Design Memorandum



TO:                  All Design Section Staff

FROM:            Bijan Khaleghi
DATE:             August 28, 2007
SUBJECT:       Web bursting Reinforcement for Curved Post-Tensioned Bridges


The effects of in-plane and out-of-plane forces of curved tendons shall be investigated during the design of post-tensioned bridges as prescribed in LRFD article 5.10.4.3. When tendons curve in two planes, the effects of in-plane and out-of-plane forces shall be added.

In addition to the above requirement, supplemental ties shall be provided to confine the PT tendons when horizontal curvature radius is less than 800 ft or the effect of in-plane and out-of-plane forces exceeds the limit shown below:

                  

          Where:

          

          R         =          radius of horizontal curvature at the considered location (ft)

The curved tendon confinement reinforcement includes as shown in the attached figure:

          #4 bars placed at1-6 maximum spacing wrapping the PT ducts and hook around shear stirrups on outside of curve.

          #4 web ties at 1-6 maximum spacing hooked around shear stirrups placed one

          


Background

The effects of in-plane and out-of-plane forces of curved tendons of curved post-tensioned bridges were not covered in the previous AASHTO STD Specifications. The current LRFD Specifications Article 5.10.4.3 addresses this issue extensively and provides guidance of how to design web reinforcement to account for effects of in-plane and out-of-plane forces of curved tendons. However, the PT contractors (based on CalTrans practice) recommend use of additional ties for tight curved bridges and include additional bars (additional cost) in their shop drawings. This design memorandum is supposed conservatively to fill the gap between the current industry practice and the design specifications.

If you have any questions regarding this issue, please contact Bijan Khaleghi at 705-7181.


cc:   Mohammad Sheikhizadeh, Bridge Construction - 47354

       F. Posner, Bridge and Structures 47340





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