Introduction to Prestressing CE 572 Purdue University School of Civil Engineering Julio Ramirez.
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Transcript of Introduction to Prestressing CE 572 Purdue University School of Civil Engineering Julio Ramirez.
Introduction to PrestressingCE 572
Purdue University
School of Civil Engineering
Julio Ramirez
CE572/Spring 04/JAR 2
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.
CE572/Spring 04/JAR 3
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.
CE572/Spring 04/JAR 4
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.
CE572/Spring 04/JAR 5
Extended Range of Application
CE572/Spring 04/JAR 6
Examples of Structures
CE572/Spring 04/JAR 7
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 (zerotensile stresses at full service load).
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Applications of Prestressed Concrete in the US and Canada – Figure 1About 200,000 tons of prestressing steel used in
US/Canada each year (~ ¼ of the total world consumption)
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Prestressing Methods
Pretensioning: suited for mass production.
*Video (PCI)
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Prestressing Methods
Post-Tensioning: jacking against the member and the eccentricity is easily varied along the length of the member.
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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)-------
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Profiles of Post-Tensioned Tendons
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Geometry of Parabolic Profiles
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Location and Eccentricity of Tendon in Duct After Stressing
CE572/Spring 04/JAR 15
Materials: Types of Prestressing Steel
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Production of Seven Wire StrandLow relaxation versus stress-relieved
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Concrete
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Hognestad Parabola
fc ( ) fcmax 2o
o
2
Calculate εo (strain corresponding to peak stress, fcmax) using the secant Modulus of Elasticity at 0.5fcmax. Thus, fitThe Hognestad expression through a point corresponding to 0.5fcmax and 0.5fcmax/Ec.
Ec 570002 fcmax( )
1000
Prestress Force Levels
Po = Jacking Force
Anchor Slip Friction Elastic Shortening
Pi = Initial (Transfer) Force
Creep Shrinkage Relaxation
Pe = Effective Force