49
SECTION 1 INTRODUCTION TO POST-TENSIONED CONCRETE DEVELOPED BY THE PTI EDC-130 EDUCATION COMMITTEE

Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

  • Upload
    haphuc

  • View
    244

  • Download
    5

Embed Size (px)

Citation preview

Page 1: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

SECTION 1

INTRODUCTION TO POST-TENSIONED

CONCRETE

DEVELOPED BY THE PTI EDC-130 EDUCATION COMMITTEE

Page 2: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

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 reinforcing for strength.

• When moment is drawn on the tension side, the diagram matches the general drape of the tendons. The tendons change their vertical location in the beam to follow the tensile 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 moment diagrams 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.

Page 3: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

REVIEW: FUNDAMENTALS OF PRESTRESSED CONCRETE

NEW: DIFFERENCES BETWEEN PRE-TENSIONING AND POST-TENSIONING

Page 4: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

REVIEW OF REINFORCED CONCRETE

Critical Point for Cracking

Stages of BehaviorUncracked Cracked (~Elastic) Ultimate

Page 5: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

REVIEW OF REINFORCED CONCRETE

Mcr

My

Mn

Mom

ent

Curvature

Large deflections due to cracking

Steel is not engaged until after cracking

Page 6: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

REVIEW OF REINFORCED CONCRETE

Reinforcement is PASSIVE

Steel crosses cracks, but does not prevent them

Page 7: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning
Page 8: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

QUESTION TO PONDERSuppose a R/C beam has too much cracking and too much 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

Page 9: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

HOW TO BUILD IT?

Prestressing: Concrete pre-compressed before loading in bending (flexural tension)

1. Pre-Tensioning: Steel tensioned beforeconcrete is placed

2. Post-Tensioning: Steel tensioned afterconcrete is hardened

Prestressing is ACTIVE – can prevent cracks from forming

Page 10: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONING

1. Tension Strands

2. Cast Concrete – Bond strands to concrete

3. Cut Strands – Transfer force to concrete

Page 11: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

1. Cast Concrete with Duct

2. Feed Strands through Duct

3. Tension Strands

4. Grout Duct (or other corrosion protection)

Section

Page 12: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING• Post-tensioning can take on any profile

• Draped configurations are much more common than straight tendons• Why?

Page 13: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONING

Post-Tensioning

Force Transfer by Steel-Concrete bond

Force Transfer at end anchor

Strain Compatibility and Force Equilibrium:

Steel held at length longer than it “wants” to be: Tension

Concrete compressed shorter than it “wants” to be: Compression

Page 14: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

Pre-Tensioned elements are often precast in a factory and shipped to the site

Post-Tensioned elements can be cast and tensioned in the final location (cast-in-place). They can also be precast.

Page 15: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGINSTALL PRESTRESSING STRANDS

Page 16: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGTENSION STRANDS

Page 17: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGSTRANDS AFTER TENSIONING

Page 18: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGINSTALL MILD REINFORCEMENT

Page 19: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGINSTALL INSERTS AND ASSEMBLIES

Page 20: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGSET FORM SIDES

Page 21: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGPLACE CONCRETE

Page 22: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGCURE CONCRETE WITH ACCELERATED METHODS

Page 23: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGREMOVE GIRDER FROM CASTING BED

Page 24: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGMOVE GIRDER TO STORAGE

Page 25: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGTRANSPORT TO JOBSITE

Page 26: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PRE-TENSIONINGGIRDERS IN FINISHED STRUCTURE

Page 27: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONINGDucts for Post-Tensioning

Page 28: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

Page 29: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

Page 30: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

Page 31: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

Page 32: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

Page 33: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

Page 34: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONINGStressing Strands:

Multiple Strands: Multistrand

Single Strand: Monostrand

Page 35: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

HOW ARE STRANDS ANCHORED?

Cast against concrete at end of beam

Page 36: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

HOW ARE STRANDS ANCHORED?

Concrete

Duct

Strand

Anchor cast in concrete

Page 37: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING:Bonded System(at high point)

Unbonded System

Grout“PT Coating” (grease)

Page 38: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

GROUTING POST-TENSIONED SYSTEMS

Grout In

Vent

Vent

Page 39: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

POST-TENSIONING

Page 40: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

STRUCTURAL EFFECT OF PRESTRESSING

True for Pre- and Post-Tensioning

Pre-Stressing

+

Applied Load

=

T C T C

Total Stress

T C

Stress Limits

Page 41: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

STRUCTURAL EFFECT OF PRESTRESSING

True for Pre- and Post-Tensioning

Pre-Stressing

+

Applied Load

=

T C T C

Total Stress

T C

+

T C

Service

Transfer

Page 42: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

ECCENTRIC PRESTRESSINGEccentricity in prestressing:

- Desirable at midspan- Not productive, even detrimental, at end of span

Strategies for pre-tensioned systems:- Draped / harped profiles

Temporarily held in place before concrete is hardened

- DebondingNot all strands are active at end of span

Strategies for post-tensioned systems:- Install ducts in desired profile

Page 43: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

COMMON CONFIGURATIONSPre-tensioning:

Draped

Debonded

Post-tensioning:

Page 44: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PROBLEM FOR THOUGHT…Where should the prestressing be placed?

Tension

Tension

Mom

ent D

iagr

am

(T)

Page 45: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PROBLEM FOR THOUGHT…Where should the prestressing be placed?

Tension

Tension

Mom

ent D

iagr

am

Option 1

Good:•Efficient at midspan•Easy to construct

Bad:•Counter-productive over support(T)

Page 46: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PROBLEM FOR THOUGHT…Where should the prestressing be placed?

Tension

Tension

Mom

ent D

iagr

am

Option 2

Good:•Efficient over support•Easy to construct

Bad:•Counter-productive at midspan(T)

Page 47: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PROBLEM FOR THOUGHT…Where should the prestressing be placed?

Tension

Tension

Mom

ent D

iagr

am

Option 3

Good:•Efficient over support•Efficient at midspan

Bad:•Difficult to construct

(T)

Page 48: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

PROBLEM FOR THOUGHT…Where should the prestressing be placed?

Tension

Tension

Mom

ent D

iagr

am

Option 4

Requires post-tensioning; very difficult to achieve by pre-tensioning

No net eccentricity

No net eccentricity

(T)

Page 49: Introduction to Prestressed Concrete Resources/130821-1-PTI... · review: fundamentals of prestressed concrete new: differences between pre-tensioning and post-tensioning

SUMMARY: PRESTRESSED CONCRETE

Efficient use of materials – concrete maintained in compression, crack control

Smaller deflections/thinner members Longer spans Corrosion resistance Less material; reduced environmental impact