Thursday, June 5, 2008

Building a bridge... 6

It rained today. Lots. Lightning as well. Usually this is fun to watch. Not quite as much when you're pile driving and the crane is 100' tall. Thankfully, it stayed the heck away from the bridge site! That wasn't funny... onto something that is:

An engineer was crossing a road one day, when a frog called out to him and said, "If you kiss me, I’ll turn into a beautiful princess." He bent over, picked up the frog and put it in his pocket. The frog spoke up again and said, "If you kiss me and turn me back into a beautiful princess, I will stay with you for one week." The engineer took the frog out of his pocket, smiled at it and returned it to the pocket. The frog then cried out, "If you kiss me and turn me back into a princess, I’ll stay with you for one week and do ANYTHING you want." Again, the engineer took the frog out, smiled at it and put it back into his pocket. Finally, the frog asked,

"What is the matter? I’ve told you I’m a beautiful princess, and that I’ll stay with you for one week and do anything you want. Why won’t you kiss me?" The engineer said, "Look, I’m an engineer. I don’t have time for a girlfriend, but a talking frog, now that’s cool."


On to battered piles. Now that we had the back row of piles driven into the ground, it's time to do the front row. This bridge was designed with two rows, with the front row being battered. In engineerese, a battered pile is one that is installed at an angle away from the vertical. For this particular bridge, the front row of piles are installed with a batter of 1 horizontal to 6 vertical. In other words, for every six feet / meters you move up the pile, the pile leans away from the vertical one foot / meter. Or, the pile is installed at an angle or roughly 15 degrees away from the vertical. The photo shows the first pile being set in place. We have not yet started driving. If you compare this photo to the last one in the previous bridge post, you will note that there is another piece to the puzzle that is the driving frame. To aid in keeping the pile in position, the contractor installed another crosspiece welded to the top of the back row of piles. Along with the other steel, this keeps the piles going where we want them to go, instead of wandering all over the river bank.

What's the purpose of a battered pile you might ask? Surely it's not just to look different. Considering that, when the bridge is done, you will never see these pile unless there is a serious problem of scour (another topic for another day) - so why batter them? Pretend that you are a force (think back to high school physics class where your eyes glazed over at the talk of force vectors) and you are being applied to the bridge. Run with me for a minute here... I'll try and not get technical and talk about statics and the strength of materials - at least, not today. The mass of the finished bridge will be bearing on these piles. Albert's good friend, gravity, will be pulling the bridge down onto the earth. Without these piles, the compacted dirt would not be strong enough to hold up the bridge - the bank would fail and the bridge would be in the water, instead of over it. So we drive piles into the ground so that, through a combination of end-bearing and skin friction, they will support the bridge.

Oh dear, more technical terms that I need to explain. End-bearing. Picture a drinking straw. Now picture taking that straw and pushing it into a milkshake. All the way to the bottom. We need a really thick shake for this analogy, so thing of one of the golden arches chemical compositions for this analogy. Once you've gotten the shake all the way to the bottom and you are now pushing against the bottom of the cup - that is analogous to an end bearing pile. The force that you are applying to the straw (you can stop now) is bearing on the end of the straw against the bottom of the cup. All of the force is taken on the end of the straw / pile. Now, picture pushing another straw into the cup, only you are going to stop pushing before you get to the bottom. When you take your hand off of the straw, the straw doesn't sink out of sight, but rather it sits rather stationary. This is because the weight of the straw is being held by the friction of the shake against the surface of the straw. This is skin friction. On a pile, the earth around the pile holds the pile in place by friction. Here in Alberta, most piling is supported by skin friction because the bedrock is far, far, far too deep to design the piles to be end bearing.

Now that that is, hopefully, cleared up - here's where it is significant. So in this bridge our piles are supported by a combination of both mechanisms. Why? We are driving them into some hard clay shale material. Thus there will be a portion of the pile capacity from both bearing mechanisms. The dead weight of the bridge will be held by the piles. Dead weight is the weight of all of the concrete and steel that constitutes the bridge structure. Live load is the force applied by vehicular traffic, the wind, seismic forces, and Marvin the Moose to the bridge. Dead weight generally follows the direction of gravity - straight down. The vertical piles are assumed to function primarily in a vertical direction and don't resist a lot of lateral force.

Remember the golden rule - for every action there is an equal and opposite reaction? Ok, so it's the golden rule of structural science, sorry. For every force, there is a resisting force to counter it. When the resistance is not equal to the force - something has to move. If the piles were not able to resist the force applied to them, the bridge would sink.

And now Mrs. Spit tells me that I've been typing too long, so this will continue next time! Any maybe, if you're nice, I'll finish explaining a battered pile to you! D'oh! So much for trying to not go into too much depth... if you'll pardon the pun. ;-)

1 comment:

spragujs said...

Holy moly, Mr. Spit! Not only did I learn a lot (nice analogy with the straw, btw), but I also laughed a whole bunch! (Still chuckling about "you can stop now"!) Ever thought of teaching?