Clark857
Here’s terminology question for you:
I find it curious that people are calling this a “truss” rather than a “cantilever” My understanding is that a cantilever is suspended out from the piers, tension on top, compression on the bottom. A truss, on the other hand, has the compression members on the top, the tension on the bottom.
Here’s a diagram, of the Quebec Bridge, a cantilever bridge with a suspended truss. The diagram shows the reversal of tension and compression members where the cantilever arms end and the truss section begins. http://www.brantacan.org.uk/QuebecSide2A.jpg
The transition between the truss segments and cantilever segments of cantilever bridges with suspended spans is pretty easy to see. In “through cantilevers” like the Quebec bridge or the first Carquinez Strait bridge in California, the truss sections are arched in typical truss fashion. With “deck cantilevers” the truss sections can be spotted where the thick compression members of the cantilever transition into the thin tension members of the truss, see http://www.cvrma.org/pictures/MISC/
dfrr5_170_so_young’s_high_bridge_tyrone_ky_1977.jpg (cut and paste link)
So, here’s a terminology question: What would you call
* The Quebec Bridge in Quebec? (I’d call it a “through cantilever with suspended truss”) http://www.hvq.com/fr_location.htm
* The Queensborough Bridge in New York? (I’d call it a “through cantilever without suspended truss”) http://www.brantacan.co.uk/QueensBoroughB.jpg
* The Aurora Bridge in Seattle? (I’d call it a “deck cantilever with suspended truss”) http://www.art.com/asp/display-asp/_/id—6725/Seattle.htm
* And I’d call the 35W bridge a “deck cantilever without suspended truss.”
So, back to your statement that it’s not a cantilever, seems that a truss would have the compression members on top, tension on the bottom, which is clearly not the case. So I’m curious as to the terminolgy used by bridge engineers—can you elaborate? If it was a “through” bridge rather than a “deck” bridge so it looked like the Queensborough Bridge in profile, would you still call it a “truss” or would you call it a cantilever?
Inquiring minds want to know
Kwuntongchai
BTW, posting links is easy—just paste in the location info of the page you’re steering us to.
Through and deck trusses are so named by the location of the driving surface. If the driving surface sits on framing connected to the bottom chord, it’s a through truss. If, as in the case of the Minneapolis bridge, the driving surface is supported above the top chord, then it’s a deck truss. There are also trusses that make up an arch form which are called truss arches. The bowed form of the bottom chord of the MN bridge doesn't make it an arch. Truss arches can be deck structure, i.e. all below the deck. There are also truss arches that rise well above the deck and support the deck via cables.
The Quebec Bridge is a through truss composed of anchor, cantilever and suspended segments.
The Qeensborough Bridge is also a through truss. More particularly it is a continuous through truss. The continuity is evidenced by the additional depth at the towers. The MN bridge was a continuous deck truss.
The Aurora Bridge may be a deck truss arch. The depth of the lower chord arc may be such that the action is more arch than truss, i.e. the bottom chord remains in compression even at mid-span.
Continuous trusses are distinguished by depth at the towers as opposed to a series of simple span trusses. Simple trusses were often used for railroad bridges. Railroads cared less for structural efficiency than they did for maintenance. If there was a problem with one span, it didn't affect the others. See the Big Four Railroad Bridge (now abandoned) http://en.structurae.de/structures/data/index.cfm?ID=s0001373.
This site hosts a considerable database of bridges.
