Overview of Segmental Bridge Construction

Overview ofSegmental Bridge Construction

Construction Practices Webinar SeriesFebruary 2022

What is Segmental Bridge Construction

Overview• What is segmental construction?• Major Types of Segmental Bridges• Casting Yard• Geometry Control• Post-Tensioning• Erection Equipment


What is Segmental Bridge Construction• Fabrication and Erection of a

Structural Element Using Individual Elements (Segments), Which May be Either Precast or Cast-in-Place

• The Completed Structural Element Acts as a Monolithic Unit Under Some or All Design Loads

• Post-Tensioning is Typically Used to Connect the Individual Elements

• Individual Superstructure Elements are Typically Short Box-Shaped Segments with Monolithic Flanges That Comprise the Full Width of the Structure

Terminology• Typical Segment• Pier Segment• Diaphragm• Variable Depth Segment• Precast Substructure


Terminology• Segment Joint• Top Flange / Slab• Bottom Flange / Slab• Web• Shear keys• Tendon Anchors• PT Blisters• PT Bar Blisters


Segmental Bridge Types


Cast-in-Place Segmental• Segments Cast In-Situ• Usually Progressive Placement on Previously Built Structure• Specialized Forming Required


Balanced Cantilever• Often Single Crossings of Difficult

Terrain / Congestion• Segments are Built Out from

Central Pier, Limiting Unbalanced Weight

• Adjacent Cantilevers Meet in the Middle, are Connected with a Closure


Balanced Cantilever• Top “Cantilever” tendons resist

negative bending while the cantilever is built outwards.


Balanced Cantilever• Bottom “Continuity” tendons

provide positive bending resistance in the completed structure.


Cable-Stay Segmental• Major Crossings• Similar in Concept to

Balanced Cantilever• Wider Variety of Sections• Specialized Equipment and

Forming to Accommodate Cables

Segmental Bridge TypesCable-Stay Segmental• Stay Anchorage


CIP Arches• Single Direction Cantilever• Relatively Uncommon• Sure Do Look Good


Incremental Launch• Segments Cast in Fixed Form at

Abutment• After Each Cycle, Bridge is

Pushed Forward• Typically Shorter Spans Where

Falsework is Impossible


Incremental Launch• Post tensioning is heavier,

primarily axial compression.

Segmental Bridge TypesCast-in-Place Segmental• Continuous rebar across joints


Pre-Cast Segmental• Segments Precast Off-Site• Segments Transported to Site and Assembled• Specialized Equipment for Casting and Erection• Used in High Volume / Low Access Situations

Segmental Bridge Types• Span-by-Span, Simple• Single, Simple Span Erected

Between Piers• Simplest, Fastest Way to

Build• Span Lengths on the Shorter

End of the Spectrum

Segmental Bridge Types• Span-by-Span, Simple• Articulation

Segmental Bridge TypesSpan-by-Span Continuous• Spans Erected One at a Time but

Made Continuous with Post-Tensioning

• Slightly Longer Erection Cycle but Increases the Span Length Due to Continuity

Segmental Bridge TypesSpan-by-Span Continuous

Exp. Jt.

CIP Closure

CIP Closure

CIP Closure

CIP Closure

CIP Closure

CIP Closure

CIP Closure

CIP Closure Exp.

Jt.

Pier

Pier

Pier

Pier

Pier

Pier

Span 1 Span 2

Span 3 Span 4

Segmental Bridge TypesSpan-by-Span Continuous

Segmental Bridge TypesPre-Cast Segmental• Similar in Concept to CIP

Balanced Cantilever• Precast Segments Used Instead• Erected in Balanced Pairs• Segment Transportation is the

Limiting Factor

Segmental Bridge TypesBalanced Cantilever on Land• Segment / span typical• Lengths up to ~300’• Closure pours where

cantilever tips meet• Potentially closures at pier

segment interface• Delivery often limiting

factor

Segmental Bridge TypesBalanced Cantilever Over Water• Delivery of much larger segments

possible

• Opens the door to longer spans with fewer segments


Pre-Cast Cable-Stayed• Major Crossing• Similar to Balanced Cantilever• Segment Transportation

Planning a Must


Substructures• Columns Can be Precast and

Assembled Vertically• Typically in Low-Seismic

Environments


Pre-Cast Segmental• Shear and alignment keys

Segmental Bridge TypesEpoxy• Currently required for all precast bridges

Segmental Bridge TypesEpoxy Squeeze• Wet epoxy is compressed to create a tight match with no gaps

Casting YardApplies to Precast

Forms• Short-Line vs. Long Line• Short-Line Far More Common

Casting Yard

Forms• Long line forms mimic the entire span

Casting Yard

Forms• Short line forms capture one

segment at a time

Casting Yard

Short-Line Forms• Fixed Bulkhead• Exterior Forms• Core Forms• Hydraulics• Manipulator

Casting Yard

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Casting Yard

Match Casting• The “Fourth Wall” of the

Form is the Adjacent or Match Cast Segment

• This Ensures the Segments Will Fit Together in the Field

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Casting Yard

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Casting Yard

Rebar Jigs• Minimal Cycle Time is

Important in the Casting Yard

• Reinforcement is Pre-Tied in Steel Jigs That Mimic Segment Geometry

• They Are Then Lifted Into an Adjacent Form

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Casting YardStorage• Segments are Moved from the Forms to

Storage – and from Storage to Transportation

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Casting YardStorage• Segments are Stored on Dunnage

and are Occasionally Stacked* to Increase Storage Capacity

*AS APPROVED BY THE ENGINEER

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Casting YardStorage• Three point support• Prevents permanent

twist

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Casting YardDelivery• Transport by land on trucks• Limiting condition for height• Water transportation has more

flexibility

Don’t forget about these!

