11.2 Incremental launching and segmental construction

Incremental launching and segmental construction are game-changing techniques in bridge building. These methods allow engineers to construct bridges in challenging locations with minimal environmental impact and disruption to traffic.

Both approaches involve building bridges in sections, either by pushing from one end or assembling in place. They require specialized equipment and careful planning but offer speed, precision, and flexibility in bridge design and construction.

Incremental Launching Principles

Assembly and Pushing Process

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Top images from around the web for Assembly and Pushing Process

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  Push up | Hydraulic jacks pushing the fire truck up and stab… | Flickr View original

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  River Esk Bridge Launch (3) © Andrew Tryon cc-by-sa/2.0 :: Geograph Britain and Ireland View original

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  Together we stand & lift!... | Launching of Flyover bridge s… | Flickr View original

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• Incremental launching assembles bridge superstructure on one side of obstacle and pushes it longitudinally into final position
• Superstructure constructed in segments on casting yard behind abutment, with each new segment cast against previous one
• Launching nose (lightweight steel structure) attached to front of bridge reduces bending moments and deflections during launching
• Hydraulic jacks push bridge forward in small increments (15-25 meters at a time)
• Temporary supports or piers along launching path reduce span lengths and control deflections
• Bridge deck prestressed longitudinally withstands varying stress conditions during launching and final position

Monitoring and Control

• Careful monitoring of alignment, deflections, and stresses crucial throughout launching process
• Ensures structural integrity and proper positioning of bridge
• Utilizes advanced surveying equipment (total stations, laser scanners)
• Real-time data analysis allows for immediate adjustments during launching
• Strain gauges and accelerometers monitor structural behavior
• Regular inspections of bearings and temporary supports conducted

Design Considerations

• Bridge geometry must accommodate launching process (relatively straight alignment preferred)
• Cross-section design optimized for both construction and final service conditions
• Prestressing system designed to handle varying stress states during launching
• Launching nose design critical for reducing cantilever moments at leading edge
• Temporary and permanent bearings must accommodate horizontal movements during launching
• Consideration given to thermal effects and time-dependent deformations (creep, shrinkage)

Segmental Bridge Construction

Segment Fabrication and Assembly

• Segmental construction builds bridge in small sections (segments) assembled to form complete structure
• Segments precast off-site or cast-in-place (precast offers quality control and reduced on-site time)
• Balanced cantilever method adds segments symmetrically on both sides of pier for equilibrium
• Post-tensioning connects segments and provides structural strength and continuity
• Specialized equipment (segment lifters, launching gantries) transports and places segments accurately
• Match-casting ensures precise fit between adjacent segments
• Epoxy applied at segment joints for additional bonding and waterproofing

Advantages and Applications

• Allows rapid bridge assembly, minimizing traffic disruption and environmental impact
• Particularly advantageous for long-span bridges or viaducts in challenging terrain
• Enables construction of complex geometries (curved alignments, variable depth)
• Reduces formwork requirements compared to traditional cast-in-place methods
• Facilitates standardization and repetition in segment production
• Well-suited for water crossings and environmentally sensitive areas
• Allows for simultaneous work at multiple locations along bridge alignment

Construction Techniques

• Span-by-span method assembles entire spans sequentially using underslung gantries
• Progressive placement technique adds segments one by one from piers towards midspan
• Precast segmental balanced cantilever construction ideal for long-span bridges
• Cast-in-place segmental construction using form travelers for site-cast segments
• Hybrid systems combining precast and cast-in-place elements for optimized construction
• Use of temporary stays or props to support cantilevers during construction
• Closure pours connect cantilevers at midspan, ensuring continuity

Incremental Launching vs Segmental Construction

Construction Approach

• Incremental launching builds entire cross-section in segments and pushes forward
• Segmental construction assembles individual segments in their final position
• Launching requires casting yard and equipment at one end of bridge
• Segmental construction may utilize multiple construction fronts simultaneously
• Launching subjects bridge to varying stress conditions during construction
• Segmental methods offer more flexibility in span arrangement and bridge geometry
• Both reduce need for falsework and formwork in sensitive or difficult-to-access areas

Structural Considerations

• Incremental launching results in more uniform bridge appearance due to continuous casting
• Segmental construction may have visible joints between segments
• Launching requires careful design of temporary and permanent prestressing systems
• Segmental bridges typically have higher post-tensioning requirements
• Launching method limited to straighter alignments and more uniform span lengths
• Segmental construction allows for variable depth and curved alignments more easily
• Both methods suitable for concrete and composite steel-concrete bridges

Equipment and Resource Requirements

• Incremental launching needs specialized launching equipment (hydraulic jacks, rollers)
• Segmental construction requires segment transport and placement equipment (gantries, cranes)
• Launching method demands larger casting yard area at one end of bridge
• Segmental approach may require multiple precast yards or on-site casting facilities
• Both methods benefit from skilled workforce familiar with specialized construction techniques
• Launching often requires less formwork but more temporary supports
• Segmental construction may have higher initial equipment investment but offers more versatility

Factors Affecting Bridge Construction Selection

Site Conditions and Constraints

• Bridge length and span configuration influence choice (uniform spans favor launching)
• Site accessibility and available construction space impact decision
• Soil conditions affect foundation design and temporary support placement
• Presence of obstacles (waterways, existing structures) may limit construction options
• Climate and weather patterns influence construction schedule and method suitability
• Seismic considerations may favor one method over another

Project Requirements and Objectives

• Desired construction speed and project timeline influence method selection
• Environmental considerations (minimizing impact on ecosystems) affect choice
• Bridge's horizontal and vertical alignment complexity may limit incremental launching
• Aesthetic requirements and desired bridge appearance factor into decision
• Future maintenance and inspection needs considered in construction method selection
• Project budget and financing structure impact feasibility of different methods

Technical and Economic Factors

• Structural system and materials (concrete, steel, composite) influence method compatibility
• Economic factors (equipment availability, labor costs, material transportation) impact selection
• Local expertise and contractor experience with specific techniques guide decision-making
• Availability of specialized equipment and materials in project location
• Long-term durability and performance expectations affect choice
• Potential for standardization and prefabrication opportunities considered
• Risk assessment and mitigation strategies for different construction methods evaluated