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Introduction to Prestressing CE 572 Purdue University School of Civil Engineering Julio Ramirez.

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Presentation on theme: "Introduction to Prestressing CE 572 Purdue University School of Civil Engineering Julio Ramirez."— Presentation transcript:

1 Introduction to Prestressing CE 572 Purdue University School of Civil Engineering Julio Ramirez

2 CE572/Spring 04/JAR2 Definition of Prestressing  It consists of preloading the structure before application of design loads in such a way so as to improve its general performance.

3 CE572/Spring 04/JAR3 Objectives of Prestressing  Control or eliminate tensile stresses in the concrete (cracking) at least up to service load levels.  Control or eliminate deflection at some specific load level.  Allow the use of high strength steel and concrete.

4 CE572/Spring 04/JAR4 Net Result of Prestressing  Improved performance of concrete in “ordinary” design situations (compared to R/C).  Extended range of application of structural concrete (longer spans).  Innovative forms of structures.

5 CE572/Spring 04/JAR5 Extended Range of Application

6 CE572/Spring 04/JAR6 Examples of Structures

7 CE572/Spring 04/JAR7 Partial Prestressing  Intermediate state, in which tension and usually flexure cracking is allowed at full service load level. Some advantages are: -Prevents excessive camber at more typical loads less than full service loads. -Reduced prestressing force (less $$$) compared to full prestressing (zero tensile stresses at full service load).

8 CE572/Spring 04/JAR8 Applications of Prestressed Concrete in the US and Canada – Figure 1  About 200,000 tons of prestressing steel used in US/Canada each year (~ ¼ of the total world consumption)

9 CE572/Spring 04/JAR9 Prestressing Methods  Pretensioning: suited for mass production. *Video (PCI)

10 CE572/Spring 04/JAR10 Prestressing Methods  Post-Tensioning: jacking against the member and the eccentricity is easily varied along the length of the member.

11 CE572/Spring 04/JAR11 Post-Tensioning Sequence  Place reinforcing cage and post-tensioning ducts inside the formwork.  After casting and curing of the concrete, the tendons are tensioned and anchored with special jacks that react against the member.  Unless unbonded tendons are being used, the duct is then grouted to complete the post- tensioning operation.  Slides (PTI)-------

12 CE572/Spring 04/JAR12 Profiles of Post-Tensioned Tendons

13 CE572/Spring 04/JAR13 Geometry of Parabolic Profiles

14 CE572/Spring 04/JAR14 Location and Eccentricity of Tendon in Duct After Stressing

15 CE572/Spring 04/JAR15 Materials: Types of Prestressing Steel

16 CE572/Spring 04/JAR16 Production of Seven Wire Strand  Low relaxation versus stress-relieved

17 CE572/Spring 04/JAR17

18 CE572/Spring 04/JAR18

19 CE572/Spring 04/JAR19 Concrete

20 CE572/Spring 04/JAR20 Hognestad Parabola Calculate ε o (strain corresponding to peak stress, fcmax) using the secant Modulus of Elasticity at 0.5fcmax. Thus, fit The Hognestad expression through a point corresponding to 0.5fcmax and 0.5fcmax/Ec.

21 Prestress Force Levels Po = Jacking Force Anchor SlipFriction Pi = Initial (Transfer) Force CreepShrinkage Pe = Effective Force Relaxation Elastic Shortening

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