Modeling of Post-Tensioned Segmental Box Girders

Modeling of Post-Tensioned Segmental Box Girders Cast Study – Calgary West LRT Neon Koon, P.Eng. July 12 2012

Overview    

Post Tensioned Segmental Concrete Bridges Case Study – Calgary West LRT Modeling Techniques and Applications in Design Process Conclusion

Post Tensioned Segmental Concrete Bridges

What Are Segmental Concrete Bridges?  Built in short pieces of concrete sections, one piece at a time  The concrete segments can be cast-in-place or precast  Large size cranes, referred to as gantry, are typically used to erect the precast segments or slip forms are used to produce cast-in-place concrete  The segments are joined together by applying a high compressive force from high strength bundled wires, referred to as strands

Post Tensioned Segmental Concrete Bridges

What Are Segmental Concrete Bridges?

Precast segment at batch plant

19-strand tendon

Segments being erected using gantry

Tendon stressing in operation

Post Tensioned Segmental Concrete Bridges

Issues To Be Considered in Segmental Bridge Construction       

Span length Project size Site restrictions Local labor and material costs Aesthetic/Appearance Quality Assurance / Quality Control (QA/QC) Construction Methods and Continuing Engineering Services During Construction

Post Tensioned Segmental Concrete Bridges

Various Types of Segmental Bridge Construction    

Span-by-Span Balanced Cantilever Incremental Launching Cable Stayed

Post Tensioned Segmental Concrete Bridges

Various Types of Segmental Bridge Construction Span-By-Span

Courtesy: VSL Systems (CZ) Ltd.

Post Tensioned Segmental Concrete Bridges

Various Types of Segmental Bridge Construction Balanced Cantilever

Courtesy: VSL Systems (CZ) Ltd.

Post Tensioned Segmental Concrete Bridges

Various Types of Segmental Bridge Construction Incremental Launching

Courtesy: VSL Systems (CZ) Ltd.

Post Tensioned Segmental Concrete Bridges

Various Types of Segmental Bridge Construction Cable Stayed

Post Tensioned Segmental Concrete Bridges

Typical Concrete Box Girder Section  Single cell box preferable due to its high torsional resistance, ease of construction and inspection access  For constant girder depth, span to depth ratio ranging from 15 to 30, with optimum value around 18 to 20  Top flange width is preferably limited to 6 times the box depth and can be pushed up to about 18 m  Web spacing normally between 4.6 m to 7.6 m

Post Tensioned Segmental Concrete Bridges

Typical Concrete Box Girder Section  Minimum top flange thickness = 200mm  Minimum ribbed web thickness = 180mm  Very wide bridge deck can be accommodated by using several box girders with a joint between the two box sections

Post Tensioned Segmental Concrete Bridges

Typical Concrete Box Girder Section

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders Tendons – Longitudinal post-tensioning vs transverse post-tensioning – External tendons vs internal tendons Typical Tendon Profile for Longitudinal Post-Tensioning

Courtesy: FHWA

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders Longitudinal External Tendons Mid Span Deviator

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders Transverse Post-Tensioning

Courtesy: FHWA

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders  Prestressing Strands in Tendons in bridge structures – – – – – –

Each strand composed of 7 bundled wires, low relaxation Tensile strength = 1860 MPa Size typically 13 mm or 15 mm diameter Nominal cross sectional area = 99 mm2 or 140 mm2 Duct – at least 2.5 times > the net area of post-tensioning strands Corrosion protection • • • • • •

Water-proofing membrane/sealant at the exterior surface of concrete Cover HDPE duct Grout Sheathing/coating Proper design detailing and construction procedures

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders Levels of Corrosion Protection to Internal Tendons

Courtesy: FHWA

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders Levels of Corrosion Protection to External Tendons

Courtesy: FHWA

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders  External Deviators – Function as a intermediate anchorage point to transfer the vertical component of the post-tensioning force

 Anchorage Blocks – Provide an anchorage point at the ends of the span to develop the required post-tensioning forces – Provide a jacking area for the post-tensioning equipment – Special attention and design details required to increase the levels of corrosion protection at anchorages

Post Tensioned Segmental Concrete Bridges

Major Components of Segmental Box Girders Stressing of Strands in Process at Anchorage Block

Post Tensioned Segmental Concrete Bridges

Design Aspects of Post-Tensioned Box Girders  Design Codes – AASHTO LRFD Bridge Design Specifications – AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges

 Ultimate – Flexure, Shear, Torsion, Axial – Bursting at anchorages, jacking of superstructure during launching

 Service – – – –

Stress within concrete segments and at segment joints Fatigue Deflection Seat width

Post Tensioned Segmental Concrete Bridges

Factors Affecting the Expected Performances        

Concrete mix Temperature Creep and shrinkage Curing methods End restraints (secondary effects) Stress losses in tendons Construction sequence Construction practice/load

Case Study – Calgary West LRT

Case Study – Calgary West Light Rail Transit

Case Study – Calgary West LRT

Case Study – Calgary West Light Rail Transit  Total length – 8 km extending from west end of 7th Ave Downtown to 69th St S.W.  Track works on elevated guideway, trenches, tunnels, and grounds  Expected opening schedule in early Spring 2013

Case Study – Calgary West LRT

Case Study – Calgary West Light Rail Transit  Total length of the elevated guideway is about 1.5 km  Comprised of standard 30m, 33m, and 36m single span segments and two-continuous spans constructed using the span-by-span method as well as a four-span continuous structure using the balanced cantilever method

