The process that decides this involves a lot of people. And some of it is politics which is not my forte, but most of it is engineering, and that is. The road layout, the position of sidewalks, the width of the traffic lanes all factor into positioning the bridge. Some people seem to think that we basically show-up, see a space, and throw a bridge in there.
This is not the case: the roadway is planned out with several considerations. First, of course, is the size of the road. Traffic patterns are predicted about 25 years in advance and total lanes needed to accommodate that traffic are estimated and (if in budget) provided. Sight lines horizontally and vertically are checked to ensure that national codes are met. These relate to the amount of time a driver needs to see an object of a given size and stop (or otherwise avoid it). Time to merge or turn is calculated based on road design speed (which is different from the posted speed limit) and things like crash protection and shoulder size is determined based on estimated traffic speeds.
Every aspect of the bridge is laid out to the nearest 1/8th of an inch. It's a bit of a surreal feeling to be sitting in a building, miles away, specifying where the edge of a concrete barrier will be to the nearest eighth of an inch.
Every bridge has pages of plans drawn up specifying every bit of concrete, every single piece of rebar, every bolt and patch of rubber. Those plans are public record, but they aren't posted yet. I do have a copy, but I'm not sure of what my legal right is to share them, so ... I'll show one page here. From an old set.
|One of what ended up being about 40 pages of the plan set|
From the roadway engineering perspective, it's essentially a question of fit. They want to accomodate 'x' number of lanes going in 'y' direction, passing over/under 'z' road. The department of transportation for Wisconsin (WisDoT) already owns some amount of land, and while it's expensive, they can use eminent domain to buy more. But the goal is to help out the area, not destroy homes or business, so an attempt is made to buy and clear as little land as possible.
The roadway engineer (which I'm not) will work with the structural engineer (which I am) to make sure that a bridge will fit in that area, and have enough space to support it's weight. Position of columns, decks and girders are taken into account and a horizontal and vertical profile is created that is supposed describe all the driving surfaces in all three dimensions.
Once the geometry of the bridge is determined (and this is actually an evolving process, that goes back and forth over the course of the design, but in theory it's linear) the structural design starts. In construction the bridge is of course built from the ground up. But it's designed from the top down. Meaning the function is considered first (function being the driving surface) and then what's needed to make that function is designed.
Once again the process is iterative, as the design of the substructure (the piers/columns supporting the bridge, abutments or supports at either end, and anything in the ground) may influence what has to be done on the superstructure (anything above the substructure). Part of being a good engineer means being able to make good estimates for things you haven't designed. For example, if I was told that the desire was to span about 100 feet, I'd guess that we'd need a superstructure that was 5 feet deep to do so. A roadway engineer could then design her vertical profiles to ensure that there was enough clearance between the bridge and the road beneath it. As I refine my design, I may discover that I only need 4 feet and 9 inches. The choice can then be made to lower the bridge, or to leave the excess clearance in.
|Some bridge terminology|
But I'm getting ahead of myself. While the design came before anything was done in the field, the field had to start off by clearing the old one. The road was going to be widened but not re-routed, so the destruction of the old bridge needed to be done before a new bridge could be built.