Cross-Frame Design Questions!!!

[bridgeengineer2007]

Hello All,

I am designing my first cross-frame (K-type) for a straight, two-span continuous bridge under LRFD specs. I am looking at a typical intermediate cross-frame, and a pier cross-frame. I have several questions regarding each:

Intermediate: Per AASHTO LRFD, it says that wind loading on the top half of the superstructure is transmitted directly to the deck. Therefore, I was designing the bottom strut for wind on the lower half of the beam. I have solved the diagonals as truss members and found their equivalent loads. Is this the only load I should consider?

Pier: Do I need to design the top strut and diagonals for transfer of forces (WS and WL) back down from the deck to the bearings? Or are the girders considered stiff enough to transfer the load?

It seems like cross frames on a straight bridge are only for counteracting wind during the construction phase before the deck is hardened, but I wanted some other opinions.

Finally - what is the purpose of designing the abutment concrete diaphragms for wheel loads? Isn't the deck designed to carry loads to the beams through the bridge? I never understood why two wheel loads are placed over the end diaphragms...


THANKS!

 

[RHTPE]

Gwynn, I definitely understand about the personal info. You can reach me thru my web site - just add a dot com and no spaces.


Ralph
Structures Consulting
Northeast USA

 

[OSUCivlEng]

I have found myself designing cross frames for a 3 span continuous plate girder. I have always sized the cross frames members based on two loads: the wind load on the girder and a force applied to the top flange of the girder during the deck placement. I usually use 2% of the construction load for the force applied to the top flange of the girder. I am also going to look at the torsion applied to the exterior girder by the overhang brackets.