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AASHTO-LRFD Concrete Stresses after Transfer of Prestressing Forces :
AASHTO-LRFD Concrete Stresses after Transfer of Prestressing Forces for superstructure tutorial’s content is;
- stress check at prestress transfer,
- end zone stresses,
- force distribution with debonding strands and,
- lastly concrete stresses at top and bottom fiber of girder at prestress transfer.
Stress Check at Prestress Transfer ;
Firstly, at the time of prestress transfer, only prestressing force and dead load are acting on the girder. Number of strands are selected in accordance with the maximum stresses developed at the mid-span. The stresses induced by dead load at the end zones is negligible. Furthermore, the only source of stress is due to prestressing that will not be balanced by dead load. Thus, top and bottom fiber stresses developed just after transfer of prestressing along the girder need to be checked against stress limits at prestress transfer. Moreover, the end zones of the girders can crack at top flanges of the girder due to excessive stresses supplied by prestressing that cannot be suppressed by low compression stresses supplied by the dead load effects.
Secondly, at end zones, some of the strands need to be deactivated by the action called “debonding”. Debonding can be achieved by use of pipes that will prevent bonding of the prestressing steel to the concrete. Additionally, wedges are used to grab the strands at its position after a certain elongation is applied.
Moreover, harping is an alternative to debonding but has been abandoned in many countries due to durability problems and excessive loss of prestressing at deviation points. Due to these disadvantages of harping, harping practice leaves its place to debonding.
End Zone Stresses ;
Another content for the prestrestressed strands, the prestress force will be fully effective in the girder after a distance called transfer zone. Indeed the effective prestress force at the end of girder is zero and along the transfer length, strand gains the prestress force linearly. Transfer length is usually accepted as 60 times strand diameters.
Force Distribution with Debonding Strands ;
Lastly, transfer zone also exist for debonded strands. Therefore, a debonded prestressing strand will fully transfer the prestressing force after the debonded length plus 60 diameter of strands.