Post-Tensioned Bridge Decks Convention/Swartz...precast deck panels. ... 2007 pti technical conference session 7- post-tensioned bridge decks as a cost-effective durable alternative

Post-Tensioned Bridge Decks

Brian Swartz, M.S. Candidate

Andrea Schokker, Principal Investigator

The Pennsylvania State University

2007 PTI TECHNICAL CONFERENCE SESSION 7- POST-TENSIONED BRIDGE DECKS AS A COST-EFFECTIVE DURABLE ALTERNATIVE BRIAN SWARTZ & ANDREA SCHOKKER

(C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 1 of 30

• The Post-Tensioning Institute• VSL

• Photos: Greg Ehmke, Univ. of Wisconsin

Acknowledgements



Benefits of Post-TensioningLong-Term Durability

Less Cracking/Closes Cracks

Less infiltration by water, salt, oxygen

Structural ResponseUncracked section behavior:

Thinner membersIncreased Girder Spacing

ConstructabilityPrecast Deck Panels



When to Use Post-TensioningDurability

Thinner Members

Rapid Construction – Precast Panels

Pro Con

Maintain Traffic Less Familiarity

Minimize Site Time Lacking Guidelines

Better Quality Control No Industry Standards

Economy through Scale

Equipment / Personnel



General Terminology

Longitudinal PT

Longitudinal JointTransverse PT

Transverse JointShear Connector Pocket

Shear Pocket Detail:

Haunch



Construction Procedure

Reroute traffic

Demolition

Prepare for Panel Placement

Panel Erection

Install Tendons

Grout Joints

Install Shear Studs

Post-Tension & Grout Tendons

Grout Pockets & Haunches

Cast Closures

Install Parapet

Surface Finish



Construction Procedure

Reroute traffic

Demolition


Panel Erection

Install Tendons

Grout Joints

Install Shear Studs



Cast Closures

Install Parapet

Surface Finish

Demolition


Panel Erection

Install Tendons

Grout Joints

Install Shear Studs



Cast Closures

Install Parapet

Surface Finish



Review of Previous ProjectsDoor Creek Bridge, WI

Woodrow Wilson Bridge, MD/VA

Skyline Drive, NE

Successful Projects

Vast Range of Details

Lack of Standardization



Review of Previous Projects-Door Creek Bridge, WITwo 83’ simply supported bridges

Transverse PT – Longitudinally staged

Joint Type



Review of Previous Projects-Door Creek Bridge, WILongitudinal PT – monostrand tendons

Shear stud pockets spaced at 4’

(AASHTO maximum 2’)

Epoxy Overlay

Cost comparison – precast option much higher

No economy of scale

Numerous subcontractors

Combined bid



Review of Previous Projects-Woodrow Wilson Bridge, MD/VACapital Beltway – cannot be closed for redecking

Continuous spans – non-composite

Lightweight concrete = redecking + widening

Transverse Joint Detail Hold Down Detail

-Lutz & Scalia-Lutz & Scalia



Review of Previous Projects- Woodrow Wilson Bridge, MD/VA



Review of Previous Projects-Skyline Drive, NE (NUDECK)Two-Span Continuous Bridge – Composite Girders

Transverse Pretensioned/Longitudinally Post-Tensioned

6” panel thickness with 2” CIP concrete topping

Complex Longitudinal PT anchorage

Open gap and concrete topping not prestressed

-PCI -PCI



Precast Panel Design Details-Panel DimensionsDimensions typically controlled by panel weight

8”-10” thickness most common

Typical Length: 8’ Typical Width: 40’-50’

Example: 8’x50’x8” thick – 20 tons

Panel homogenization is key



Precast Panel Design Details-JointsMain Objectives:

Transfer shear

Seal against leaks

Easily constructed

Compression is KEY

Female-to-Female preferred

Duct splicing required



Precast Panel Design Details-Post-tensioningTransverse PT:

Accommodates crowned profile

No development length requirements

Before or after form removal

Net effect = concentric force

Longitudinal PT:

Concentric

Tendons or bars

Must be stressed before

made composite



Precast Panel Design Details-Composite ConnectionsShear studs easily welded to top flange through blockout

Concrete girders:

- very suitable for new construction

- mechanical anchors needed for redecking

Pockets grouted with non-shrink grout

AASHTO maximum spacing 2’

- 4’ spacing under study

- concerns:

composite action

stress concentrations



Cast-in-Place PT DecksStructural response and durability benefits

No major construction advantages over traditional methods

Most suitable for applications where geometry is not repetitive for precast construction

Example: Wacker Drive, Chicago

- needed greater clearance (thinner slab)

- durability was a priority

- not suitable geometry for precasting some areas

- city allowed complete closure of some sections



Structural DesignNot unlike other bridge deck schemes

Design for service stresses and ultimate strength

Construction staging is very important

- construction loads may control

Stress limit across joint: No Tension

Stress limit away from joints: 3√f’c



Design Example

-FHWA

-FHWA



Design ExampleTransverse Post-Tensioning

Design Chart: 1.35 Strands/ftFinal Design: 2.00 Strands/ft

Note: Final Design was driven by the need for symmetry within each panel

9.75 ft

1.35 Strands/Foot



Design ExampleLongitudinal Post-Tensioning

⎥⎦

⎤⎢⎣

⎡−⎟⎟⎠

⎞⎜⎜⎝

⎛= σ

xs nSMtP 112



Design ExampleLongitudinal Loads – Dead Load

Span = 120 ft

MDL1 = -2200 k-ft



Design ExampleLongitudinal Loads – Live Load

Span = 120 ft

MLL = -3800 k-ft



Design ExampleLongitudinal Loads – Distribution Factor

Span = 120 ft

gI = 0.69Spacing = 9.75 ft



Design Example

( )SDLDLILLIII MgMM ω18.0 −+=

( )( ) ( ) ftkklfklfftkftkM III −−=⎟⎟⎠

⎞⎜⎜⎝

⎛−−+−−= 3310

1553.0220069.038008.0

Longitudinal Loads – Design Moment



Design ExampleGirder Properties

Girder Depth

Girder Moment of Inertia

Girder Area

Girder Neutral Axis

Slab Thickness

Modular Ratio

Composite Properties

Sx(comp) = 7085 in3



Design ExampleLongitudinal Post-Tensioning ⎥

⎦

⎤⎢⎣

⎡−⎟⎟⎠

⎞⎜⎜⎝

⎛= σ

xs nSMtP 112

Design Chart: 2.3 Strands/ft * 46.875 ft = 108 StrandsFinal Design: 112 Strands

nSx = 6.175*7085 in3 = 44000 in3

MIII = -3310 k-ft

2.3 Strands/ft



ConclusionPost-Tensioned Decks (especially precast panels) are a viable solution for both redecking and new construction

Economy will improve with:

- Project Scale

- Industry Standardization

- Guidelines for owners and designers



Thank you!

Questions?



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Post-Tensioned Bridge Decks Convention/Swartz...precast deck panels. ... 2007 pti technical conference session 7- post-tensioned bridge decks as a cost-effective durable alternative