post-tensioned concrete u-girder design - midas...
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Post-Tensioned Concrete U-Girder Design
Midas Elite Speaker Series
Doug Midkiff, PE
AECOM
May 23, 2017
Doug Midkiff
Structural Engineer III
AECOM
Master of Science, Civil Engineering
Bachelor of Science, Civil Engineering
Colorado School of Mines
E d u c a t i o n
POST-TENSIONED CONCRETE U-GIRDER BRIDGE DESIGN
(I-49 LAFAYETTE CONNECTOR)
• SH392 over I-25, Windsor, CO (Pre-tensioned concrete U-girders)
• Horseshoe Project, Dallas, TX (Post-tensioned concrete bulb tee)
• Illinois Tollway, Downers Grove, IL (Concrete I-Girder Design standards)
PA S T P R O J E C T S
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Reproduction, distribution, display and use of the presentation without written
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© MIDASoft Inc.,2013
Copyright Materials
Post-Tensioned Concrete U-Girder Design
Midas Elite Speaker Series
Doug Midkiff, PE
AECOM
May 23, 2017
I-49 Lafayette Connector
Concrete U-Girder Design
May 23, 2017
Project SummaryDesign CriteriaGirder SectionMidas Modeling
Project Location
May 23, 2017
Post-Tensioned Concrete U-Girder
Design Page 7
CHICAGO
MADISON
PHOENIX
EDMONTON
CALGARYREGINA WINNIPEGVANCOUVER
TORONTO
OAKLAND
ALBANY
SAN DIEGO
SACRAMENTO
BLOOMFIELD
ALEXANDRIA
AUSTIN LAFAYETTE
DENVER
Louisiana Department of Transportation and Development (LaDOTD)
Project Summary
Future 5.5 mile segment of limited access highway that will
extend I-49 through I-10 to the Lafayette Regional Airport
– Features 2.75 miles of elevated freeway
May 23, 2017
Post-Tensioned Concrete U-Girder
Design Page 8
Project Goals
– Determine the best-value bridge and structure alternatives
• U-Girder design is part of a larger Bridge Development Report that also
studied steel girders and concrete segmental
• Work within the confines of a Context Sensitive Solutions (CSS) design that
is intended to obtain input from all stakeholders
– Utilize a closed, trapezoidal shaped structure for the mainline
structures
– Maintain a minimum span length of 150-feet
May 23, 2017
Post-Tensioned Concrete U-Girder
Design Page 9
Project SummaryDesign CriteriaGirder SectionMidas Modeling
Design Criteria
– Prepared by AECOM with input and acceptance by LADOTD
• “LA Spec” – LADOTD Standard Specifications for Roads and Bridges (2006)
• “BDEM” – LADOTD Bridge Design and Evaluation Manual (2015)
• “AASHTO” – AASHTO LRFD Bridge Design Specifications, 7th Edition (2015)
– Design Static Loads
• Post-Tensioned Concrete γ = 155pcf
• Future Wearing Surface σ = 25psf
• 42” F-Shape barrier w = 521plf
May 23, 2017
Pre and Post-Tensioned Concrete
U-Girder Design Page 11
Design Criteria
– Design Live Loads
• Louisiana Design Vehicle Live
Load 2011 (LADV-11)
o Product of the force effects
produced by AASHTO HL-93 and
a BDEM magnification factor
o LADV-11 is a multiplier on the load
magnitudes
• AASHTO Lane definition and
multiple presence factors
• Live load deflection limited to
L/800 using either Design Truck
alone or 25% Design Truck with
Design Lane
May 23, 2017
Pre and Post-Tensioned Concrete
U-Girder Design Page 12
Design Criteria
– Temperature Loads
• Uniform Temperature by AASHTO Procedure A, base temperature of 68°
• Neglect Temperature Gradient
– Creep and Shrinkage Loads
• Relative Humidity 75%
– Wind Loads
• Per AASHTO Section 3.8
– Centrifugal Force
• Per AASHTO Section 3.6.3, design speed of 60mph (Urban Freeway)
– Braking Force
• Per AASHTO Section 3.6.4
– Dynamic Live Load Allowance
• Per AASHTO Section 3.6.2
May 23, 2017
Pre and Post-Tensioned Concrete
U-Girder Design Page 13
Design Criteria
– Load Factors
• BDEM has a load factor table that supersedes AASHTO Tbl 3.4.1-1
o Service III LL factor 1.00, increased from AASHTO 0.80
o Extreme Event I factor 0.25, decreased from AASHTO 0.50
o Redundancy Load Factor 1.10 – for girder spacing greater than 12-ft
– Materials
• Post-Tensioned concrete f’c = 6ksi (LADOTD Class P)
• Precast-Prestressed concrete girders f’c = 8.5ksi (LADOTD Class P)
• Elected to use f’c = 10ksi (LADOTD Class P)
o Release strength of f’c = 7.5ksi
May 23, 2017
Pre and Post-Tensioned Concrete
U-Girder Design Page 14
Design Criteria
– Assume shoring towers at every splice for support until girders are
made continuous by post-tensioning
– Neglect curvature of superstructure in the Midas models
• Girders will be chorded along the unit with kinks at the splice locations
• The curvature is large enough that centrifugal forces are minimal
– Post processing of Midas results
– Allowable stresses were calculated based on AASHTO and BDEM
– Load calculations prepared in advance of model building
May 23, 2017
Pre and Post-Tensioned Concrete
U-Girder Design Page 15
Project SummaryDesign CriteriaGirder SectionMidas Modeling
Girder Section
PCI Zone 6 U84 Girder (Post
Tension)
May 23, 2017Post-Tensioned Concrete U-Girder
DesignPage 17
– bf = 6’-0”
– W = 10’-9”
– tw = 10”
– tf = 1’-9”
– Weight = 2.529klf (γ = 150pcf)
Girder Section
Colorado U84 Girder (Prestress) Florida U72 Girder (Prestress)
May 23, 2017Post-Tensioned Concrete U-Girder
DesignPage 18
Area = 10.40sf
Weight = 1.612klf (γ = 155pcf)Area = 10.27sf
Weight = 1.592klf (γ = 155pcf)
Girder Section
LADOTD U84 Girder
May 23, 2017Post-Tensioned Concrete U-Girder
DesignPage 19
– Use the FDOT standard and
extrapolate the shape to 84”
deep and increase web
thickness to accommodate post-
tension ducts
– Web thickness increased to the
inside of the tub
– Area = 14.92sf
– Weight = 2.313klf (γ = 155pcf)
Post-Tensioning
LADOTD U84 Girder
May 23, 2017Post-Tensioned Concrete U-Girder
DesignPage 20
– Use 12-0.6”Φ strand ducts
– Grade 270 low lax strands
– Web thickness by FDOT Table
4.5.6-1
Post-Tensioning
LADOTD U84 Girder
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DesignPage 21
