how do architects calculate if a structure like a bridge is stable?
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Civil Engineer here. There are plenty of great answers here that dig into a lot of important details, but in the spirit of ELI5, I will try to do specifically that…
Different parts of a structure will be pushed or pulled or twisted in different ways depending on a bunch of different factors. This pushing, pulling, or twisting can cause things to break. An engineers job is to figure out all of the different pushes, pulls, and twists the structure will have to deal with under the most extreme cases, figure out what parts are most likely to break, and then choose the proper shapes, sizes and materials of all the parts to make sure nothing breaks under the most extreme cases.
Civil Engineer here. There are plenty of great answers here that dig into a lot of important details, but in the spirit of ELI5, I will try to do specifically that…
Different parts of a structure will be pushed or pulled or twisted in different ways depending on a bunch of different factors. This pushing, pulling, or twisting can cause things to break. An engineers job is to figure out all of the different pushes, pulls, and twists the structure will have to deal with under the most extreme cases, figure out what parts are most likely to break, and then choose the proper shapes, sizes and materials of all the parts to make sure nothing breaks under the most extreme cases.
r/realcivilengineer would be so mad right now.
Architects don’t do the calculations, engineers do.
That’s more engineering.
But from an architecture point of view; you should have a rudimentary grasp of material strengths (tensile strength of building materials) and structural strengths (like arches, load bearing pillars or pylons)
I’m an engineer and have worked on the foundation for bridges. When we design the foundation, we get loads (weights, etc) from the structural engineer. We use these to check different possibilities that could cause things to fail. For most of the ones I’ve done this ends up being 4 main ways.
The first one is having a super heavy truck drive over it, this is the strength condition. The max weight it will take. The elementary school experiment is putting a ruler between two books and setting something on the ruler, when it breaks, that’s the strength.
Next would be service, if you ever drive over a bridge and feel it flex, this is service. It may not fail, but if it flexes/bounces too much is is uncomfortable. For foundations this includes settlement. For the ruler, now you only want the ruler to bend so much.
We have earthquakes here, so we need figure out how big it will be. The geologic survey does that for us. Then we look at what the dirt is like. Now sit you books and ruler on something. Something like pudding will shake more than rock. This is called an extreme event, we don’t want it to fall down, or if it’s important, it needs to be usable after the earthquake. In a building this would be the difference between you being about to get out of your house and not be dead vs the hospital that still needs to be the hospital after the earthquake.
After this there are a bunch of other extreme events. For rivers we look at how bad the flood will be. The geologic survey gives us an idea too. We have to see that it will be ok for a 500 year flood, a flood that has a 0.2 percent to happen each year. Will that flood wash away all the dirt around our foundation? This actually is usually our worst case in our area when we cross a river, even though we expect a 7.0 earthquake.
Other possibilities include:
What if there’s ice in the river in the spring that hits the bridge? Or gets stuck and won’t let the water by?
What if a truck runs into the bridge?
What if the wind makes the bridge vibrate like a guitar string? (This is what killed the Tacoma narrows)
What if the steel used for the foundation rusts?
Bridges are hard, but the challenge is fun. On the other hand, you are not going to do it alone. We always have someone else in our office independently check our “homework.” Then, in most cases, the state department of transportation checks it themselves. Even if it’s not for the state or feds, cities and counties just don’t have the expertise.
One super nice thing is the building code for bridges. It is both very complex and very simple at the same time. The old binder copy I have is 6 inches thick, and the new one is bigger. But it is very step by step. And has lots of footnotes and references. It’s very step one,step 2. If you have something unusual, get this book out.
Most of the math is not super involved, but you need to understand what it’s doing. Design for this heavy of a truck, the tires go here, the beam can only bend this much. Now check if the next thing happens. Most of it can actually be done with paper and a calculator if it’s a simple bridge. Computers help you do the hard math, but you need to be able to tell if the answer makes sense.
r/realcivilengineer would be so mad right now.
Architects don’t do the calculations, engineers do.
I’m an engineer and have worked on the foundation for bridges. When we design the foundation, we get loads (weights, etc) from the structural engineer. We use these to check different possibilities that could cause things to fail. For most of the ones I’ve done this ends up being 4 main ways.
The first one is having a super heavy truck drive over it, this is the strength condition. The max weight it will take. The elementary school experiment is putting a ruler between two books and setting something on the ruler, when it breaks, that’s the strength.
Next would be service, if you ever drive over a bridge and feel it flex, this is service. It may not fail, but if it flexes/bounces too much is is uncomfortable. For foundations this includes settlement. For the ruler, now you only want the ruler to bend so much.
We have earthquakes here, so we need figure out how big it will be. The geologic survey does that for us. Then we look at what the dirt is like. Now sit you books and ruler on something. Something like pudding will shake more than rock. This is called an extreme event, we don’t want it to fall down, or if it’s important, it needs to be usable after the earthquake. In a building this would be the difference between you being about to get out of your house and not be dead vs the hospital that still needs to be the hospital after the earthquake.
