Post on 23-Mar-2023
Overview• What is segmental construction?• Major Types of Segmental Bridges• Casting Yard• Geometry Control• Post-Tensioning• Erection Equipment
What is Segmental Bridge Construction
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
What is Segmental Bridge Construction
Terminology• Segment Joint• Top Flange / Slab• Bottom Flange / Slab• Web• Shear keys• Tendon Anchors• PT Blisters• PT Bar Blisters
What is Segmental Bridge Construction
Segmental Bridge Types
Cast-in-Place Segmental• Segments Cast In-Situ• Usually Progressive Placement on Previously Built Structure• Specialized Forming Required
Segmental Bridge Types
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
Segmental Bridge Types
Balanced Cantilever• Top “Cantilever” tendons resist
negative bending while the cantilever is built outwards.
Segmental Bridge Types
Balanced Cantilever• Bottom “Continuity” tendons
provide positive bending resistance in the completed structure.
Segmental Bridge Types
Cable-Stay Segmental• Major Crossings• Similar in Concept to
Balanced Cantilever• Wider Variety of Sections• Specialized Equipment and
Forming to Accommodate Cables
Segmental Bridge Types
CIP Arches• Single Direction Cantilever• Relatively Uncommon• Sure Do Look Good
Segmental Bridge Types
Incremental Launch• Segments Cast in Fixed Form at
Abutment• After Each Cycle, Bridge is
Pushed Forward• Typically Shorter Spans Where
Falsework is Impossible
Segmental Bridge Types
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 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 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
Segmental Bridge Types
Pre-Cast Cable-Stayed• Major Crossing• Similar to Balanced Cantilever• Segment Transportation
Planning a Must
Segmental Bridge Types
Substructures• Columns Can be Precast and
Assembled Vertically• Typically in Low-Seismic
Environments
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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
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 YardDelivery• Transport by land on trucks• Limiting condition for height• Water transportation has more
flexibility
Don’t forget about these!
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
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
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
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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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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
Basic Erection Equipment
Form Traveler• Forms the Segment in CIP
Construction• Is Advanced Forward with Each
Cycle and Secured• Often Accommodates Varying
Geometry• Horses
Basic Erection Equipment
CIP Cable-Stayed• Similar Concept• Increased Complexity• Accommodating Stays – Both
as Forces and as Obstacles
Basic Erection Equipment
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
Basic Erection Equipment
Underslung Gantry• Similar Function as Overhead• Supported on Brackets• Can be Relatively Low Tech (and
Cheaper) When Circumstances Allow
Basic Erection Equipment
Articulated Gantry• Variation of overhead gantry• Has articulated joint in middle• Allows tighter turns in congested
areas
Basic Erection Equipment
Lifter (Beam and Winch)• Single Segment Lifts on Balanced
Cantilever or Cable-Stayed• Usually on Larger Bridges• Lower Investment, but Limited
Reach
Basic Erection Equipment
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