Losses in prestressed concrete structures
Prestressing Losses
Basis and Notation
Recall that transfer is the moment at which the concrete first feels the prestress. Losses can be before or after transfer:
• Before-transfer losses are the difference between what is applied by the hydraulic jack, jack P and what the concrete feels at transfer, Pt ;
• After-transfer losses are the difference between the transfer force, P and the force at SLS, Ps =α Pt .
The calculation of losses is different for pre- and post-tensioned PSC
Notation
Losses in Pre-Tensioned PSC
1. Elastic Shortening Loss
In pre-tensioning, the strands are stressed before the concrete is cast. Therefore, after jacking ( Pjack ) has been applied, at a level y, the concrete is subjected to the stress:
At the level of the strands, y = e. Hence, the stress in the concrete at that level is:
And the concrete at the level of the strands has a strain of:
Since the strands are bonded to the concrete, they undergo a loss of strain of the same amount, εc,e , with associated stress and force losses of Ep εc,e and Ap Ep εc,e . Hence
The concrete never feels the full jacking force since the strands shorten as the concrete strains. Hence, elastic shortening loss in pre-tensioning is ‘before-transfer’.