Document Information

Document Type: FAQ

Product(s): LEAP CONSPAN

Version(s): All

Original Author: Bentley Technical Support Group -VM

QUESTION: How the program is calculating Losses due to Relaxation of Prestressing Strands?

ANSWER:

The Prestress loss due to relaxation of prestressing strands between time of transfer and deck placement ∆fpR1 is calculated as per AASHTO LRFD (7th edition,2014) Article 5.9.5.4.2c-1.

This example shows calculation for Low Relaxation Strands.

KL = 30 for Low Relaxation Strands and 7 for other prestressing Steel.

fpu = Tensile Strength of Prestressing Strand = 270 ksi for Low relaxation strands.

fpy = Yield Strength of Prestressing Strand = 0.9 x fpu = 0.9 x 270 = 243 ksi.

fpt = Stress in Prestressing strands immediately after transfer.= 0.75 x 270 = 202.5 ksi.

∆fpR1  =  (fpt / KL ) x ( (fpt / fpy) - 0.55 )

∆fpR1  = (202.5 / 30 ) x ( ( 202.5 / 243)  - 0.55) = 1.91 ksi

Reference : AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS, SEVENTH EDITION ,2014.