Load moving horizontally across bridge, speed affect loading

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Discussion Overview

The discussion centers on the effects of a load moving horizontally across a bridge on the vertical forces experienced by the bridge. Participants explore whether the duration of the load's presence on the bridge influences the structural integrity and loading capacity compared to a stationary load.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions if the time a load is on the bridge affects the vertical force on the bridge, suggesting that a bridge might support a greater load if the load is only present for a short time.
  • Another participant notes that the deformation of a structure under load varies with time, which complicates the question and suggests that the specifics depend on the structure in question.
  • A participant expresses doubt about their memories of balsa wood bridge experiments, stating that engineering equations for structural failure do not account for time dependence, despite an intuitive belief that shorter load durations might cause less damage.
  • One participant references a tensile strength video showing steel deforming over time, implying that time may play a role in structural failure.
  • Another participant expresses interest in conducting an experiment with a breaking force applied for a very short duration, hypothesizing that the material might not fail under such conditions.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the duration of the load affects the structural integrity of the bridge. Multiple competing views and uncertainties remain regarding the influence of time on loading and structural failure.

Contextual Notes

Participants acknowledge limitations in existing engineering formulas that do not incorporate time dependence, which may affect their understanding of the problem. There is also uncertainty regarding the accuracy of personal recollections from past experiments.

houlahound
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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.
 
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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.
 
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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.
 
houlahound said:
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.
 
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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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