LOSSES Flashcards
occur during stressing operation and include anchor seating, elastic shortening, and friction between prestressing steel and post-tensioning ducts or tendon deviators and harped pretensioned strands.
Initial losses
occur because of viscoelastic material effects and include concrete shrinkage, creep, and
tendon relaxation.
Long-term losses
recognized as the founder of modern prestressed concrete
Eugene Freyssinet
All prestressed members are subject to losses resulting from
elastic shortening,
shrinkage, creep, and relaxation.
The following loss mechanisms have historically been listed by the code for consideration:
(a) Prestressed reinforcement seating at transfer (initial)
(b) Elastic shortening of concrete (initial)
(c) Creep of concrete (long-term)
(d) Shrinkage of concrete (long-term)
(e) Relaxation of prestressed reinforcement (long-term)
(f) Friction loss due to intended or unintended curvature in post-tensioning tendons
(initial)
Methods available for this task vary in complexity and accuracy. In general, these methods are?
- Lump sum
- Detailed
- Time-dependent
the total combined losses in the prestress based on experience or historical data, to select the initial prestress.
Lump Sum Losses
is a function of the duct material.
WOBBLE FRICTION
is a function of the angle of curvature α intentionally designed into the duct placement and the coefficient of friction μ between the tendon and the duct with a total effect equal to μ
CURVATURE FRICTION
Transfer of the prestress force from the tendon to the concrete results in
ELASTIC SHORTENING
is the continued deformation of the concrete under sustained loads.
CREEP
e is the volume reduction of the concrete due to hydration of the cement and loss of water from the concrete as it cures.
SHRINKAGE LOSS
