Abutment Sesimic Design for Single Span Bridges

[lawqasim]

AASHTO LRFD and Standard Specifications call out to neglect sesimic design for single span bridges regardless of any sesimic zone. Does this depict that No Sesimic Analysis and Forces are to be considered for the desing of the abutments and wingwalls for a single span bridge. Please share your experience. This info is required for a single span bridge design one in NY and one in NJ. Any help is greatly appreciated.

 

[bridgebuster]

You still have to design the abutment for the lateral earth pressure force due to a seismic event and the connection of the bearings to the superstructure.

 

[lawqasim]

Bridgebuster thanks for your feedback. The AASHTO Standard Specs 17th Edition Section 3.1 states for single span bridges no sesismic analysis is required for the sub structure other than the connections to be designed between the super structure and the bearings and the minimum support length requirements. Even AASHTO Section 3.11 states that detailed seismic analysis pertaining to section 4,5,6 & 7 is not required other than the connection design. Could you please elaborate on the requirement for seismic analysis for the abutments for single span bridges. Thanks for your help.

 

[bridgebuster]

For NYSDOT bridges you provide the seismic reinforcement no matter what. When you design your abutment stem you'll have to do a load case where you apply the seismic force due to the backfill. You can use the Mononobe-Okabe method, which essentially is calculating Ka due to seimic and applying the resultant force at H/2.

 

[lawqasim]

bridgebuster thanks for the valueable info. Would you know what applies for the NJ Bridges (County Bridges) for applicable sesimic code for single span bridges. I did not find any specific reference material from NJDOT other than the applicable sections of AASHTO. And AASHTO calls out to eliminate seismic analysis for single span bridges. Please kindly update. Appreciate your help & advise.

 

[tumbleleaves]

I did not comment on this post because it did not specify the type of abutments (piles, spread footing, seat, diaphragm, etc), and was not posted in the bridge forum--too vague.

I believe that in California LRFD is superseded by the Caltrans Seismic Design Criteria, and that also abutments are still designed using WSD and conventional analysis techniques.

It's certainly a good idea to design correctly in spite of the LRFD code.

 

[lawqasim]

tumbleleaves the bridge is a single span bridge on abutments with spread footings (due to very close proximity to rock and the rock Allowable Bearing Pressure from the Geotech Report is 5 TSF). So the AButment Footings shall either bear directly on rock or on concrete fill upto to the bedrock stratum. NJDOT Bridge & Design Manual 5th Edition 2009 and earlier versions Section 38.3.2 calls out that single span bridge substructure (abutments) need not be designed for seismic other than the provision for minimum support lengths and connection forces. Please let me know based on your experience in the NJ area if the seismic analysis is required for abutments and even u configured wing walls bearing on rock/concrete fill. Thanks fo ryour help.

 

[bridgebuster]

lawqasim - It sounds like from your description NJDOT doesn't require anything other than convential abutment design. Check the standard sheets to see if they call for seismic reinforcement details.

If you apply a seismic force to the backfill it's still a convential abutment desgn.

 

[tumbleleaves]

In my very limited and distant past experience with seismic design I do not remember designing for seismic earth pressures.

The only earth pressures I remember that were specifically seismic related were the mobilized passive earth pressures. Generally the abutment was engineered to allow the superstructure to yield and/or not constrain superstructure.

I agree with bridgebuster from what you said there is probably not any analysis that goes into an abutment in NY or NJ. Whatever is necessary is likely already addressed in the standard bridge details.

If anyone has a reference, link, or knowledge of an abutment failure due to active seismic pressures it would be news to me and I would be much appreciative for the new information.

 

[bridgebuster]

Hopefully I was clear in my previous posts regarding the seismic force.

We perform an analysis for a sesimic load case using the Mononobe-Okabe method to determine the active pressure. No modelling; no sprngs, nothing out of the ordinary; just plain old-fashioned stability analysis and concrete design.