Monday, June 2, 2008

Building a bridge... 3

When last we left you, we had the abutment area excavated and the pile locations laid out. Now it's time for pile driving. But first, this message:

Hello. My name is Mr. Spit, and my wife approves of this joke:

An architect, an artist and an engineer were discussing whether it was better to spend time with the wife or a mistress. The architect said he enjoyed time with his wife, building a solid foundation for an enduring relationship. The artist said he enjoyed time with his mistress, because of the passion and mystery he found there. The engineer said, "I like both."


Engineer: "Yeah. If you have a wife and a mistress, they will each assume you are spending time with the other woman, and you can go to the lab and get some work done."

Back to the narrative...

The piles in question for this bridge are HP310x63. I know, outside the world of heavy construction, this looks like so much code. Unlike people's commonly held misconception, we don't use "I" beams anymore. Though the steel might look like an "I", it's really more of an "H" shape nowadays. So, the piles are HP type (it's a steel thing, these are under the category HP in the Canadian Institute of Steel Construction) lengths of steel. These are 12.192 m / 40' long. The 250 refers to the nominal depth of 250 mm / 10", and the 63 tells us that this steel weighs in at 63 kilograms per metre. So the designation says that we are using standard lengths of steel that weigh in at roughly 3/4 of a metric tonne. In reality, this is a fairly small piece of steel for a bridge. As a result, as this bridge has some hard material below the regular dirt, we need to reinforce the ends of the piles. If you look at the photo, there are plates welded onto the ends of the piles for just this purpose. This strengthens the pile so that when we drive them into hard shale they will not be damaged. Damaging the primary foundation for a bridge? Not a cool thing... so we reinforce...

Now, during pile driving, it is the job of the engineer to monitor the progress of the steel into the ground. The government is rather sticky about maintaining records of pile driving. They want to come back in 50 years and know how deep the foundation runs, and as well, how much strength they can expect from the foundation when they change the legal loads that can drive across the bridge. How is this done? That stalwart of mathematical prowess... counting. Yeah, I'm serious! We count how many times the pile gets hit with the hammer. We monitor how many of these blows occur for every 0.25 m of the length of the pile. How do we do this? We talk nicely to the Contractor, and they measure out and mark the piles with this interval. During driving, I stand nearby and do the counting, recording it for each pile. It's remarkably easy - when things are going well. It's only when something weird happens that this becomes difficult. Thankfully, this isn't that often.

The last important aspect of pile driving is to keep the piles properly aligned in their proper locations and relative positions. This is done using a driving frame. In this photo, you can see a welder setting up the primary frame for this bridge. It is basically a couple of heavyweight H-beams connected with angle iron along the length and H-beams chunks at the ends. The angle iron is placed such that when the piles are placed between them, they will be held at the spacing required by the design. In this case, the spacing between piles is 1600 mm.

Next time I'll go into the actual setting up of the frame and getting started on pile driving.

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