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Student Project - Bridge Questions

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I'll type responses in caps:

The daughter of an aquaintance of a client who bought a dog from my
technician's cousin, has been requested to contact structural engineers
regarding suspension bridges for a school project.  Unfortunately, I do
not deal with bridges, but I told them that I would help her find some
answers.

Any comments/opinions on the following list of questions would be
beneficial to her.

Thank you in advance.

1. How do suspension bridges allow you to build longer bridges?

IF YOU THINK OF THE CABLE AND DECK AS FLANGES OF A BEAM, THE SUSPENSION
BRIDGE TOWER ALLOWS YOU TO BUILD A VERY DEEP BEAM, WHICH REDUCES THE
CABLE (i.e. FLANGE) FORCE TO MANAGEABLE LEVELS.  ON MOST SUSPENSIONS
BRIDGES, THE CABLE FORCE IS BALANCED BY THE ANCHORAGES RATHER THAN THE
DECK, BUT THE PRINCIPLE REMAINS.

OR, LOOKED AT ANOTHER WAY, THE VERTICAL COMPONENT OF THE CABLE FORCE
RESISTS THE VERTICAL LOAD OF THE DECK.  THIS COMPONENT COMES INTO PLAY
WHENEVER THE CABLE CHANGES DIRECTION.  THE TALL TOWERS GIVE THE CABLE
ROOM TO CHANGE DIRECTION ENOUGH TO KEEP THE FORCES MANAGEABLE.  BOTH OF
THESE EXPLANATIONS MAY BE TOO TECHNICAL TO BE USEFUL--PERHAPS SOMEONE
ELSE CAN DO BETTER.

IN ADDITION, THE CABLE IS MADE OF VERY HIGH-STRENGTH STEEL, MUCH
STRONGER THAN THAT FOUND IN MOST BEAMS OR TRUSSES.

2. Are suspension bridges harder to design than other types? If so why? 

MUCH MORE DIFFICULT, BECAUSE THEIR FLEXIBILITY RENDERS SIMPLER ANALYSES
(PREDICATED ON SMALL DEFLECTIONS) INADEQUATE.

3. How do towers and cable bridges support the weight of traffic? 

BY HANGING THEM FROM THE TOWER WITH THE CABLE.  SEE THE SECOND PART OF
NUMBER 1 ABOVE.

4. Where do you see the future of suspension bridge design?

IN VERY LONG SPAN BRIDGES--THE BERING STRAIGHT, THE STRAIGHT OF
GIBRALTAR, AND SIMILAR VERY LONG CROSSINGS. 

5. What are some of the priciple of geometry used in bridge design?

MUCH OF IT IS BASED ON TRIGONOMETRY, BOTH IN THE LAYOUT OF THE BRIDGE,
AND IN THE RESOLUTION OF FORCE VECTORS. 

6. What would be the strongest design for a spagetti noodle bridge? 

IF YOU'RE ALLOWED TO USE GLUE TO MAKE CONNECTIONS, I WOULD TRY BUILD A
TRUSS.

Respond to the list, or privately to me, if you wish.

Thank you,
Doug

HTH.  SORRY TO BE SO TECHNICAL.

MIKE HEMSTAD
ST. PAUL, MINNESOTA

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