 Short-span bridges: Vital corridor links
<<Sounds of car passing>> Narrator Says: They are small and they might seem pretty insignificant in the grand scheme of the county’s transportation infrastructure and many people don’t even know they just crossed one. But short-span bridges, by definition those that are shorter than 20 feet in length, keep vital travel corridors in tact, corridors that provide critical access for those in rural parts of the county. With that in mind, the King County Road Services Division is in the midst of a massive overhaul of short-span bridges all across the county. The program began in 2005 by evaluating 50 short-span bridges countywide and identifying which should be replaced first. Jamie O’Day is an engineer in the Road Services Division; she says the new bridges will alleviate many of the shortcomings of the old bridges. King County Bridge Engineer Jamie O’Day Says: The timber typically is at the end of its design life, at the end of its useful life. You can see that it’s still bleeding creosote, years later. This particular bridge has some erosion, so this is a repair to fix the erosion, so some sheet pile’s been driven because the river scoured out below the bottom back wall plank, which left these abutments susceptible to erosion. This concrete encasement, there’s a timber pile; there’s a hollow timber pile within this. And the concrete encasement is meant to prevent the timber pile from brooming out and collapsing because it is hollow. So these are all elements designed to keep this bridge safe, but it is at the end of its useful life and it’s really time to replace this bridge with a wider bridge. Narrator Says: A total of four bridges will be removed and replaced in 2008. For the sake of saving time and money, the roads division selected four bridges around the fall city area that are close to one another. The bridges were built in the 1950’s out of timber, and are reaching the end of their predicted lifespan, about 50 years. King County Bridge Engineer Jamie O’Day Says: We have a tremendous number of bridges that were built in the same couple year time span. so we're trying to get some cost efficiency in bundling these bridges in similar locations so we can have work crews that are moving from one bridge to another so we can get cost efficiency, crew efficiency. We’re having design efficiency by trying to use the same type of design for as many bridges as possible, which is what they did in the 50's so we're doing the same thing they did 50 years ago. Narrator Says: On top of efficiency with time and money, the replacement of short-span bridges in the county provides an opportunity to update the environmental friendliness of the bridges. For one thing, the old timber supports were treated with a wood preservative called creosote, that today is considered harmful to humans and animals. King County Bridge Engineer Jamie O’Day Says: We want to do the best thing for the environment, and removing this creosote-treated timber really does help the environment out. We widened the span, we always just about double the span when we replace these bridges. And what that does is it opens the channel up and reduces stream velocities. So when water goes through a small constriction it's like when you put your thumb over a garden hose, the flow accelerates. so in having a wider span the velocities reduce and that's just good for the environment because we have less erosion. Narrator Says: On top of complying with modern safety standards, the new short-span bridges won’t need replacement for about 80 years because the new bridges are built of steel and concrete, and work on all 50 should be complete in about 15 years.
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