Inspired by Calvin’s same question where his Dad answers “They drive bigger and bigger trucks over the bridge until it breaks. Then they weigh the last truck and rebuild the bridge,” how do engineers determine the actual load limit on bridges in the real world?
In: Engineering
His dad’s answer is in the right direction, it’s just missing something.
The ‘bigger and bigger trucks’ is instead stress testing, and it allows engineers to know how strong a material is. But this can be done without building a bridge; they just need to have a piece of the materials (which will include the connections), and then they can use other tools such as hydraulic presses to do the testing. They will also do erosion testing to see how the material will do after it’s been in use and in weather for a while.
Once they do this with a piece of the material they can plug the numbers they got into the shape of the bridge. This returns a number that will break the bridge, and then they use legal code calculations to determine how much smaller than the ‘break’ value they need to list the ‘load’ value.
So in short, they ‘drive’ pretend trucks over a ‘pretend bridge’ until it breaks and then write a smaller number on the bridge they actually build.
Materials:
The materials you use to make bridges all have known strengths. The steel is rolled in a mill and sampled for testing, and is reliably very close to spec, often a modest amount above to ensure statistical confidence that even the “dud” parts of any production run meet spec. The concrete strength is estimated based on extensive experience with the mix designs (ratios of cement, sand, gravel, water, and the specific properties of the sand and gravel available to the manufacturer). This is a more variable material than steel, so each batch is tested at placement to ensure it meets the spec properties. If you’re using aluminum and wood the same deal applies.
Engineering:
You use engineering analysis to determine how the loads on the bridge translate into forces in the structural components, then you make sure the structural components are stronger than the demand on them. There are also design checks for deflection and how much the bridge vibrates and so on.
This has been a known process for over a century, so covers most bridges. With older bridges a retroactive analysis can be applied along the same principles, often taking small material samples so you know the strength of it all.
The final load rating also includes a bunch of safety factors, adding up to a factor of safety of about 2 on the theoretical design load, and about 3-4 on the actual highest load the bridge will see. This is for a few reasons, but part of it is to give some slack to cover gaps in maintenance, construction errors that might not be caught in a difficult environment to inspect, and because almost every bridge is a prototype – it’s not like designing a jet engine where you build a bunch of test engines and test them to destruction. You build *one* and put it into service. With virtually every one being unique sometimes something in the design throws a curveball so it’s nice to have some spare capacity.
Experiments and maths.
You need to break stuff to know what they can handle but we talk about smaller parts like putting a concrete sample in a hydraulic press and check what pressure it can handle. Reinforced concrete can made into a beam and you check what force is require fo it to crack. The same for metal or other material that is stretched, compressed, bent etc to see how much it can handle. This include joints between diffrent parts. Destructive testing like this is important to build up a understanding of material. But you do not need a complete bridge, we can separate individual parts.
You can then with maths calculate all the forces on a bridge to determine how much load it can hold.
That is a bit idealized because experience from previous bridged are included the mathematical model used determine the strength.
The methods have improved over time. There was no math models in the past just expedience what worked. at the same time there was no heavy vehicles and bridges spans was not very long. But it have change with out understanding of engineering.
It has been said that “Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.” The point is it is not had to make a bright that do not have to lon a span stong enough just use a lot of material. Arch bridge have been build since antiquity and some have remain to this day. The problem is the huge amount of material and work that need to be done and limits in bridge spans. Today we make bridges that can hold more for a lot less because we know what is just enough
He is kind of right but obviously we don to build an entire bridge. Instead we take a single beam and bend it until it breaks. That gives us the load limit for that beam. We then multiply by how many beams we use for the bridge. [Here is a video showing this process on an airplane wing.](https://www.youtube.com/watch?v=zcaaznmp4hQ)
This is fine when building a simple bridge. But it gets expensive when you need big long beams just to test the strength of them, especially if you find out that the beam is too weak and have to order bigger ones. So instead we have made tables of different beam sizes, types and lengths. And we have found the patterns for how these parameters affect the strength and have made mathematical models. So if you want a certain strength and length of a bridge we can simply calculate what size beams you need.
For bridges where a single beam is not efficient we can instead calculate the forces applied. You want each beam to stay still so all the forces on it needs to cancel each other out, and the forces going through the beam needs to be smaller then its maximum strength. So you can calculate how the forces of a load on the center of the bridge transfers through all the beams and therefore calculate how much force each beam needs to handle. You then look up in the tables above or use the models to calculate these same values. Then you know the size of the beams you need.
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