Foundation, Concrete and Earthquake Engineering

Classification of Prestressed-Concrete Structures

According to ACI Committee on Prestressed Concrete:

Concrete in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from given external loadings are counteracted to a desired degree. In reinforced-concrete members the prestress is commonly introduced by tensioning the steel reinfocement.The main difference between reinforced concrete and prestressed concrete is the fact that reinforced concrete combines concrete and steel bars by simply putting them togther and letting them act together as they may wish. Prestressed concrete, on the other hand, combines high-strength conrete with high-strength steel in “active” manner.

Prestressed-concrete structures can be classified in a number of ways, depending upon their features of design and construction. These are disscussed as follows:

1. Externally or internally prestressed

2. Pretensioning and post tensioning

3. End-Anchored or Non-End Anchored Tendons

4.Bonded or unbonded tendons

5. Precast, cast-in-place, composite construction

6. Partial or full prestressing

1. Externally or internally prestressed
Although the design of prestressed-concrete structures is mainly concerned with internally prestressed, presumably with high-tensile steel, it must be mentioned that it is sometimes possible to prestress a concrete structure by adjusting its external reactions. Theoretically, a simple concrete beam can be externally prestressed by jacking at the proper places to produce compression in the bottom of the fibre and tension in the top of fibres, fig -1, thus even dispensing with steel renforcement in the beam. Such an ideal arrangement, however, cannot be easily accomplished in practice, because, even if abutments favorable for such a layout are obtainable, shrinkage and creep in concrete may completely offset the artificial strains unless they can be readjusted. Besides, such a site would probabaly be better suited for an arch bridge.

For the statically indeteminate structure, like a continuous beam, it is possible to adjust the level of supports, by insurting jacks, for example, so as to produce the most desirable reactions, fig-2. This is sometimes practical, though it must be kept in mind that shrinkage and creep in conrete will modify the effects of such prestress so that they must be taken into account or else the prestress must be adjusted from time to time.

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