Sunday, June 1, 2008

Building a bridge... 2

The last post gave some background on the big bridge. Of course, we've now moved onto construction. As mentioned, we can't start on the pier construction until summer. However, this has no effect on our ability to build the abutments.

What's an abutment? Glad you asked. It's where the bridge abuts on the ground. Every bridge has two abutments. These are the ends of the bridge where the load from the bridge is transferred to the ground. Bridge foundations generally have abutments and, if they are more than one span, piers. At these locations the bridge deck load is transfered to the foundation substructure, and thence to the earth. I say this in a general way, because there are different systems for carrying the bridge deck. The deck is the system that carries traffic across the bridge. The substructure is everything below the bearings. And the bearings are what supports the bridge girders on top of the substructure. Lastly, the substructure rests on a foundation. So, bottom to top we have the foundation, the substructure, the superstructure (includes the bearings and load carrying elements), and the traffic. Pretty simple - right?

Sure, and we all love math and physics and go by the term - enginerd. At least, everyone that I work with in my office does!

Moving along before my sense of humour fails to tickle your funny bone. Here's an applicable joke to sate you before the dirt moving starts:

As priest, a doctor, and an engineer were waiting one morning for a particularly slow group of golfers.

Engineer: What's with these guys? We must have been waiting for 15 minutes!

Doctor: I don't know, but I've never seen such ineptitude!

Priest: Hey, here comes the greenskeeper. Let's have a word with him.

Priest: Hi George. Say, George, what's with that group ahead of us? They're rather slow aren't they?

George: Oh yes. That's a group of blind fire fighters. They lost their sight while saving our club house last year. So we let them play here anytime free of charge!


Priest: That's so sad. I think I will say a special prayer for them tonight.

Doctor: Good idea. And I'm going to contact my ophthalmologist buddy and see if there's anything he can do for them.

Engineer: Why can't these guys play at night?

Now that you've got a better idea of the engineer's mindset, let's carry on.

Mr. Contractor showed up onsite and proceeded to tear apart the lovely job of roadbuilding that was performed several years ago. For, while the slopes comply with the design in every way - they don't have space for the abutments. For while the road was built to grade, the slopes don't have any allowance for the roughly 3 m / 10 ' depth of abutment structure, girders, and deck that have to be build.

The eagle eyed among you will notice that the gravel bar has disappeared. It rained a couple days before this photo and presto - the water has gone up. Isn't nature grand? The hillside has been excavated for two purposes. 1) to expose the ground to a level just below the final depth of concrete that will be placed for the abutments, as this corresponds to the level just below the top of the steel piles that will carry one quarter of the weight of the bridge. 2) a ramp was built to enable the pile driving contractor to access the abutment area to drive the piles.

Here's a closer look:The surveyor has laid out the locations of the 14 steel H-piles that will be driven into the ground to support the abutment. In a month or so, this will be a seething mass of reinforcing steel prior to the concrete pour.

That's all for today. If you're interested in where in the world this is, punch the following into Google Earth: 53.16361N,115.9114W and you'll be taken to an aerial view straight away.

1 comment:

excavator said...

Thought I'd hop on the bridge-building tutorial.

Love that engineering joke--cut through all that human-obligation-entanglement to the chase and figure out how everyone can get what they want (except for that pesky problem that everyone knows that the firemen being relegated to the course at night negates the idea of paying the debt to them). From the joke in #3 I get the idea that the joke engineers tell on themselves is about missing that oh-so-delicate element of unspoken social cues.

Anyway, when I saw your post I thought I'd hang around and learn something.

Notwithstanding the fact that I had to further break down some of the more basic terms (such as 'truss' and 'span')your explanation is very clear and fun to read.