Memorandum

I have a concern about positive superstructure moment at an interior pier of a continuous concrete bridge during an earthquake. For example, our current details require only nominal longitudinal reinforcement to be carried into the X-beam extending from the bottom of the PS Girders. This nominal reinforcement may be inadequate for seismic forces and moments.

John VanLund’s design unit (Pat Clarke) recognized this concern and utilized the attached details in the design of a portion of the Ebey Slough Bridge on SR 2. Their design philosophy was to design the positive reinforcing extension to resist 120% of the column plastic hinging moment distributed to each adjacent span in accordance with their relative stiffness.

I think this is subject for the Seismic Steering Committee to discuss. In the meantime, I would like to see the following design procedure used for all concrete superstructure bridges:

• Provide positive moment reinforcing extending from the bottom of the concrete superstructure in to the X-beam.
• This positive reinforcing would be designed for one of the following two conditions, whichever is less:

1. Forces and moments resulting from 120% of the column plastic hinging moment.
2. Forces and moment resulting from elastic seismic responses of the bridge.

• The forces and moments used would be based on a frame stiffness distribution with equal participation from each girder.

Please implement this design procedure as quickly as possible. However, we will not impose change orders on bridges that are already under construction.
This requirement applies only to Seismic Performance Category C and D Bridges.

CCR

Attachment 1

Attachment 2

cc:     M. M. Lwin

         F. Higgins (w/attach)