Thursday, July 31, 2008

Building a small bridge - 1

It's the time of year when sensible people are in their yards, on the golf course, camping in the mountains, enjoying the myriad festivals that cavort through the city every summer. Sensible people apparently don't know diddly about how we make the province run.

As everyone in the north knows, deep down in the best insulated bone of their bodies, there are 4 seasons. Now, before you assume that they are the mundane spring, summer, fall and winter... think again - we call this the "frozen north" with good reason! Yes, here we have before winter, winter, after winter, and construction season. So here I am, staying in another hotel (ok, same hotel that I was in when I wrote the first segments on bridge construction, but who's counting?) writing about my job.

The subject of this destruction project is a quaint little bridge in the middle of a small Alberta town. The bridge is 8.5 m (28 ') long carrying a rather busy provincial highway across a rather small creek. The photos in today's blog were taken in April, when it hadn't warmed enough to either melt all of the ice, nor green the grass.

Here we have the old abutment. Timber piles steel caps holding precast concrete girders. Pretty standard fare on most small bridges built in Alberta from the early 1950s to the early 1980s. A whole whack of these bridges are coming to the end of their lifespan, so the job of inspectors like me is pretty safe for the foreseeable future. Lots of work in the repair / rehabilitation / replacement of these old bridges.

Not the best photo in the world, but it shows nicely what we have to deal with. A two-lane (one-way) highway running through town. As you can see, the RCMP detachment is just around the corner. The other fact of note here is the vast expanse of lush green grass to the south of the road. Ok, so you're going to have to use your bountiful imagination on that one, but trust me, it's green right now.

This greenery is the pride and joy of this little town of about 7,000 residents - the park. Life would have been sooooooooo much easier if the town didn't get their way. Had we been able to detour traffic through the park, this would be a two month job. But no, they didn't want to deface their park, so you the viewer get to watch as a bridge gets built using staged construction.

Staged construction. Let's build a bridge in two parts. First we'll build half of it, then put traffic on the new bridge and build the other half. Yeah, it's like building two bridges for the price of one.

The new structure is going to be 10 m (32.8') long and 19.5 m (64') wide. Two lanes of traffic, shoulders on each side, and a 2.2 m sidewalk beside that. The abutment will consist of cast-in-place concrete bearing on steel piles. The span consists of prestressed concrete girders bearing on the concrete abutment walls, with a cast-in-place composite concrete deck topped with 80 mm (3 1/4") of asphaltic concrete pavement.

Bear with me and I'll show you the process of this little bridge getting put together.

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