Geometry Control

Geometry Control

Concept• All Types of Segmental Construction Require Special Attention to

Geometry in Order to Achieve the Intended Shape of the Bridge• CIP Construction is a Moving Target• Precast Construction Locks in the Bridge Shape One Segment at a Time

Geometry Control

CIP• Most People are Familiar with Camber

from Traditional Beam Construction• CIP Balanced Cantilever is Also Swaying

Back and Forth as Segments are Cast• The Question is “Where Do I Set the

Forms Today, So That This Joint Ends Up on Profile After the Next 25 Steps?”

Geometry ControlCIP• Analytical Procedure,

set tip at “0”• This gives the total

movement over time• That value is then used

to set the elevation in construction

• Account for traveler deflection

Geometry ControlPrecast• Segments are Cast Miles Away

from the Site, and 10’ at a Time• The Geometry of the Finished

Product is Mapped Onto the Form Setup

• Changes in Plan Curvature, Vertical Curves and Crossfall are Achieved Through Slight Adjustments of the Match Cast Segment

Geometry Control

Geometry Control

Terminology• Fixed Bulkhead• New-Cast or Wet-Cast Segment• Match-Cast Segment• Bolts / Hairpins / etc.

Geometry ControlCamber• Camber means that you are casting the bridge above or below the theoretical

profile

• This can compensate for anything that will cause displacements over time

• Primary sources include:– Creep and shrinkage– Post-tensioning or stay cable effects– Built-in displacements from the erection sequence

Geometry ControlCamber• Camber for precasting means

an adjustment to the target vertical curve.

• It is combined with the vertical profile

Post-Tensioning

Importance of Post-Tensioning• Heavy use with segmental• Pre versus post tensioned

Post-Tensioning

Pre-stressed

Post-tensionedPre- or Post- of what?

Concrete set.Pre-stressed is stressed against a form or bulkhead, then released after the concrete is poured.Post-tensioned is installed and stressed against the hardened concrete.

Post-tensioning required special embedded hardware to transfer the force.

Terminology• Strand• Duct• Anchor / Trumpet• Wedge Plate / Wedge• Spiral / Confinement• Cap

Post-Tensioning

Anchorage Hardware Life Cycle

Post-Tensioning

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Internal and External• Internal Duct is Surrounded

by Concrete

Post-Tensioning

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Internal• Blocks / blisters• Internal deviations /

curvatures / detailing

Post-Tensioning

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Internal• Duct are corrugated plastic

(now typical) or galvanized (older, or maybe in California)

Post-Tensioning

Internal• Duct Couplers• Used as part of a more

aggressive corrosion protection system

• Seal the ducts at the joints• Decrease potential for water

intrusion or grout communication

Post-Tensioning

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Internal and External• External Duct is Not Part of

Concrete Section, but is Inside the Box Girder for Protection

Post-Tensioning

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External• Deviator Segments

Post-Tensioning

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External PTPost-Tensioning

Diabolos

Pipes

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External• Smooth HDPE Duct

Post-Tensioning

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Transverse Post-Tensioning• Compresses deck in

transverse direction• Smaller tendons, 4-strand

commons• Oval duct to reduce

thickness

Post-Tensioning

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Post-Tensioning and Grout• Grout Fills the Space Between the

Tendon and the Duct• It is a Key Component of the

Corrosion Protection• Cementitious Grouts are Frequently

Pre-Packaged to Ensure Quality and Performance

• Flexible Fillers (Wax and Grease) are an Emerging Technology

Post-Tensioning

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Post-Tensioning and Grout• Anchorage Protection• Varies by location and owner• Usually has a plastic, grout-

filled cap• May also include a secondary

pour-back and / or coatings

Post-Tensioning

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Post-Tensioning and Grout• Shameless ASBI plug

Post-Tensioning

Basic Erection Equipment


Form Traveler• Forms the Segment in CIP

Construction• Is Advanced Forward with Each

Cycle and Secured• Often Accommodates Varying

Geometry• Horses


CIP Cable-Stayed• Similar Concept• Increased Complexity• Accommodating Stays – Both

as Forces and as Obstacles


Gantry• Used in Span-by-Span and Balanced

Cantilever Construction• Holds Segments In Place Until Post-

Tensioned and Self-Supporting• Moves Segment Along Length,

Adjusts Geometry• Self-Launches Forward to Next Span


Overhead Gantry• Simple span versus BC gantries

Basic Erection EquipmentGantry• Construction cases are significant


Underslung Gantry• Similar Function as Overhead• Supported on Brackets• Can be Relatively Low Tech (and

Cheaper) When Circumstances Allow


Overhead Gantry• Typically self-

launching• Supports move with

the gantry


Articulated Gantry• Variation of overhead gantry• Has articulated joint in middle• Allows tighter turns in congested

areas


Lifter (Beam and Winch)• Single Segment Lifts on Balanced

Cantilever or Cable-Stayed• Usually on Larger Bridges• Lower Investment, but Limited

Reach


Segment Hauler• Sometimes Customized Segment

Movements are Needed• May Need to be Tailored to

Bridge Geometry to Minimize Forces

• Nothing Wrong with a Flatbed Truck and Crane – If They Work


Segment Handling• Integrate equipment

Questions?

Ben Soule, PE, SE

Overview of Segmental Bridge Construction

Documents

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