Friday, July 1, 2011

The measure of a bridge

Measure twice, cut once, woodworkers say.

Accurate and precise measurement are the keys to 90-degree square corners, even legs, and generally, pieces fitting together properly. On a small project it's usually possible to correct errors  by cutting, planing, sanding, gluing, or using wood putty.

Now imagine you are building an enormous bridge, highways, and on and off ramps over New Haven Harbor to replace the old bridge.

The materials are way too big to measure with a tape. Can a steel fabricator guarantee that a 50-foot girder will be within 0.001 inch of the blueprint girder?  When construction workers pour concrete into stories-high wooden molds,  how are they able to make the finished product exactly the right size? Also, all of the angles have to be correct, because the bridge and associated sky ways are curved.

Who is out there with a yard stick making sure the project is built according to specifications? A cursory inspection does not reveal anyone with prospecting tools, and you do not see the workers looking at plans. Maybe this all takes place in a trailer somewhere.

Still, who determines the correct height of the mold and how is this applied in practice? How do they make the towers plumb?

Sure, there are project engineers, bridge engineers, and all of those people. You ever see them measuring anything?
Maybe they are doing the equivalent of drawing out a tape measure and marking a spot, but using unrecognizable equipment. Infrared or laser beams? Global positioning systems? 
But even if an engineer can determine that a particular support needs to be 35 feet, six and half inches tall, how do the workers possibly fabricate something that big and with such precision?

It's not easy to make six two-by-fours all exactly eight feet long, for example. However, in theory, the job is straight forward. What theory do builders use when six pieces of concrete have to be exactly 50 feet long?

Maybe all highway support columns and other construction elements are a standard size, and so all of the fabrication equipment is automatically gauged properly.  That does not seem plausible. 
Or maybe the crews just eyeball the whole thing and modify as they go. That doesn't seem plausible  either.

Same holds true for skyscrapers, dams, stadiums,  oil tankers, and all enormous man-made things.

Suppose 100 men and women are working at a site, and each one is accurate down to a hundredth of an inch. At the end, won't whatever they're building be off by at least an inch?

1 comment:

  1. Wonderful post. I've wondered the same thing (allbeit in much less detail). Fortunately for us, these bridges usually hold up. They must be doing something right.