Modeling of Post-Tensioned Segmental Box Girders

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Neon Koon, P.Eng.

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 Case Study – Calgary West LRT

 Modeling Techniques and Applications in Design Process  Conclusion

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 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

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Precast segment at batch plant Segments being erected using gantry

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 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

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 Span-by-Span

 Balanced Cantilever  Incremental Launching  Cable Stayed

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Span-By-Span

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Balanced Cantilever

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Incremental Launching

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 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

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 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

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Tendons

– Longitudinal post-tensioning vs transverse post-tensioning – External tendons vs internal tendons

Courtesy: FHWA

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Longitudinal External Tendons

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Transverse Post-Tensioning

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 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

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Levels of Corrosion Protection to Internal Tendons

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Levels of Corrosion Protection to External Tendons

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 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

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 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

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 Concrete mix  Temperature

 Creep and shrinkage  Curing methods

 End restraints (secondary effects)  Stress losses in tendons

 Construction sequence

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 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

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 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

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Courtesy: City of Calgary

Mewata Bridge Bow Trail SW

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using span-by-span method

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 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

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 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

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

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 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

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 Used AutoCAD to accurately define the tendon profile

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 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

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

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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

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 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

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 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

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 Simulate the construction staging and review stresses and forces at each stage

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

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 Advantages of Post-Tensioned Box Girder Constructed Using Segmental Construction Methods

 Design Issues

 Challenges Encountered During Construction of CWLRT  Useful Modelling Techniques

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 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

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Figure

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