PC PRELIMS Flashcards
Reduction in force, addressed as part of the design of a prestressed member.
Partial prestress loss
Prestress losses are divided into two broad categories:
- Initial
- Long-term losses / Time-dependent effects
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, was successful because he recognized the value of high-strength prestressing materials and successfully incorporated high-strength materials into his design.
Eugene Freyssinet
All __________ are subject to losses resulting from elastic shortening, shrinkage, creep, and relaxation.
Prestressed members
Are subject to losses resulting from anchor set and friction.
Post-tensioned members
The __________ requires prestress losses to be considered in the calculation of effective tensile stress in the prestressed reinforcement, fse.
ACI Building Code (ACI 318-14)
6 types of prestressing losses
- Prestressed reinforcement seating at transfer (initial)
- Friction loss due to intended or unintended curvature in post-tensioning tendons (initial)
- Elastic shortening of concrete (initial)
- Creep of concrete (long-term)
- Shrinkage of concrete (long-term)
- Relaxation of prestressed reinforcement (long-term)
Are calculated at the tendon centroid and include consideration of the stress and strain in the tendon.
Losses
For _________, the losses are usually calculated at the critical sections, which are typically at midspan and the end of the member.
Precast pretensioned beams
For __________, the critical sections are at the member end, maximum positive moment locations, typically close to midspan, and maximum negative moment locations, usually over the supports.
Post-tensioned members
The ratio of modulus of elasticity of the prestressing reinforcement divided by the modulus of elasticity of concrete.
Modular ratio, n
If the calculated losses are __________, that is higher than actual losses, then more prestressing reinforcement is used and camber increases.
Too high
If the calculated losses are __________, the beam sags and cracks under service load.
Too low
__________ should neither be overly conservative nor ignored.
Loss calculations
3 methods in calculating losses of prestress
- 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
Initial stress for post-tension
0.80fpu
Maximum initial prestress force for post-tension
0.70fpu
Initial strand stress for pretensioning
0.75fpu
A small amount of movement when a prestressing tendon is released from the jack.
Set
Anchor set of __________ is common for single strand systems and larger values are for some center plug multistrand systems.
1/4 to 3/8 in.
The amount of movement ranges between ____ and ____ depending on the anchorage system.
1/8 in, 1 in