no projectile motion.
  2. jcsd
  3. Jun 5, 2015 #2


    Staff: Mentor

    I think that is a complex question. A structure under load will deform as a function of time. Deformation alters the stresses on other parts of the structure. But the details depend on the exact structure, so I believe the your question is difficult to ask or answer for the general case.
  4. Jun 5, 2015 #3
    interestingly I have looked at a number of engineering equations for failure of a particular structure/material, none of them I have seen have any time dependence. I am starting to question if my recollections of the loaded balsa bridges is accurate.

    it intuitively seems that a load applied for a microscopic time will do less damage to a structure, this is not reflected in engineering formulas for failure of a structure due to a load.
  5. Jun 5, 2015 #4


    Staff: Mentor

    Look at this tensile strength video. You can clearly see the steel deforming with time before breaking.
  6. Jun 5, 2015 #5
    indeed, I would like to see the same experiment with that final breaking force applied for a micro-second. I am guessing the rod would not fail.

    cool clip, thanks.
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