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Debonding strands or draped strands are used to relieve the excessive
tensile stresses in the beam ends during the release of the prestressing
force.  The AASHTO limits the allowable tensile stress to 7.5xSQR(f’ci).
The number of debonding strands and their debonding distance from beam ends
are determined so that the calculated P/A+M/S stresses are less than
allowable.  Ohio DOT prefers strand pattern is straight with no debonding
at beam ends.  In order to control the excessive tensile stress at beam
ends, additional strands are placed above the neutral axis, deboned at
midspan if necessary for service loads.

I have developed some handy programs to do the work.  The program will
determine the required number of strands for service loads and then
calculate the required number of draped strands or debonded strands with
debonding lengths.  The program will also check and readjust the input
concrete strength at release if necessary.  I have another nice PIER
program.  It will analyze pier loads and design pier cap and run all AASHTO
loading combinations for DL, LL, Wind/Seismic loads, and Shrinkage and
Temperature changes.  It will print out the results of the controlling
loading group.

Let me know if you like to try them out and use the computer output to
check your hand calculations.  These programs are free.

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