The discussions of the “kingpost” and lower chord near the pier being an insignificant members are off track. As alluded to in some other posts, the truss is a three-span continuous structure. (See the general plan and elevation on the previously referenced MnDOT page) Tension in the top chord and compression in the bottom chord over the piers. Reverse that as you progress toward mid-span. The tension diagonals on either side of the "kingpost" are also significant members (as are almost all members in a truss). Those diagonals transmit the accumulated load to the “kingpost” which in turn transmits the reaction to the bearing.
The speculation regarding the SE bearing is interesting. I have yet to develop an opinion that it is either a cause or a consequence.
I do have a suggestion. Think in more than one plane. Most of this discussion has focused on the primary truss mechanics. Look again at the general plan and elevation from the MnDOT web site. Note the Section drawn near Pier 7 in the lower left corner. I have been wondering if there was a failure in a floor truss or the lateral bracing which allowed the trusses over the south pier to roll. The floor trusses are the smaller trusses which span between and cantilever over the main trusses supporting the deck stringers.
The term cantilever is being overused. The only cantilevers I have noticed are the single panel cantilevers at the north and south ends of the deck truss which catch the ends of the multi-girder approach spans and the cantilevers of the floor trusses outside the main trusses to support the deck.
Some have wondered about differences in the bearings over Piers 6 and 7. According to the drawings, the bearings over pier 7 (north) are fixed with expansion bearings over pier 6 (south). Hence the toothed rollers in the Pier 6 bearing assemblies.
Way over my 2 cents worth here. Thanks for tolerating. Yes, I am a bridge engineer.
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.
