Post Tension Field Elongations

[strguy11]

For projects with unbonded, monostrand tendons in buildings, the PTI manual and ACI indicates that the measured field elongations shall be within 7% of the calculated elongations.

My question is that are these compared to the elongations using the theoretical E and Area of the tendons, or should those be revised based on actual E and A based on the mill certifications?

The shop drawings have the elongations based on the theoretical E and A. I guess the question is does the 7% tolerance account for the variations in the E and A, or should these be accounted for, then checked for the 7%?

 

[bhiggins]

Has the stressing engineer submitted elongation and stress loss calculations? How much does E and A differ from theoretical values? It's not normal procedure to re-calculate elongations based on actual E and A, but if you believe they will have a significant effect you should contact the stressing engineer to maybe do some spot checks to see if there is any significant changes in elongations.

 

[strguy11]

Yes, we have received the calcs and they are based on the theoretical numbers. I want the mill certifications to review prior to stressing, but just wasnt sure if the 7% accounted for the differences, which it sounds like it does.

For some reason i remember making the adjustments on a bridge job awhile back, but it had multistrand tendons, so i think that is why it was done.

 

[Ingenuity]

Most PT shop drawings and calcs are based upon close-to-reality strand properties, and code-based friction and wobble values/ranges.

Typically, after stressing and when the EoR receives the field elongation report, IF the results are > +/-7% then you re-calc the elongations based on actual A and E from the strand mill cert.etc.

Personally, I think the PT elongations should NOT be on the shop drawings for field personnel to know before/during stressing. Stress the tendons, issue the report and have the EoR review using the strand mill cert strand A and E. If they are outside that range then investigate why - drape, friction, wedge slip, cement paste wedge cavity etc.