Load moving horizontally across bridge, speed affect loading

[houlahound]

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.

 

[anorlunda]

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.

 

[houlahound]

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.

 

[anorlunda]

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.

Look at this tensile strength video. You can clearly see the steel deforming with time before breaking.

 

[houlahound]

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.