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AASHTO LRFD Prestress Losses
The prestress losses usually develop due to combined effect of shortening of the concrete member and relaxation of prestressing steel. It is important to select the type of prestressing steel as low-relaxation one.
The initial jacking force will be reduced by prestress losses that will develop over time. The time-dependent changes in prestress forces will result in a change in concrete stresses.
Some part of losses can develop immediately after prestressing due to elastic shortening of member. Other losses will develop over time.
Applying prestressing force to strands results in elongation of tendons. When the prestressing force is applied member will shorten and pre-elongation of steel will be reduced resulting in immediate prestress loss.
The concrete stresses at the level of prestressing need to be determined to compute elastic shortening. Computation of stresses needs some iterations.
The concrete stress surrounding the prestress is the function of elastic shortening which needs to be determined. Therefore, an initial guess for initial prestress will be made and convergence on elastic shortening and initial prestress force after the loss will be checked.
Computation of concrete stress surrounding the prestressing steel will be performed on girder section with the inclusion of moment due to girder dead load and prestressing force as well as prestressing force itself.
Prestress losses also develop due to time-dependent effects such as shrinkage, creep and relaxation. Prediction of these effects are very complex in nature.
AASHTO-LRFD specifications have recommended a simple method that combines creep, shrinkage and relaxation in one single equation.