Bridge Steel
Posted March 15, 2008
By admin
Why do some non-suspension bridges have a big heavy steel structure above them ie) Tappan Zee Bridge NY?
Why do some have these steel skelatons above road level and others do not? I understand why suspension bridges do but in this case I am talking about non-suspension bridges such as the Tappan Zee Bridge in New York or the old Jamestown Bridge in RI?
The roadway has to be held up by *something*. It's too thin and flimsy to be self-supporting, so it's got to have something else.
The choices are suspension cables or a framework. The cables of a suspension bridge carry weight up to a catenary cable, which is carried on towers which transmit the weight forces to the ground. Framework bridges work by carrying the weight as shear forces in the side trusses, and the top and bottom carry tension and compression respectively.
It's hard to describe this without pictures, but here's a short description in words:
1.) Weight wants to pull the roadway down.
2.) The ribbon of roadway has tensile strength, but if tension forces are alone the whole thing will fall inward and hang from the towers.
3.) If the roadway is put in compression it still can't hold itself up against the torque forces of weight; it isn't stiff enough to work as a cantilever.
4.) You can use a vertical beam from the tower to support a diagonal beam pulling upward on the roadway some distance out from the tower, holding that part of roadway flat. The beam pulls inward and up, the roadway pushes outward, and the net force is up (against weight).
5.) You can repeat this, but instead of building another tower you set up another vertical beam going up from the roadway itself. The top of this beam is tied back to the upper end of the tower through a horizontal beam, and you have another diagonal beam going down to the roadway again some distance further out. The extra weight and forces require the section next to the tower to be stronger, but you have just increased your free span a lot.
6.) Repeat in smaller steps until you're halfway to the next pier, where you're met by trusses coming the other way.
The horizontal X-bracing helps resist forces from wind, vibration from vehicles, and the like.
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