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

Posted by PITHOCRATES - August 7th, 2013

Technology 101

(Originally published March 6th, 2013)

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.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , ,

Side Streets, Downtown Streets, Highways, Parkways and Freeways

Posted by PITHOCRATES - July 31st, 2013

Technology 101

In 20th Century our Subdivision Planners shifted from Automobile-Friendly to People-Friendly Designs

The automobile changed how we live.  Where once we crowded into crowded cites and worked close to where we lived today we don’t.  Instead choosing to live in sleepy suburbs.  Away from the noise and congestion of city life.  Where we can relax after work.  And on the weekend.  Enjoying a beer in the shade in our backyard.  Our little Shangri-La.  Come Monday morning, though, it’s back to the grind.  So we back our car out of the garage.  And drive out of our little residential community.

If you live in an older suburb that would be a drive down a straight road.  Running either north and south.  Or east and west.  Bringing you efficiently to a larger road.  That you can efficiently take to a larger road yet.  With a higher speed limit.  With many of us eventually taking that road to an onramp of an interstate freeway.  For that morning commute.  Quick.  And efficiently.  Thanks to our city and suburb planners making our cities and suburbs so automobile-friendly.

Soon everyone was driving so much that these roads got congested.  Including the ones in our sleepy little subdivisions.  With people racing down our side streets to get to those bigger roads.  Filling our little Shangri-La with the sounds of traffic.  And making it unsafe for our kids to ride their bicycles in the street.  Which is why somewhere around the middle of the 20th century our subdivision planners shifted from automobile-friendly to people-friendly.  Instead of grids of straight lines crossing other straight lines at neat right angles our roads in our subdivisions began to curve.  If you ever tried to cut through a subdivision and got so turned around that you ended up where you entered this is why.  To discourage people from driving through our sleepy little streets.  So we can relax with that beer in the shade.  And our kids can ride their bicycles safely in the streets in front of our homes.

Design Speed is the First Consideration when Designing a New Road

Cars are big and heavy.  Trucks are even bigger and heavier.  Yet millions of them safely share the same roads every day.  And few in a small car look twice at a semi truck and trailer stopped next to them at a traffic light.  Or give a second thought to an even bigger and heavier freight train crossing the road ahead of them while they sit at a railroad crossing.  All because of lines painted on the road.  Speed limit signs keeping us driving at the same speed.  And stop signs and traffic lights.  Which people observe.  And give the right-of-way to others.  While they wait their turn to proceed.  Except for trains.  They always have the right-of-way.  Because trains can’t stop as easily as a car or a truck.  And they pay a lot of money for that right-of-way.

As we left our neighborhoods and got onto the bigger roads and drove to the interstate freeway the speed limit got higher and higher.  And the faster large things go the more kinetic energy they build up.  Making it harder to stop.  And to control.  That’s why trains don’t stop for cars.  Cars stop for trains.  Emergency vehicles, like fire trucks and ambulances, get the right-of-way, too.  When we see their lights flashing and/or hear their sirens we pull to the curb and stop.  Because they’re speeding to an emergency and need a clear road.  But also because they are often traveling faster than the design speed of the road.

Yes, design speed.  Not the speed limit.  Two completely different things.  It’s the first consideration when designing a new road.  How fast will traffic travel?  Because everything follows from that.  Curves, grades, visibility, etc., these are all things that vary with speed.  Engineers will design a downtown street with a lot of vehicular and pedestrian traffic for lower speeds than they’ll design a country highway that connects two towns.  Also, lane width in a downtown street can be as narrow as 9 feet.  And they can have sidewalks adjacent to the curbs.  Allowing narrower streets for pedestrians to cross.  Freeways, on the other hand, have lanes that are 12 feet wide.  And have wide shoulders.  Because faster vehicles need more separation.  As they tend to waver across their lanes.  So this is another reason why we pull aside for emergency vehicles.  As they may approach or exceed the design speed of a road.  So we give them wider lanes by pulling over.  As well as giving them a less obstructive view of the road ahead.

The Modern Interstate Freeway System is Basically an Improved Parkway

Old 2-lane country highways had narrow lanes and narrow shoulders.  Making it easy to drift across the center line if distracted.  Or tired.  Into oncoming traffic.  If a person hugs the shoulder because he or she is nervous about fast-moving oncoming traffic they could drift over to the right.  Out of their lane.  And drop off of the shoulder.  Which could result in a loss of control.  Even a rollover accident.  And if you were stuck behind a slow-moving truck on a grade there was only one way around it.  Moving over into the lane of oncoming traffic.  And speeding up to get ahead of the truck before a car crashes head-on into you.  In fact, there used to be a passing lane.  A 3-lane highway with one lane traveling one direction.  One lane traveling in the other direction.  And a lane in the middle for passing.  Which worked well when only one person passed at a time.  But did not work so well when cars from each lane moved into the passing lane at the same time.  Running head-on into each other.  That’s why you won’t see a passing lane these days.  They are just too dangerous.

In the 20th century we started making roads for higher speeds.  Parkways.  The traffic travelling in either direction was separated by a median.  So you couldn’t drift into oncoming traffic.  There were no intersections.  Crossroads went over or under these parkways.  So traffic on the parkways didn’t have to stop.  They also had limited access.  On ramps and off ramps brought cars on and off, merging them into/out of moving traffic.  And unlike the old 2-lane country roads there were 2 lanes of traffic in each direction.  So if you wanted to pass someone you didn’t have to drive into oncoming traffic to go around a slower-moving vehicle.  And there was a paved shoulder.  So if a car had a flat tire they could limp onto the shoulder to change their tire.  Without interrupting the traffic on the parkway.  Of course, being on the shoulder of a parkway was not the safest place to be.  Especially if some distracted driver drifted onto the shoulder.  And crashed into your broken down car.

The modern interstate freeway system is basically an improved parkway.  They have wider lanes and wider shoulders.  Along the median and the outside right lane.  Instead of the typical Windsor Arch of the parkway they have bridges of concrete and steel.  Allowing greater spans over the roadway.  Keeping those shoulders wide even under the overpasses.  Grades are less steep.  And curves are less sharp.  Allowing little steering inputs at high speeds to control your vehicle.  Making for safer travel at even higher speeds.  And a much greater field of vision.  Even at night where there are no streetlights.  The road won’t change grade or curve so great beyond the length of your headlights.  Safely allowing a high speed even when you can’t see what’s up ahead.  Little things that you’ve probably never noticed.  But if you exit the interstate onto a curvy 2-lane highway with steep grades you will notice that you can’t drive at the same speed.  Especially at night.  In fact, you may drive well below the posted speed limit.  Because you can’t see the summit of the next hill.  Or the curve that takes you away from a sharp drop-off to a ravine below.  Like you find around ski resorts in the mountains.  The kind of highways you can’t wait to get off of and onto the safer interstate freeway system.  Especially in a driving snow storm.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , , , , , , ,

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.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , ,