River Traffic, Road Traffic, Ferry Crossings, Vertical Lift Bridges and Bascule Bridges

Posted by PITHOCRATES - March 6th, 2013

Technology 101

Bridges that rise High Enough for Shipping Traffic to Pass Underneath need Long Approaches

As civilizations expanded they followed rivers inland.  People traveled on the river and founded new cites on sites further upstream.  Which they could supply from cities downstream.  Including the materials to build a waterwheel and lumber mill.  They can go upstream to fell trees and float them downriver to the lumber mill.  They can use this lumber to expand the city.  Out away from the river.  On a growing network of roads.  On both sides the river.

As this city grows cross-river traffic increases.  A road on both sides of the river end at a dock.  Between these docks runs a ferry.  That can transport people, horses, carts, wagons, etc.  Allowing people and goods to travel anywhere within this city on the river.  Over time cross-river traffic increases causing backups at the ferry crossing.  Eventually cars replace horses.  Concrete replaces dirt roads.  And vehicular traffic increases.  While railroads connect our cities.  All of which has to cross the river.  While at the same time allowing boats to continue to navigate the river.

If you ever driven on a bridge over a navigable river with shipping traffic you probably noticed a couple of things.  First of all, when you crossed the navigable portion of the waterway you were pretty high in the air.  Second, there was a long approach to that portion of the bridge that allowed you to reach that height over a gradual incline.  And you started that incline about a mile or so away from the river.  Which is fine for an interstate that can rise above a city until it reaches a sufficient height to cross the river without impeding river traffic.  But it’s a bit of a problem for the roads at the river’s edge.  For it is just not practical to drive a mile or so away from the river, cross over on the bridge, and then drive a mile or so back to the river.  Not to mention the incredible cost of such a bridge that would provide only one river crossing.  It would be far more practical and less costly to build multiple bridge crossings at the current elevation of the roads at the river’s edge.  But that would, of course, block river traffic to most commercial shipping.

The Vertical Lift Bridge can lift Heavier Road Sections than Bascule Bridges with the same Size Counterweights

The solution is the moveable bridge.  A bridge at the elevation of the local roads so a car can cross from one shore to the other in the shortest possible distance.  And one that can move to create an opening in the roadway to allow a ship to navigate the river at the bridge crossing.  Because vehicular traffic is greater than river traffic vehicular bridges are normally in a position to allow vehicular traffic to cross.  In places where river traffic is greater than rail traffic rail bridges are normally in a position to allow river traffic to pass.  Which can be a problem for trains that ignore a red signal to stop.  For a train can drive right off the tracks and into a river.  And have.

Two of the most common moveable bridges are the vertical lift bridge.  And the bascule bridge.  Each has benefits.  Each has its faults.  The vertical lift bridge raises a portion of the roadway over the shipping channel.  At each end of the lifting section is a tower.  Inside these towers are counterweights.  The counterweights equal the weight of the section of the moveable roadway.  Because the roadway and the weights are balanced it doesn’t take much force to raise or lower the bridge.  Like an elevator in a building.

Older bridges had a bridge operator that rode up and down with the bridge.  As a ship approached traffic signals would stop traffic.  Once all traffic was off the bridge gates came down blocking further traffic from entering.  Once all vehicles and pedestrians were off the lift portion a signal sounded to warn people the bridge would begin to move.  Then it moved.  The section of roadway traveled up between the two towers.  Creating a safe passage for the ship below.  Most bridges today are automated and unmanned.  The big advantage of the lift bridge is the size of the counterweights.  They only have to equal the weight of the span. Allowing heavier road sections to be lifted.  Making them good for rail bridges.

The ‘Chicago’ Bascule Bridge is the most common Moveable Bridge in the World

The drawback to the vertical lift bridge is that there is still a maximum height of ship that can pass underneath.  Which isn’t a problem for most shipping.  But it can be an issue for some oversized loads or ships with tall masts.  Also, those tall towers can be unsightly.  Consider a city like Chicago.  Which has a lot of tall buildings right on the banks of the Chicago River.  Where a lot of bridge towers at all of those river crossings could really ugly up the Chicago skyline.  So in Chicago you won’t see vertical lift bridges spanning the Chicago River.  Instead you’ll see bascule bridges.

The typical bascule bridge you see in Chicago is a double-leaf bascule bridge.  Bascule is French for seesaw.  Think of a playground seesaw.  When one side goes up the other side goes down.  Each leaf of a bascule bridge is a seesaw.  A teeter-totter.  One side of the seesaw is a metal roadbed.  The other side is a counterweight.  When the bridge opens the counterweight teeters down below the road elevation while the other end teeters up above the road elevation.  Creating an opening over the river for ships to pass through.  To span a river two seesaws are connected together with their metal roadbeds pointing towards each other.  And their counterweights pointing away from the river.  Because the leaf is longer than the counterweight the counterweight has to be heavier than the bridge leaf.  To equal the torque between the leaf and the counterweight.  So it takes the same turning force to raise and lower the bridge leaf.  Keeping both sides of the seesaw balanced so it takes little power to operate a bascule bridge.  Just as it takes little power to raise and lower a vertical lift bridge.

The greater weight of the counterweights makes the bascule bridge more costly than the vertical lift bridge.  But in return for the added cost you get a cleaner bridge installation with no unsightly towers.  And when the bridge is opened there is no limit to how tall a ship can pass through the bridge crossing.  Because there is an open gap in the roadway.  Which creates one additional challenge for the bascule bridge over the vertical lift bridge.  When a lift bridge rises cables can rise with it like in an elevator.  Keeping both ends and the moveable roadway connected to each other electrically.  Both power and communication.  This is not possible in the bascule bridge.  With nothing going over the river crossing when the bridge is open there is only one option for electrically connecting the two ends of the bridge.  If cables can’t go over a ship they must go underneath a ship.  With a bascule bridge submarine power and communication cables interconnect the two bridge ends.  Requiring a diving crew to lay this cable.  Making both the bridge cost and maintenance more costly on a bascule bridge than a vertical lift bridge.  But in return you get a less unsightly installation.  And a shorter time to open and close the bridge.  Making the ‘Chicago’ bascule bridge the most common moveable bridge in the world.



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