Semi-integral Abutment Design

[bridgebuster]

We have a project under construction. 175' long integral abutment bridge. Due to a number of issues related to pile driving at one abutment, the owner asked about changing one abutment to semi-integral and using drilled-in micropiles. The designer said it was impossible because under an extreme event, the integral abutment will deflect 24" towards the semi-integral abutment. The bridge, as designed, deflects less than 1" under extreme event. I believe the designer has an error in the semi-integral model. See below. I asked why he wasn't including soil springs for the semi-integral abutment back wall. He said it's never done, which doesn't make sense to me. Can some one give me a sanity check or am I wrong?

 

[BridgeSmith]

I think you're right. It's perfectly legitimate to use the passive resistance of the soil against the semi-integral end diaphragm. If the design uses steel-reinforced elastomeric bearings at the semi-integral abutment (and I know of no good reason not to), the bearings will provide a considerable amount of restraint to the superstructure movement, also.

 

[STrctPono]

How can that bridge displace 24"!? Tell the designer to pull his head out of his rear end. Our office always considers soil passive resistance on both integral and semi integral abutment designs. I thought everyone did. That's one of the main benefits from an analysis standpoint. From a practical standpoint, the superstructure on the semi-integral side cannot move that far into the backfill. We limit our soil spring displacements to 1.25".

Now, seeing things from the designer's perspective... I would hate to assume the liability of a pretty major design change during construction.... maybe that's why he is feeding you a load of BS. Not sure if his superstructure is designed using the benefits of the integral abutments but this change could affect other aspects of his design.

 

[bridgebuster]

Thanks for the replies; as I said above I needed a sanity check or perhaps a sounding board. My initial response to him was "B@!!$#|+" There's enough passive pressure to resist 80% of the seismic force and I wasn't even considering shear resistance of the elastomeric bearings. Then he proceeded to lecture me about my ignorance. Surprisingly, the client didn't question the results. There are a lot of constructability problems on this project. The designer doesn't want any changes to his work. Whatever, I've moved on from this.

 

[BridgeSmith]

I've moved on from this.

Sounds like a really good idea. I was about to suggest if at all possible run as far as you can get from this disaster in the making.

 

[AS_PEng]

I'm assuming the 24" is due to seismic loads. It's my understanding that AASHTO recognizes passive abutment resistance as a part of the Earthquake Resistance System but also recognizes it as permissible with the owner's approval. I believe some owners in US, such as WSDoT don't permit this approach. In BC, Canada, passive resistance is based on only 70% of ultimate capacity the the passive resistance.

 

[bridgebuster]

Correct, but it assumes the integral abutment is a free standing column, which seems overly conservative to me. Meanwhile, the longitudinal deflection for the bridge as currently designed is less than an inch.