11 th St SW

Mewata Bridge Bow Trail SW

Bow River

Courtesy: City of Calgary

Courtesy: City of Calgary

Launching Trusses

Launching Trusses Stored on Ground

Cured End Segment at Batch Plant

Steel Form for End Segment Casting

Preparation of Reinforcing Cage

Standard segment for spans constructed using balanced cantilever method

Standard segment for spans constructed using span-by-span method

Erected girder segments using the span-by-span method

Erected girder segments using the balanced cantilever method

Crews applying stressing to tendons at end segment

Two Span Continuous with Integral Connection at Straddle Bent

Two Span Continuous with Integral Connection at Straddle Bent

Crews working in a heated tent in winter

Courtesy: City of Calgary

Modeling Techniques and Applications in Design Process

Modeling Demonstration of a Two-Span Continuous Structure

Modeling Techniques and Applications in Design Process

Modeling Demonstration of a Two-Span Continuous Structure Plan View

Modeling Techniques and Applications in Design Process

Modeling Demonstration of a Two-Span Continuous Structure Elevation View (Longitudinal)

Modeling Techniques and Applications in Design Process

Modeling Demonstration of a Two-Span Continuous Structure Elevation View (Transverse)

Modeling Techniques and Applications in Design Process

Defining the Geometry  Can be either imported from Midas Civil, CAD, or other structural software such as SAP or Lusas  Define nodes and elements manually using the geometry defining features under the Menu Tab

Modeling Techniques and Applications in Design Process

Defining the Geometry

Modeling Techniques and Applications in Design Process

Defining Material  Non-time dependent material  Time dependent material including creep and shrinkage as well concrete strength development – used in construction staging analysis  Plastic material for non-linear analysis  AASHTO and Canadian Codes implemented in material database

Modeling Techniques and Applications in Design Process

Defining Material  Various guidelines such as the CEB-FIP (1990), ACI, PCA, and AASHTO implemented to predict the creep and shrinkage effects

Modeling Techniques and Applications in Design Process

Defining Sections  Sections can be found from the database or manually enter dimensions  Sections like basic bare steel, generic sectional properties, hybrid sections, prestressed concrete section, taper section, and composite section can be defined  Additional stress points can be defined using the Section Manager feature  General sections using SPC

Modeling Techniques and Applications in Design Process

Box Section Developed by SPC

Modeling Techniques and Applications in Design Process

Defining Tendons  Used AutoCAD to accurately define the tendon profile  Simply copy & paste the coordinates in 2D / 3D into Midas Civil

Modeling Techniques and Applications in Design Process

Defining Tendons  Tendon property was defined based on the CEB-FIP  Tendon diameter, relaxation coefficient, tensile strength, and anchorage set, and first jacking force were all defined in the model

Modeling Techniques and Applications in Design Process

Defining Groups  Structure, Boundary, Load, and Tendon Groups were defined accordingly for construction stage analysis and subsequent data manipulation

Modeling Techniques and Applications in Design Process

Defining Construction Stage Analysis & Challenges Defining Construction Stage Analysis  Construction staging had to be accurately defined to capture accumulated stresses built up during construction  Tendon stresses at service after all the losses, secondary moment effects, and stresses at the segmental box girders and other structural components were all examined

Challenges  Integral connection between the straddle bent beam and each end of the spans, resulting consideration of secondary moment effect  Excessive amount of flexural reinforcing originally required in the bent beam, resulting issue of proper concrete placement

Modeling Techniques and Applications in Design Process

Solution...  Accurately defined all the member section properties, their design locations, time dependent materials, tendon properties, stressing and construction sequences in the construction staging analysis  Used internal post-tensioning to replace the flexural reinforcing

Modeling Techniques and Applications in Design Process

Construction Staging Rendering

Modeling Techniques and Applications in Design Process

Defining Moving Load  Canadian and AASHTO LRFD available  Traffic lanes can be defined based on beam or plate elements  Program can consider a number of sub-load cases and perform independent analyses of each sub-load case and provide the maximum and minimum results at a particular location or run combined analyses of all sub-load cases and provide the maximum and minimum results  Dynamic load factor in Canadian code

Modeling Techniques and Applications in Design Process

Post-processing Information  Simulate the construction staging and review stresses and forces at each stage  Report forces and stresses in tendons over time

Modeling Techniques and Applications in Design Process

Post-processing Information  Moving load tracer used to find out location of live load to produce the maximum/minimum forces/stresses at a certain point

Modeling Techniques and Applications in Design Process

Conclusion  Advantages of Post-Tensioned Box Girder Constructed Using Segmental Construction Methods  Design Issues  Challenges Encountered During Construction of CWLRT  Useful Modelling Techniques

Modeling Techniques and Applications in Design Process

Case Study – Calgary West Light Rail Transit http://www.westlrt.ca/contentdesign/constructiondashboard.cfm

Modeling Techniques and Applications in Design Process

Segmental Concrete Bridge Design Resources  AASHTO LRFD Bridge Design Specifications  AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges  American Segmental Bridge Institute (AASHTO-PCI-ASBI Segmental Box Girder Standard Drawings)  ASBI Recommended Practice for Design and construction of Concrete Segmental Bridges  FHWA  Post Tensioning Manual, PTI

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