Pedestrian Bridge FE model boundary conditions

[bar7h]

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.

 

[kelowna]

Before you go that route, is the model correct? Are you sure you did not release transversally the expansion joint bearings? That would explain the high forces on the fixed end.

 

[bar7h]

Yes, I did check that, and I check the deflected model and it behaves as expected. Thinking of a beam that is fully fixed at one end and "roller" at the other, you still get quite a large moment reaction at the fixed end which seems to be what is giving me the large couple forces at the bearing. I'm just not sure if it's realistic.

 

[asixth]

How is the deck constructed? Are you relying on a concrete diaphram to transfer the horizontal reaction back to the bearings or do you have steel cross bracing.

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?

 

[bar7h]

This bridge has a timber plank deck, so I'm using steel bracing in the horizontal plane. The bearing is a typical bearing plate on a setting plate - teflon coated at expansion end.

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.

 

[MichSt]

If the two truss chords are braced together, wouldn’t one chord what to stretch and the opposite chord what to compress under lateral loading?? Similar to beam bending. Seems like the displacement of the two truss chords would cause the longitudinal reactions you are questioning at the fixed end. You can check your member forces in the truss and bracing elements to verify.

 

[asixth]

33kips is a large longitudinal force for the bearings. I would model the stiffness of the bearings to eleviate some of this reaction and then design the bearings for this force.

 

[bar7h]

Thanks for the input. I'll give that a shot.

 

[wiktor]

The problem is with the modeling of the bridge. When you are "fixing" your bearings at one end, you should do it only in the vertical plane. In horizontal plane, one could be fixed, and the other should be pinned with translation allowed alog the axis of the bridge, or you will have large longitudinal reactions.

 

[asixth]

wiktor

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).

 

[bar7h]

How do you determine the stiffness of the bearing and what spring constant would you use?

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.

 

[wiktor]

asixth
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.