Passive Earth Pressure Resistance on Bridge Abutment

[lprogers]

Hello,

I am modeling a single span prestressed I-beam bridge in CSI Bridge(Basically SAP2000). I am going to subject the model to ground motions using nonlinear time history analysis. I need a way to model the effect of the passive earth pressure resistance acting on the abutment backwall when the abutment is 'pushed' into the backfill. I currently have gap springs setup with the correct stiffness but is seems they also provide resistance in tension when the abutment moves away from the backfill, which is not what I want. I have foundation springs setup to replicate pile foundations at 40kip/in/pile in both transverse and longitudinal directions.

any ideas?

Thanks

 

[stressed]

Sounds like you forgot to define load cases as nonlinear if the gap element is acting in both tension and compression.

 

[lprogers]

Stressed,

thanks, i just checked that and both the load case and the gap link are checked as nonlinear already.

 

[stressed]

If the load case and gap element are nonlinear, then the gap should only act in compression. Did you enter a stiffness for the gap? Use a value approximating the stiffness of the backfill. Also, verify that the gap local directions are as you expect by using Set Display options to display local axes of links. Red arrow = local 1, Green arrow = local 2, Blue arrow = local 3.

By chance, did you also assign a spring or another link at the same location which could be acting in tension and compression?

 

[lprogers]

looks like i got it to work, thanks for the help!

 

[stressed]

What was the problem, and how did you resolve it?

 

[lprogers]

there were a couple problems, first, I had the gap spring oriented in the wrong direction so your advice to display local axis helped with that. Next. the stiffness i had for the gap spring was small in comparison to the stiffness of the pile foundation springs, therefore the effect of the gap spring was difficult to detect. I increased the gap spring stiffness to verify the positive orientation of the gap link.

I may have to change from using a gap link to a multilinear elastic link to provide more representative results for passive earth pressureresistance. as deformation increases the stiffness of the soil decreases. To do this I plan to define the different stiffnesses (deformation vs force) for the compression direction and 0 stiffness for the tension direction. i need to then apply this to both abutments. what are your thoughts on this procedure?

thanks

 

[stressed]

Use of multilinear link with force-deformation values is a good way to model soil behavior. Using 0 stiffness for tension direction will give you the desired compression-only behavior.