Post-Tensioned Concrete U-Girder Midas Elite Speaker Series Doug Midkiff, PE AECOM
POST-TENSIONED CONCRETE U-GIRDER BRIDGE DESIGN (I-49 LAFAYETTE CONNECTOR) Doug Midkiff Structural Engineer III AECOM E d u c a t i o n Master of Science, Civil Engineering Bachelor of Science, Civil Engineering Colorado School of Mines PA S T P R O J E C T S 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 standards)
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Post-Tensioned Concrete U-Girder Midas Elite Speaker Series Doug Midkiff, PE AECOM
I-49 Lafayette Connector Concrete U-Girder
Project Summary Criteria Girder Section Midas Modeling
Project Location EDMONTON ANCOUVER CALGARY REGINA WINNIPEG MADISON TORONTO BLOOMFIELD ALBANY KLAND SACRAMENTO DENVER CHICAGO ALEXANDRIA SAN DIEGO PHOENIX AUSTIN LAFAYETTE Louisiana Department of Transportation and Development (LaDOTD) Page 7
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 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 Page 9
Project Summary Criteria Girder Section Midas Modeling
Criteria Prepared by AECOM with input and acceptance by LADOTD LA Spec LADOTD Standard Specifications for Roads and Bridges (2006) BDEM LADOTD Bridge and Evaluation Manual (2015) AASHTO AASHTO LRFD Bridge Specifications, 7 th Edition (2015) Static Loads Post-Tensioned Concrete γ = 155pcf Future Wearing Surface σ = 25psf 42 F-Shape barrier w = 521plf Pre and Post-Tensioned Concrete U-Girder Page 11
Criteria Live Loads Louisiana 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 Truck alone or 25% Truck with Lane Pre and Post-Tensioned Concrete U-Girder Page 12
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 Pre and Post-Tensioned Concrete U-Girder Page 13
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 Pre and Post-Tensioned Concrete U-Girder Page 14
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 Pre and Post-Tensioned Concrete U-Girder Page 15
Project Summary Criteria Girder Section Midas Modeling
Girder Section PCI Zone 6 U84 Girder (Post Tension) b f = 6-0 W = 10-9 t w = 10 t f = 1-9 Weight = 2.529klf (γ = 150pcf) Page 17
Girder Section Colorado U84 Girder (Prestress) Florida U72 Girder (Prestress) Area = 10.40sf Weight = 1.612klf (γ = 155pcf) Area = 10.27sf Weight = 1.592klf (γ = 155pcf) Page 18
Girder Section LADOTD U84 Girder Use the FDOT standard and extrapolate the shape to 84 deep and increase web thickness to accommodate posttension ducts Web thickness increased to the inside of the tub Area = 14.92sf Weight = 2.313klf (γ = 155pcf) Page 19
Post-Tensioning LADOTD U84 Girder Use 12-0.6 Φ strand ducts Grade 270 low lax strands Web thickness by FDOT Table 4.5.6-1 Page 20
Post-Tensioning LADOTD U84 Girder Duct center to center spacing by FDOT Table 4.5.5-1 Page 21
Pre-Tensioning LADOTD U84 Girder 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 Page 22
Project Summary Criteria Girder Section Midas Modeling
Viaduct Decision Matrix 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 Page 24
Viaduct Decision Matrix Page 25
Girder Section Composite Girder drawn in AutoCAD for import to Midas Section Property Calculator Neglect lid slab in section and models at this level of design Page 26
Section Property Calculator Page 27
Section Property Calculator Page 28
Section Property Calculator Page 29
Basic Midas Model 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 Page 30
Basic Midas Model Page 31
Basic Midas Model Page 32
Bridge Wizard Page 33
Bridge Wizard Page 34
Bridge Wizard Page 35
Bridge Wizard Page 36
Bridge Wizard Page 37
Bridge Wizard Page 38
3-Span Unit Page 39
3-Span Unit Page 40
3-Span Unit Page 41
3-Span Unit Page 42
3-Span Unit Page 43
3-Span Unit Page 44
3-Span Unit Page 45
3-Span Unit Page 46
3-Span Unit Page 47
3-Span Unit Page 48
3-Span Unit Page 49
3-Span Unit Page 50
3-Span Unit Page 51
3-Span Unit Page 52
3-Span Unit 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 Page 53
3-Span Unit Page 54
3-Span Unit Construction Stage stress at top of girder after post tensioning Page 55
3-Span Unit Page 56
3-Span Unit Page 57
3-Span Unit Construction Stage stress at top of girder after post tensioning Page 58
3-Span Unit Construction Stage stress at bottom of girder after post tensioning Page 59
3-Span Unit Service I stress at top of girder, LADV positive moment magnification loading Page 60
3-Span Unit Service I stress at bottom of girder, LADV negative moment magnification loading Page 61
3-Span Unit Service III stress at top of girder, LADV negative moment magnification loading Page 62
3-Span Unit Service III stress at bottom of girder, LADV positive moment magnification loading Page 63
3-Span Unit Allowable stresses are calculated based on AASHTO and BDEM Stresses are checked by visual inspection of the Midas results Page 64
3-Span Unit Strength I moment about transverse axis, LADV positive moment magnification loading Page 65
3-Span Unit Strength I moment about transverse axis, LADV positive moment magnification loading Page 66
3-Span Unit Positive moment capacity relied on composite section Page 67
3-Span Unit Negative Moment Capacity Negative Moment Capacity Calculated section capacity Midas results Page 68
3-Span Unit Use strain compatibility to check moment capacity Negative moment capacity relied on girder alone Page 69
3-Span Unit Negative Moment Capacity Negative Moment Capacity Midas results Negative Moment Capacity Calculated section capacity Page 70
4-Span Unit with Flared Girders Page 71
4-Span Unit with Flared Girders Page 72
4-Span Unit with Flared Girders Page 73
4-Span Unit with Flared Girders Page 74
4-Span Unit with Flared Girders Page 75
4-Span Unit with Flared Girders Page 76
4-Span Unit with Flared Girders Page 77
4-Span Unit with Flared Girders Page 78
4-Span Unit with Flared Girders Page 79
4-Span Unit with Flared Girders Page 80
4-Span Unit with Flared Girders Page 81
4-Span Unit with Flared Girders Page 82
North Viaduct, Unit 10 Northbound Page 83
4-Span Unit with Flared Girders Service I stress at top of girder, LADV negative moment magnification loading Page 84
4-Span Unit with Flared Girders Service III stress at bottom of girder, LADV positive moment magnification loading Page 85
4-Span Unit with Flared Girders Strength I LADV positive moment magnification loading Page 86
4-Span Unit with Flared Girders Strength I LADV negative moment magnification loading Page 87
Thank you to: Jay Kwon and Angela Kim of Midas Jenny Fu of LaDOTD Questions? doug.midkiff@aecom.com