Modeling of Post-Tensioned Segmental Box Girders


Loading.... (view fulltext now)





Full text


Neon Koon, P.Eng.


 Case Study – Calgary West LRT

 Modeling Techniques and Applications in Design Process  Conclusion


 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


Precast segment at batch plant Segments being erected using gantry


 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


 Span-by-Span

 Balanced Cantilever  Incremental Launching  Cable Stayed




Balanced Cantilever


Incremental Launching


 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


 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



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

Courtesy: FHWA


Longitudinal External Tendons


Transverse Post-Tensioning


 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


Levels of Corrosion Protection to Internal Tendons


Levels of Corrosion Protection to External Tendons


 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


 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


 Concrete mix  Temperature

 Creep and shrinkage  Curing methods

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

 Construction sequence


 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


 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


Courtesy: City of Calgary

Mewata Bridge Bow Trail SW


using span-by-span method


 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


 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


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


 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


 Used AutoCAD to accurately define the tendon profile


 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


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


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


 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


 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


 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


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


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


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

 Design Issues

 Challenges Encountered During Construction of CWLRT  Useful Modelling Techniques


 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