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Introduction to PrestressingCE 572
Purdue University
School of Civil Engineering
Julio Ramirez
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Definition of Prestressing
It consists of preloading the structurebefore application of design loads in sucha way so as to improve its general
performance.
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Objectives of Prestressing
Control or eliminate tensile stresses in theconcrete (cracking) at least up to serviceload levels.
Control or eliminate deflection at somespecific load level.
Allow the use of high strength steel andconcrete.
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Net Result of Prestressing
Improved performance of concrete inordinary design situations (compared to
R/C).
Extended range of application of structuralconcrete (longer spans).
Innovative forms of structures.
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Extended Range of Application
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Examples of Structures
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Partial Prestressing
Intermediate state, in which tension andusually flexure cracking is allowed at fullservice load level. Some advantages are:
- Prevents excessive camber at moretypical loads less than full serviceloads.
- Reduced prestressing force (less $$$)compared to full prestressing (zero
tensile 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 (~ 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 memberand the eccentricity is easily varied along thelength of the member.
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Post-Tensioning Sequence
Place reinforcing cage and post-tensioning ductsinside the formwork.
After casting and curing of the concrete, the
tendons are tensioned and anchored withspecial jacks that react against the member.
Unless unbonded tendons are being used, theduct 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
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Materials: Types of Prestressing
Steel
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Production of Seven Wire Strand
Low relaxation versus stress-relieved
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Concrete
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Hognestad Parabola
fc( ) fcmax 2
o
o
2
Calculate 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 to0.5fcmax and 0.5fcmax/Ec.
Ec 57000
2fcmax( )
1000
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