• houlahound
In summary: this does seem to suggest that a load that is applied for a very short time (ie microseconds or less) might not have a significant affect on a structure's tensile strength.
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.

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.

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.

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.

The speed of a load can have a significant impact on the loading of a bridge. As the speed increases, the dynamic forces acting on the bridge also increase. This can lead to additional strain and stress on the bridge structure, potentially causing damage or failure. Therefore, it is important to consider the speed of a load when designing and constructing a bridge.

## 2. Is there a maximum speed limit for loads crossing a bridge?

Yes, there are typically maximum speed limits for loads crossing a bridge. These limits are determined by engineers during the design phase and take into account factors such as the weight and size of the load, the condition of the bridge, and the surrounding environment. Exceeding the speed limit can put unnecessary strain on the bridge and increase the risk of accidents.

## 3. Can the speed of a load affect the weight limit of a bridge?

Yes, the speed of a load can impact the weight limit of a bridge. As mentioned earlier, higher speeds can result in increased dynamic forces on the bridge, which can exceed the weight limit. This is why it is important for engineers to consider both the weight and speed of a load when determining the maximum weight limit for a bridge.

## 4. How does the shape of a load affect its speed and loading on a bridge?

The shape of a load can have a significant impact on both its speed and loading on a bridge. Loads with streamlined and aerodynamic shapes will typically have less wind resistance and therefore can reach higher speeds without causing excessive loading on the bridge. On the other hand, irregularly shaped or bulky loads can create more wind resistance and may need to travel at slower speeds to avoid putting too much strain on the bridge.

Yes, there are several techniques that can be used to reduce the loading on a bridge for high-speed loads. These include using aerodynamic shapes for the load, adding wind deflectors or streamlining devices, and implementing speed restrictions. Additionally, bridges can also be designed with features such as shock absorbers and flexible bearings to help absorb the dynamic forces from high-speed loads, reducing the overall loading on the bridge.

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