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TO: All Design Section Staff MS 47340
FROM: Chuck Ruth
DATE: December 22, 1999
SUBJECT: Stiffness of RC Column in seismic analysis

To: All Design Section Staff

It has been brought to my attention that both of cracked and non-cracked cross sections are being used to calculate the stiffness of reinforced concrete columns for seismic analysis of new bridges. With AASHTO Response Modification Factor (R-factor), both design approaches result in ductile performance.

The columns analyzed with non-cracked stiffness form the plastic hinges at higher stress levels, which minimize damage (cover concrete spalling, lateral residual deformation, etc.) in an intermediate earthquake. Currently, CALTRANS is using the non-cracked stiffness for all new bridge designs. This method results in larger footing/foundations than using the column cracked section. The seismic responses of a bridge analyzed with cracked section column stiffness have longer periods, which result in lower lateral forces and smaller size of footing/foundations. This method will result in a higher potential of damage (cover concrete spalling, lateral deformation, etc.) in an intermediate earthquake. This is a tolerable risk, in my opinion, since the overall seismic design goal is to achieve ductile performance and avoid bridge collapse.

For consistency in our bridge design, all new bridge designs should use the cracked column stiffness in the seismic analysis. This interim Bridge and Structures Office policy is effective immediately and will be incorporated into a future BDM update.



cc:         J. Weigel, MS 47340

            H. Coffman, MS 47341

            T. Allen, MS 47365

            F. Higgins, MS 47340

            E. Henley, MS 47340