PRESTRESSED CONCRETE PART 4

Definition of terms

Strand: is the high tension cable (typically more than 4 times the strength of rebar)
Tendon: agroup of strands (can be one for unbonded but it’s multiple strands inside a duct for bonded system)
Duct: a galvanized or HDPE that separates the concrete from the strand

Issues associated with PT

Restrain to shortening: this needs a special consideration for pouring sequence and connection detailing
Relaxation of strand: this can be avoided with a use of low relaxation strands

Design of Post tension members

The design of post tension elements needs a sound engineering knowledge and field skill, the amount of force applied to a single strand with a diameter of 12.7mm is about 15Ton (this is more than the weight of 7 Toyota Corollas). One can imagine how dangerous it can get if either the design or site execution are not properly done. That’s why all countries (at least the countries I am aware of) requires a specialized contractor who can do the design, execute the job and take the liability; no consulting office, no mater how big they are, are not allowed to do a PT design (this actually goes to structural steel, pile, shoring … as well)

The design of PT elements involve three steps: these are load balancing (equivalent load), service design and ultimate load design. Even though PT was developed more than a century ago and has been on practice for over 70years, only few schools teach the subject either in US or Europe. That can be attributed the amount of work require to reach to a safe, and economical design, what I mean by that is two engineers can have completely deferent designs results for a simple beam or slab.

For example, if a person calculates a required steel beam size of 305x127x42 but decide to use a size 356x171x45, that beam memeber is still safe, even though it might not be the most economical. But in case of PT, a beam with 12 strands can be safe but the same beam with 20 strand might not be safe. In short term more strand does not mean safe, that makes the design of any pt members trickier and as for me enjoyable

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