a few bridge questions

[nkp_174]

I have a degree in Civil Engineering, so I might be able to help a little.

Basics: designs are usually optimized to minimize a valuable resource. Can you guess what is the most valuable today?

Every material has its advantages and disadvantages...rigidity, elasticity, tensile strength, compressive strength, shear strength, price, and ability to be joined. Steel is wonderful in tension, and ok in compression. Steel also has a valuable property known as strain hardening...it gets stronger AND lets you know before it fails. Concrete and masonry are wonderful in compression but worthless in tension and bending. I've seen a half a brick pulverized in a massive press...it is incredible...and yet I can snap it with my bare hands under a bending moment.

Bridges are not as stationary as you think. They usually are pinned at one end and have rollers elsewhere. When traveling across a highway bridge, notice the teeth at one end for an expansion joint? There are rockers or some other type of roller device underneath. This isn't just for temperature changes, but also for deflection under loads.

All bridge spans are compatible with one another...but workers whom deal with steel usually don't know much about wood, concrete, brick, etc. That is a major reason why bridges are usually built to use as few different materials and construction techniques as possible. Note how, typically, all joints on a steel bridge are either riveted, bolted, or welded?

Piers typically only need to resist compression. Therefore, masonry or concrete is an excellent material...especially if the pier is going to be subject to moisture. Steel, on the other hand, is excellent for a truss. Masonry and concrete bridges require arching, and so they usually are far more expensive to build.

Wood, on the other hand, is usually too expensive for both maintenance and labor. It is a bad choice in modern times.

As sumpter has pointed out, curved bridges generally don't exist. Think about the amount of effort required to bend a piece of lumber for such a bridge...worth it? No way! Same thing with steel. And as for casting? That would require special forms. It has been done before, usually in concrete or brick, but they are unnecessarily expensive to build. I would also play havoc on the mechanics of a truss if the parts weren't straight. Trestles and trusses usually fail when deflection under load causes parts to curve. Curved wooden trestles only exist on model railroads. Real "curved" trestles are actually a series of straight bridge spans with the stringers only touching 3 trestle bents. The neighboring stringer ends every other bent, allowing a constant piece above each bent allowing the stringers which end to be pinned through it.

Next time you see a photo of a (edit: damaged) trestle (a bridge consisting of more than one span), notice how only the affected spans are down. These is because each span is structurally independent of the others under normal conditions.

As a footnote, railroad bridges are rated according to the Cooper Bridge Rating system. It was designed in the late 19th century and assumes a 2-8-0 to be the locomotive. The Cooper bridge rating will vary for every span. For shorter spans, shear is the enemy. For longer spans, bending is the enemy. For an NKP berk, the worst Cooper ratings aren't when the entire locomotive is on the span...I know this from having seen the ratings on one of them. The speed is also a factor. Today, those SD70s pulling autoracks are allowed to cross a bridge dependent upon a rating formula which uses a 2-8-0 as the model.

This has gotten a touch long, so I've probably missed some stuff.

Michael

EDIT: wooden bents are probably taboo because they require quite a bit of maintenance. They also would be built to shorter spans than would be necessary for steel. I'm feeling too lazy to go consult my handbook on the compressive strength of wood. If it is worthwhile to replace the deck or truss, it is probably worth it to replace the bents underneath.