Friday, August 9, 2013

To Build a Bridge Part VI: The Law of Bridges

You may or may not be familiar with building codes.  Across the world, just about every country has some kind of "code" that determines how and what you're allowed to build.  Want a shed in your backyard?  There will be laws about how high it can go, what it can be made out of, and how strong it has to be.  In the United States these codes are written on the state and local level (counties and cities get into the game too).  Well, building bridges has the same kind of rules and laws associated with it as building a shed in your backyard.  A little more complex but the idea is the same: the government has some minimum requirements for structures put up under its jurisdiction.  If you want to build your house out of straw you're probably going to get a visit from an unhappy inspector to inform you that it won't stand-up to the wind loads it's likely to see and maybe you should consider brick.

The Three Little Pigs is actually a story about the importance of government regulations and shoddy contractors
How do you determine what kind of traffic loading the bridge will see?  How do you decide what stresses your concrete or steel will experience?  How do you handle cracking in the concrete?  How do you know the section your designing is strong enough?  These may seem like questions that should be answered by just having an education in structural engineering, but they're actually variable depending on all sorts of assumptions.

You asked for concrete that can handle a compressive load of 4,000 pounds per square inch (that's called "4ksi concrete" and is pretty standard stuff, if a bit low-end) but what will actually show up?  You need to protect the reinforcing steel in the concrete from the elements so it doesn't corrode.  You decide to embed it far enough to be protected, but how far is that?  How long does it need to last and how fast do invasive chemicals penetrate the concrete?

When the concrete around the rebar falls off (typically this process is called "spalling") the rebar is exposed to the humidity and any other chemicals around, which for bridges, means the highly corrosive agents put down to prevent icing.  The rebar can deteriorate rapidly and loose the strength it needs to support the concrete.

That's what the code is for.  Every single state in the union has their own bridge design code because, apparently, physics changes depending on what state borders you're currently in.  It's really annoying.  But it's also not as bad as it sounds.  A group called AASHTO (The American Association of State Highway and Transportation Officials) has put together, and updates, a national code that determines all these things.  It is 1,600 pages long, and generally looks like this:

Knowing and following 1,600 pages of this is my job.  Now you know why engineering has such a high attrition rate.
This code is not the law, it's a document put out by an organization who has no legal authority.  At least not in that way.  But what most states do is adopt the AASHTO code as a body, and then have their own manual that goes on top of it to either clarify, add, remove, or otherwise change the provisions in AASHTO.  You can see Wisconsin's bridge manual here.  Once the state adopts any document as their code: it becomes law.  So most states say something like: "Except when our manual we've written differs from it, the AASHTO code is law". 

When we design a bridge, we have to "stamp the plans", which means literally stamping plan-sets and calculations with a "PE" stamp.  That's a professional engineering stamp that you get when you're licensed   Because each state has a different code, you have to be licensed in every state you want to work in separately   Most states, if you are licensed in another one, just require some paperwork.  Some states, in particular those with a lot of earthquakes (like California or Washington) will require a lot more than that.  By stamping the plans, you're taking personal responsibility for the design of that bridge.

Because the code is law: should something happen to the bridge, you don't necessarily have to prove that your design would work in the world, but rather that it conforms to the local code.  It's pretty rare that something designed to code falls down, but it happens.  In extreme events (mostly earthquakes)  and in some other, rare, instances like the Tacoma Narrows bridge.

The code is created after a lot of research and discussion.  A full discussion of code creation is a massive topic, which I will not be covering.  Instead I'll say that there are right ways and wrong ways to put together a code.  Bridge code isn't necessarily done the wrong way, but it's not the right way either.  Code for building design is, to me, a lot more elegant and efficient and produces a more consistent and usable code.  This of course changes from country to country: I'm only knowledgeable about US code.

There are various considerations when designing a code about how to lay it out, what to include, what to leave to the designer, etc...  One of the important descriptions of a building code is if it's "prescriptive" or "performance based".  A totally prescriptive code will tell you exactly what to do in all cases.  "For spans between 45 and 50 feet, use a prestressed, concrete girder of exactly these dimensions with exactly this reinforcing pattern and exactly this..."  A performance based code will tell you what is expected of the structure: "After a magnitude 7.5 earthquake, the girder can have cracks no larger than 1 inch".

The problem with prescriptive codes is that they can't actually cover everything, and so are both really expensive to build to (since they have to design for worst case uses which are probably rare) and leave little guidance or direction for uncovered cases.  Or just result in the under-design of situations the code-creators didn't consider.  Their advantages are that they're hard to screw-up a design with and are easy to use.  If boring.

If this bridge was designed to a rigid, prescriptive code, it likely would've fallen down.

The problem with performance based code is that it can be complicated to design with such little guidance, some aspects of design may not be considered when not spelled out by the code, and they open the designer up for more legal issues.  The advantages are they're much more flexible and allow for more accurate and efficient designs.  As well as giving a lot more guidance when working with unusual cases.  (When prescriptive code tells you that there must be rebar every foot of concrete no matter what that's what you put.  Performance based code would tell you that cracks can't develop that are larger than "x".  Thus, when looking at an odd scenario you have guidance from the performance based code about what you're trying to accomplish, and just a bar spacing from the prescriptive code which may or may not work).

No code is entirely one or the other once you get past assembling Lego models of Star Wars vehicles: they're all on a spectrum.  AASHTO is more prescriptive than some of the more sophisticated codes (I understand the Japanese have a very advanced, performance based code) but it has performance based elements as well.  And it's the law every bridge in the US is built to, or a modified version of it anyway.

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