Those Unasked Q's on 40 Year Old Bridges

One report indicated that the 40-year old MN bridge was an older design whic lacked additional over-designing of later technology which would have prevented a multiple failure. This makes one ask:

a) Isn’t a 40-year old bridge really like 50 years old? Unlike nanotechnology, you’d be amazed how long it can take technological advances to trickle down from the drawing board into implementation.

b) Are 40 year old bridges ever revamped over time for today’s more abusive traffic? A load (i.e., a weight) is not a load because we’re talking a live load. It bounces causing vibration which accelerates fatigue, etc. Do we as a nation really have the funds to keep up with the maintenance and inspections?

What say you, fellow bridge users?

  • Jinx

There are bridges hundreds of years old; 40 years old is nothing.

(a) Most bridges and other structures don’t tend to use the latest technology, largely because designers and builders want to be very sure that any new approach is going to work and work for a long time, so you’re right in that technological advances do take a long time to make it into practice.

(b) Bridges are designed to account for the “live” load and to deal with the fatigue issue. There can be an issue of what the bridge was designed to do vs. what is approved later in terms of vehicle sizes and loads. This has been an issue here in North Carolina, where there has been a lot of pressure on the state to approve increased truck capacity, especially for construction (dump trucks), on roads that were not designed for those heavier loads. Whether that has happened here, I have no idea. Interstates are usually designed for very high traffic loads and heavy truck weights, so that may not be an issue.

Do we have the funds to make the repairs? Not all at once, but I can’t see how we don’t have the funds in this country to do what is needed to maintain the roads. The problem is that it’s always more popular to build new roads than to simply maintain what’s already there. No one supports higher taxes to address the problems, but everyone wants all the work done. Most of us tend to think that once a bridge is built, then it’s as permanent as bedrock, but that’s not the case.

In general, disasters like this are typically not the consequence of a single failure. Although there are exceptions, it’s usually a combination of factors. I’ve read (but can’t come up with the cite immediately) that most (by far) mature technologies can withstand a design flaw, a construction or maintenance problem/shortcoming, or a human/operational error. It’s when there’s a combination of two or more that the technology fails. I think this comes out of studies of aircraft accidents, but can be applied to buildings and bridges as well.

Older bridges are often a lot stronger than more modern ones, either because of the limited materials available for construction (if you have to make it out of stone, it’s pretty much going be immensely strong, or just fall down straight away), or because the builders didn’t have our methods of oconomising on materials by designing it to be just a bit stronger than it needs to be, so they built them to be indestructable.

It is possible to upgrade existing bridges and it is done. I’d say the money is available to upgrade and fix everything; the question is whether the politicians who make that decision are willing to make the committment to do whatever it takes (i.e., raise taxes).

I can’t find a hard number for this, but a lot of the inadequate bridges do not have structural problems but just weren’t designed for the loads and traffic counts they now must bear. Whence the weight limits you see on many rural bridges. Of course, enforcement is a problem. There is this interesting statistic: while nationally about 26 percent of bridges are either structurally deficient for functionally obsolete, in Minnesota it is just 12 percent.

One problem with new designs is it can take a couple of decades for a design to prove itself inadequate - usually the hard way.