suspension bridges

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                             SUSPENSION BRIDGES

                          What do all suspension bridges have in common?  They all have 2 towers, at least 2 main cables, lots of suspenders, a deck and a couple of anchorages.


  The Golden Gate Bridge was finished in 1937. The chief engineer was Joseph Strauss.


The chief engineer of the St. Johns Bridge (1931) was David Steinman-

All suspension bridges have 2 towers. Then there are the 2 thick main cables that start from 1 end of the bridge,go up and over the 1st tower, then sag down before going up and over the 2nd tower and finally down to the opposite end of the bridge.


You'll notice these 2 cables never connect with the deck (or roadway) of the bridge. They get pretty close at the middle of the bridge, but they never touch. 


It's easier to see here with this "human" suspension bridge.


The 2 yellow main cables are supported by the 2 towers.-Sacha and Frank are 1 tower and Tim and Eric are the other.  The deck (with Megan) is hanging from the main cables by 6 smaller SUSPENDERS.


These suspenders take all the weight of the deck and whatever is on the deck (people,cars, snow etc.) and transfer this weight to the  main cables.


Here's a shot of the 4 suspenders going up to the main cable- 500 feet up.




What about the main cables? On the Golden Gate Bridge, the main cables are over 3 feet in diameter.   



Each of the over 27,000  wires in the main cable, is about the size of a pencil.


I got the chance to go to the top of the tower on the Golden Gate. The weather was clear and the water calm.


You can see the other tower 4200 feet away.


The suspenders hang from the main cables in groups of 4.


You can see the main cable coming close, but not connecting with the deck. 

The weight of the deck and the cars, people, etc. pull down on the suspenders, which pull down on the main cable.  So what's preventing the main cables from being pulled down off the tops of  the towers?

The end of the main cable is attached to an ANCHORAGE- which is something  big and heavy that can resist the pull on the main cables.----like an oak tree. 


The anchorage for the St. Johns Bridge in Cathedral Park is  53 million pounds of concrete inside this structure.



Now that you know all about suspension bridges, you can use the BRIDGE BEAMS to build your own model. 


The towers are built with a pair of #6 beams, supported by 2 #3 beams. The base is made of 4 #4 beams while the brick (the anchorage) sits on a pair of #3 beams.  The main cables are 1/8" line (string) and the suspenders


are 4 J-bolts that hang from the main cables.


We ran out of bricks for the anchorage, so we had to improvise- we used a can filled with metal scrap at 1 end and a pineapple at the other end.