nonlinear soil spring at abutment

[BlastResistant]

Hello all,

I am trying to determine the lateral deflection of a bridge abutment into the backfill material, where the backwall shears off. I am doing a seismic analysis on an existing 3-span concrete bridge with longitudinal girders and bearing seat type abutment. The geotech has given me soil spring values for the passive resistance of the abutment fill. After the seismic force has been determined, do I subtract the force required to shear off the backwall to determine the lateral deflection (and backfill soil pressure), or do I ignore the force required to shear off the backwall and use the full seismic force to calculate lateral deflection and backfill pressure? In this particular case, it makes the difference between meeting the allowable backfill pressure (5 ksf) and not. I'm not sure what the typical standard of practice is regarding this.

Thanks in advance.

 

[crossframe]

Do you have / can you get a copy of the Caltrans Bridge Design Aids Manual? I think Section 14 will have what you're looking for.

Also check out Caltrans' Memos To Designers Manual. Section 20 in Vol. 2 deals with seismic. These are more informal memos and are issued more frequently.

I think both are available for download on the Caltrans website.

 

[DanTex]

After the backwall shears off... the EQ continues. Thus.. the backwall essentially becomes irrelevant. Caltrans does not have a specific policy on how exactly to analyze this situation as it is very complex and even the best analytical models you might come up with are still just going to give approximate results which may or may not be dependable.

One solution is to neglect the abutments resistant to movement entirely and make sure that your columns can handle the expected deflections. This is conservative, but safe. If you 'must' rely on resistance at the abutment, etc. at least make sure that in the worst case scenario, you don't have the possibility of a collapse situation due to girder unseating at the abutments. And never lose sight of the fact that the best analysis you do is still only an educated guess. For instance, with respect to the soil springs you were given... don't make the mistake of assuming that they are even close to the actual resistance that you may expect to get from the soil during an actual EQ. On the other hand... you know you're getting some resistance just due to the girder sliding over the abutment seat... but how much?

Use good judgment and you'll be fine.

Dan