130821 1 pti edc 130 pt introduction 54.pdf
TRANSCRIPT
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NOTE: MOMENT DIAGRAM
CONVENTION In PT design, it is preferable to draw moment diagrams to
the tensile face of the concrete section. The tensile face
indicates what portion of the beam requires reinforcingfor strength.
When moment is drawn on the tension side, the diagram
matches the general drape of the tendons. The tendonschange their vertical location in the beam to follow thetensile moment diagram. Strands are at the top of the
beam over the support and near the bottom at mid span.
For convenience, the following slides contain momentdiagrams drawn on both the tensile and compressive face,denoted by (T) and (C), in the lower left hand corner.
Please delete the slides to suit the presenter's convention.
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REVIEW:FUNDAMENTALS OF
PRESTRESSED CONCRETE
NEW:DIFFERENCES BETWEEN
PRE-TENSIONING ANDPOST-TENSIONING
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REVIEW OF REINFORCED CONCRETE
Critical Point for Cracking
Stages of Behavior Uncracked Cracked (~Elastic) Ultimate
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REVIEW OF REINFORCEDCONCRETE
Mcr
My
Mn M o
m e n
t
Curvature
Large deflections due to cracking
Steel is not engaged until after cracking
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REVIEW OF REINFORCEDCONCRETE
Reinforcement is PASSIVE
Steel crosses cracks, but does not prevent them
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QUESTION TO PONDERSuppose a R/C beam has too much cracking and toomuch deflection. How might you propose to fix it? (i.e.not replace it)
Tension (bending) + Compression (squeezing) =
Net Zero Stress
Sqeezed Before Loading (Pre-compressed):
Pre-Compression ( prestressing ) + Tension (bending) =
Net Zero Stress
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HOW TO BUILD IT?
Prestressing : Concrete pre-compressed beforeloading in bending (flexural tension)
1. Pre -Tensioning: Steel tensioned beforeconcrete is placed
2. Post -Tensioning: Steel tensioned after concrete is hardened
Prestressing is ACTIVE can prevent
cracks from forming
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PRE-TENSIONING
1. Tension Strands
2. Cast Concrete Bond strands to concrete
3. Cut Strands Transfer force to concrete
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POST -TENSIONING
1. Cast Concrete with Duct
2. Feed Strands through Duct
3. Tension Strands
4. Grout Duct (or other corrosion protection)
Section
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POST -TENSIONING Post-tensioning can take on any profile
Draped configurations are much morecommon than straight tendons
Why?
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PRE-TENSIONINGINSTALL PRESTRESSING STRANDS
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PRE-TENSIONINGTENSION STRANDS
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PRE-TENSIONINGSTRANDS AFTER TENSIONING
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PRE-TENSIONINGINSTALL MILD REINFORCEMENT
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PRE-TENSIONINGINSTALL INSERTS AND ASSEMBLIES
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PRE-TENSIONINGSET FORM SIDES
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PRE-TENSIONINGPLACE CONCRETE
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PRE-TENSIONINGCURE CONCRETE WITH ACCELERATED METHODS
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PRE-TENSIONINGREMOVE GIRDER FROM CASTING BED
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PRE-TENSIONINGMOVE GIRDER TO STORAGE
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PRE-TENSIONINGTRANSPORT TO JOBSITE
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PRE-TENSIONINGGIRDERS IN FINISHED STRUCTURE
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POST- TENSIONINGDucts for Post-Tensioning
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POST- TENSIONING
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POST- TENSIONING
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POST- TENSIONING
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POST- TENSIONINGStressing Strands:
Multiple Strands: Multistrand
Single Strand: Monostrand
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HOW ARE STRANDSANCHORED?
Concrete
Duct
Strand
Anchor cast in concrete
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POST- TENSIONING:Bonded System
(at high point)Unbonded System
Grout
PT Coating (grease)
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GROUTING POST- TENSIONEDSYSTEMS
Grout In
Vent
Vent
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POST- TENSIONING
STRUCTURAL EFFECT OF
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STRUCTURAL EFFECT OF
PRESTRESSINGTrue for Pre- and Post- Tensioning
Pre-Stressing
+
Applied Load
=
T C T C
Total StressT C
Stress Limits
STRUCTURAL EFFECT OF
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STRUCTURAL EFFECT OFPRESTRESSING
True for Pre- and Post- Tensioning
Pre-Stressing
+
Applied Load
=
T C T C
Total Stress
T C
+
T C
Service
Transfer
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COMMON CONFIGURATIONSPre-tensioning :Draped
Debonded
Post-tensioning :
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PROBLEM FOR THOUGHTWhere should the prestressing be placed?
Tension
Tension
M o m e n
t D i a g r a m
(T)
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PROBLEM FOR THOUGHTWhere should the prestressing be placed?
Tension
Tension
M o m e n
t D i a g r a m
Option 1
Good:Efficient at midspan
Easy to construct
Bad:Counter-productive over
support(T)
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PROBLEM FOR THOUGHTWhere should the prestressing be placed?
Tension
Tension
M o m e n
t D i a g r a m
Option 2
Good:Efficient over support
Easy to construct
Bad:Counter-productive at
midspan(T)
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PROBLEM FOR THOUGHTWhere should the prestressing be placed?
Tension
Tension
M o m
e n
t D i a g r a m
Option 3
Good:Efficient over support
Efficient at midspan
Bad:Difficult to construct
(T)
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PROBLEM FOR THOUGHTWhere should the prestressing be placed?
Tension
Tension
M o m
e n
t D i a g r a m
Option 4
Requires post-tensioning; very difficult to achieve by pre-tensioning
No neteccentricity
No neteccentricity
(T)
SUMMARY: PRESTRESSED
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SUMMARY: PRESTRESSEDCONCRETE
Efficient use of materials concrete maintained
in compression, crack controlSmaller deflections/thinner membersLonger spansCorrosion resistanceLess material; reduced environmental impact