After this there are a bunch of other extreme events. For rivers we look at how bad the flood will be. The geologic survey gives us an idea too. We have to see that it will be ok for a 500 year flood, a flood that has a 0.2 percent to happen each year. Will that flood wash away all the dirt around our foundation? This actually is usually our worst case in our area when we cross a river, even though we expect a 7.0 earthquake.
Other possibilities include:
What if there’s ice in the river in the spring that hits the bridge? Or gets stuck and won’t let the water by?
What if a truck runs into the bridge?
What if the wind makes the bridge vibrate like a guitar string? (This is what killed the Tacoma narrows)
What if the steel used for the foundation rusts?
Bridges are hard, but the challenge is fun. On the other hand, you are not going to do it alone. We always have someone else in our office independently check our “homework.” Then, in most cases, the state department of transportation checks it themselves. Even if it’s not for the state or feds, cities and counties just don’t have the expertise.
One super nice thing is the building code for bridges. It is both very complex and very simple at the same time. The old binder copy I have is 6 inches thick, and the new one is bigger. But it is very step by step. And has lots of footnotes and references. It’s very step one,step 2. If you have something unusual, get this book out.
Most of the math is not super involved, but you need to understand what it’s doing. Design for this heavy of a truck, the tires go here, the beam can only bend this much. Now check if the next thing happens. Most of it can actually be done with paper and a calculator if it’s a simple bridge. Computers help you do the hard math, but you need to be able to tell if the answer makes sense.
I’m an engineer and have worked on the foundation for bridges. When we design the foundation, we get loads (weights, etc) from the structural engineer. We use these to check different possibilities that could cause things to fail. For most of the ones I’ve done this ends up being 4 main ways.
The first one is having a super heavy truck drive over it, this is the strength condition. The max weight it will take. The elementary school experiment is putting a ruler between two books and setting something on the ruler, when it breaks, that’s the strength.
Next would be service, if you ever drive over a bridge and feel it flex, this is service. It may not fail, but if it flexes/bounces too much is is uncomfortable. For foundations this includes settlement. For the ruler, now you only want the ruler to bend so much.
We have earthquakes here, so we need figure out how big it will be. The geologic survey does that for us. Then we look at what the dirt is like. Now sit you books and ruler on something. Something like pudding will shake more than rock. This is called an extreme event, we don’t want it to fall down, or if it’s important, it needs to be usable after the earthquake. In a building this would be the difference between you being about to get out of your house and not be dead vs the hospital that still needs to be the hospital after the earthquake.
After this there are a bunch of other extreme events. For rivers we look at how bad the flood will be. The geologic survey gives us an idea too. We have to see that it will be ok for a 500 year flood, a flood that has a 0.2 percent to happen each year. Will that flood wash away all the dirt around our foundation? This actually is usually our worst case in our area when we cross a river, even though we expect a 7.0 earthquake.
Other possibilities include:
What if there’s ice in the river in the spring that hits the bridge? Or gets stuck and won’t let the water by?
What if a truck runs into the bridge?
What if the wind makes the bridge vibrate like a guitar string? (This is what killed the Tacoma narrows)
What if the steel used for the foundation rusts?
Bridges are hard, but the challenge is fun. On the other hand, you are not going to do it alone. We always have someone else in our office independently check our “homework.” Then, in most cases, the state department of transportation checks it themselves. Even if it’s not for the state or feds, cities and counties just don’t have the expertise.
One super nice thing is the building code for bridges. It is both very complex and very simple at the same time. The old binder copy I have is 6 inches thick, and the new one is bigger. But it is very step by step. And has lots of footnotes and references. It’s very step one,step 2. If you have something unusual, get this book out.
Most of the math is not super involved, but you need to understand what it’s doing. Design for this heavy of a truck, the tires go here, the beam can only bend this much. Now check if the next thing happens. Most of it can actually be done with paper and a calculator if it’s a simple bridge. Computers help you do the hard math, but you need to be able to tell if the answer makes sense.
People are giving great answers, but here’s my five year old response – take a popsicle stick, support it at both ends, and hang a weight from the middle. Let’s say it breaks at 2kg. Now you know the bending strength of a popsicle stick. You can do the same thing to figure out the strength in other orientations. You can even create simple shapes (say a triangle) and do it again. This creates a solid reference library for your material strength. Now use those simple shapes to create a structure, and you can use some math along with your material strength to calculate the strength of the structure. You can test that full structure to verify (or in the case of a large structure, build a scale model).
How do you know if that strength is enough? You come up with load cases. What is the structure going to be used for? How much weight should it hold? What kind of wind do you expect? What if someone uses the structure in an unintended way.
If you want to be extra careful, you design the strength to be 2, 3, 4, etc times stronger than the worst load case. This is called a factor of safety.
This all seems somewhat complicated, but humans have been designing and building structures for a very long time, so there’s a lot of experience to draw on.
Pre-computers, engineers did the designing and math on paper, but now we have computer tools that can help us make things much faster.
Architects make fancy drawings and models, Civil and Structural Engineers turn it into something that can exist in Earths gravity.
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