– Duct center to center spacing by
FDOT Table 4.5.5-1
Pre-Tensioning
LADOTD U84 Girder
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– Required for dead weight
resistance until girders are made
continuous
– Use 0.6”Φ strand ducts
– Grade 270 low lax strands
– Up to 3 layers in bottom flange
– Harp strands at girder ends as
necessary
– Maximum of 96 strands in each
piece
Project SummaryDesign CriteriaGirder SectionMidas Modeling
Viaduct Decision Matrix
May 23, 2017Post-Tensioned Concrete U-Girder
DesignPage 24
– Units were categorized by
number of spans
• 2, 3 and >3
– Modeled 4 units that were
determined to be worst case
• Longest span and greatest
girder spacing
Viaduct Decision Matrix
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DesignPage 25
Girder Section
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– Composite Girder drawn in AutoCAD for import to Midas
Section Property Calculator
– Neglect lid slab in section and models at this level of
design
Section Property Calculator
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Section Property Calculator
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Section Property Calculator
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Basic Midas Model
May 23, 2017Post-Tensioned Concrete U-Girder
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– Blank model that features all of the properties needed
• Girder section has been defined
• Concrete strengths and time dependencies defined
• Pre and Post-Tensioned strand materials defined
• Static and Live Load definitions defined
• Load Cases set up
Basic Midas Model
May 23, 2017Post-Tensioned Concrete U-Girder
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Basic Midas Model
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Bridge Wizard
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Bridge Wizard
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Bridge Wizard
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Bridge Wizard
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Bridge Wizard
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Bridge Wizard
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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3-Span Unit
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– Construction Staging, 7 stages for each unit
1. Pier sections
2. Drop-in sections
3. Post-tensioning
4. Wet deck pour
5. Composite section with dead load only
6. Composite section with live loads
7. Long term service, 10 years
3-Span Unit
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3-Span Unit
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Construction Stage – stress at top of girder after post tensioning
3-Span Unit
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3-Span Unit
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3-Span Unit
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Construction Stage – stress at top of girder after post tensioning
3-Span Unit
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Construction Stage – stress at bottom of girder after post tensioning
3-Span Unit
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Service I – stress at top of girder, LADV positive moment magnification loading
3-Span Unit
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Service I – stress at bottom of girder, LADV negative moment magnification loading
3-Span Unit
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Service III – stress at top of girder, LADV negative moment magnification loading
3-Span Unit
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Service III – stress at bottom of girder, LADV positive moment magnification loading
3-Span Unit
– Allowable stresses are
calculated based on
AASHTO and BDEM
– Stresses are checked by
visual inspection of the
Midas results
May 23, 2017Post-Tensioned Concrete U-Girder
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3-Span Unit
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Strength I – moment about transverse axis, LADV positive moment magnification loading
3-Span Unit
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Strength I – moment about transverse axis, LADV positive moment magnification loading
3-Span Unit
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– Positive moment capacity relied on
composite section
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Negative Moment Capacity
Negative Moment Capacity
3-Span Unit
Calculated section capacity
Midas results
3-Span Unit
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– Use strain compatibility to check
moment capacity
– Negative moment capacity relied on
girder alone
3-Span Unit
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Negative Moment Capacity
Negative Moment Capacity
Negative Moment Capacity
Calculated section capacity
Midas results
4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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4-Span Unit with Flared Girders
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North Viaduct, Unit 10 Northbound
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4-Span Unit with Flared Girders
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Service I – stress at top of girder, LADV negative moment magnification loading
4-Span Unit with Flared Girders
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Service III – stress at bottom of girder, LADV positive moment magnification loading
4-Span Unit with Flared Girders
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Strength I –LADV positive moment magnification loading
4-Span Unit with Flared Girders
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Strength I –LADV negative moment magnification loading
