Pedestrian Bridge FE model boundary conditions
Pedestrian Bridge FE model boundary conditions
(OP)
I am designing a single span 10'x100' Pratt Truss pedestrian bridge. I have created a 3D FE model of the bridge with the two bearings at the fixed-end modeled as "pinned" and bearings at the expansion-end free to translate longitudinally. When I analyze the model under transverse wind loads, I get very large opposing longitudinal reactions at the fixed-end bearings. It doesn't seem like those reactions represent real world behavior since the oversized holes at the bearing plates should allow enough rotation of the fixed-end of the bridge that the anchors will not see those reactions.
Should I design my fixed-end bearings for those large shear reactions or can I model the bridge so only one of the fixed-end bearing points is longitudinally fixed? Thanks for the input.
Should I design my fixed-end bearings for those large shear reactions or can I model the bridge so only one of the fixed-end bearing points is longitudinally fixed? Thanks for the input.





RE: Pedestrian Bridge FE model boundary conditions
RE: Pedestrian Bridge FE model boundary conditions
RE: Pedestrian Bridge FE model boundary conditions
Modelling the fixed-end bearings as "fixed restraints" which do not allow any horizontal movement will attract the opposing reactions as you have indicated. The reality is that the fixed-end bearings will allow some movement. You may want to model the stiffness of the bearings as well to try and eleviate some of the reaction.
What type of bearing are you using and who is the manufacturer?
What is the total transverse force on the bridge and what portion of this is taken by the fixed and expansion joint?
What is the magnitude of the large opposing reactions that you have at the fixed end of the bridge?
RE: Pedestrian Bridge FE model boundary conditions
I don't have the model in front of me now, but the opposing reactions are in the neighborhood of 33 kips. I don't remember off the top of my head exactly what portion of the transverse force is taken by each joint, but I know the larger portion is taken by the fixed end.
RE: Pedestrian Bridge FE model boundary conditions
RE: Pedestrian Bridge FE model boundary conditions
RE: Pedestrian Bridge FE model boundary conditions
RE: Pedestrian Bridge FE model boundary conditions
RE: Pedestrian Bridge FE model boundary conditions
That will then eleviate all the longitudinal reaction in the bearings which defeats detailing a fixed bearing. Shown in the attachment is a plan of the bridge deck with three boundary conditions.
1. Bridge bearings which do not allow movement (R=33kips)
2. Bridge bearings which allow free movement (R=0)
3. Bridge bearings which have a stiffness and allow some movement between 0 and free movement (R=25kips...depending on the stiffness of the bearings).
RE: Pedestrian Bridge FE model boundary conditions
It seems like there would be free movement due to the oversized bearing plate A.B. holes until the holes came in contact with the the anchor bolt, then there would be a fully fixed condition. I know some analysis software allows you to model a gap boundary condition to allow that movement - unfortunately mine does not.
RE: Pedestrian Bridge FE model boundary conditions
Thanks for nice plans.
You are right - changing your boundry conditions will not work.
The solution is much simpler - just specify the diagonals (wind braces) as "tension only" members. From the plans you provided this is your intention anyway, and typically wind braces are designed as such. Let us know, how it worked.