06 pre stressed elements
DESCRIPTION
Pres stressing design manualTRANSCRIPT
Prestressed Elements
Ben Jilk
Bridge Design Engineer
MnDOT Bridge Office 2012 LRFD Workshop – June 12, 2012
Outline
• Inverted tees
• New MW-shapes and archiving M-shapes
• Camber study
• Curved bridge design
Prestressed Elements2
Inverted Tees
• Developed in 2004 as an alternative to slabspan bridges
• Spans up to ≈45’
• Typically not used on skewed bridges
• Intended to speed up construction
• 4 generations built, 5th to be designed thissummer
Prestressed Elements3
Inverted Tees - Locations
Prestressed Elements4
3”
Inverted Tees - Geometry
Prestressed Elements5
CIP SLAB
CIP SLAB
INTERIOR BEAM
EXTERIOR BEAM
6”
12” to 18”
12” to 18”
6”
4’-0”1’-0” 1’-0”
6’-0”
1’-0”VARIES
2” CHAMFER
1” CHAMFER
Inverted Tees - Geometry
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INV-T BEAM INV-T BEAM
CIP SLAB
MASTICBOND
BREAKER
PIER6”
Inverted Tees
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POLYSTYRENE
POLYSTYRENE
POLYSTYRENE
POLYSTYRENE
PIER
ABUTMENT
ABUTMENT
• Stainless steel
• Wrapped at piers, not abutments
Inverted Tees
Prestressed Elements8
≈15 ft
CL STRUCTURE
EXTERIORINV-T BEAM
EXTERIORINV-T BEAM
EXTERIORINV-T BEAM
EXTERIORINV-T BEAM
INTERIORINV-T BEAM
INTERIORINV-T BEAM
INTERIORINV-T BEAM
INTERIORINV-T BEAM
PIER
CL PIER
TROUGH
• Beam Concrete
– f’ci = 4 ksi
– f’c = 6 ksi
• Slab Concrete
– f’c = 4 ksi
• ½” diameter 7-wire low-relaxation strands
2”
Inverted Tees - Materials
Prestressed Elements9
2”
2”(TYP.)
3”(TYP.)
Inverted Tees – Design
• LLDF calculated assuming slab-type bridge
• Additional loads:
– Restraint moment (time dependent)
– Thermal gradient
Prestressed Elements10
PRECAST GIRDERS PLACED ON SUBSTRUCTURE
REINFORCEMENT PLACED OVER PIERS
CAST-IN-PLACE SLAB POURED
CONSTRUCTION SEQUENCE FOR THREE-SPAN BRIDGE WITHINVERTED TEES MADE CONTINUOUS FOR LIVE LOADS
• Positive restraint moments
– Beam prestress creep
• Positive thermal gradient
Inverted Tees - Design
Prestressed Elements11
ABUT. PIER PIER ABUT.
POSITIVE MOMENTAPPROXIMATION
• Negative restraint moments
– Dead load creep (beam self-weight, CIP deck weight)
– Deck shrinkage
• Negative thermal gradient
Inverted Tees - Design
Prestressed Elements12
ABUT. PIER PIER ABUT.
NEGATIVE MOMENTAPPROXIMATION
Inverted Tees – Design
• Designed as simple-span
• Restraint moments and thermal gradientincluded by taking yield moment of troughreinforcement continuous over the piers
Prestressed Elements13
ABUT. PIER PIER ABUT.CONTINUOUS REINF.
RESTRAINT/THERMAL GRADIENTMOMENT APPROXIMATION
M YIELD
0 kip-ft0 kip-ft
Inverted Tees – Beam Design
• Tension at release limited to rather
than or 200 psi used for typical
prestressed beams
Prestressed Elements14
Inverted Tees – Slab Design
• Designed as continuous for loads applied afterslab cures (barrier, FWS, LL)
• Restraint moments and thermal gradientincluded by applying a factor of 1.20 to thenegative LL moment at the piers
Prestressed Elements15
ABUT. PIER PIER ABUT.
RESTRAINT/THERMAL GRADIENTMOMENT APPROXIMATION
0 kip-ft0 kip-ft
1.20 × M NEGATIVE LL AT PIER
Inverted Tees
• MnDOT is currently in the process of developingguidelines for Inverted Tees which will bereleased once completed.
Prestressed Elements16
MW Shapes
• Goal to develop:– Beams that span
farther than existingshapes OR
– Beams that could beused at a widerspacing
• 82” and 96” MWBeams
• MnDOT Memo toDesigners (2011-01),July 29, 2011
Prestressed Elements17
6” 6”3’-0”
ROUGHENED1 1
1 SMOOTH FINISH WITHBOND BREAKER
MW Shapes
Prestressed Elements18
14 DRAPED54 STRAIGHT
68 TOTAL STRANDS
MW Shapes
Prestressed Elements19
68 @ 5.6
150
160
170
180
190
200
210
220
5 6 7 8 9 10 11 12 13 14
SP
AN
LE
NG
TH
(FE
ET
)
BEAM SPACING (FEET)
PRESTRESSED CONCRETE BEAM CHART FOR MW SERIES
DESIGN CRITERIAHL-93 LOADING f'c=9ksi f'ci=7.5ksi 0.6" f STRANDS
NUMBERS ADJACENT TO LIMIT CURVES REPRESENT ANAPPROXIMATE DESIGN NUMBER OF STRANDS ANDCENTER OF GRAVITY AT MIDSPAN.96MW
82MW190’
206’
178’
193’
MW Shapes
Prestressed Elements20
68 @ 5.6
130
140
150
160
170
180
190
200
210
220
5 6 7 8 9 10 11 12 13 14
SP
AN
LE
NG
TH
(FE
ET
)
BEAM SPACING (FEET)
PRESTRESSED CONCRETE BEAM CHART
DESIGN CRITERIAHL-93 LOADING f'c=9ksi f'ci=7.5ksi 0.6" f STRANDS
NUMBERS ADJACENT TO LIMIT CURVES REPRESENT ANAPPROXIMATE DESIGN NUMBER OF STRANDS AND CENTER OFGRAVITY AT MIDSPAN.
96MW
82MW
81M
MW Shapes
Prestressed Elements21
4”
PIER
INTERMEDIATEDIAPHRAGM
INTERMEDIATEDIAPHRAGM
1 ONE INTERMEDIATE DIAPHRAGMFOR EVERY 45’ OF SPAN LENGTH(NOT INCLUDING THOSE AT PIERENDS OF BEAM)
1 1
MW Shapes
• Shipment/handling of beams - lateral instability
• Deck pour sequence should be investigated
• Camber tracking required
– Estimated cambers given in tabular form varyingwith age of girder
Prestressed Elements22
MW Shapes – Camber Example
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• Beam length on slopes
– Use “L” in plan sheets when “L” – “H” ≥ ½”
MW Shapes
Prestressed Elements24
“H”
MW Shapes – Standard Plans andB-Details Developed/Modified
• Standard Plans
– 5-397.531 82MW Prestressed Concrete Beam
– 5-397.532 96MW Prestressed Concrete Beam
• B-Details
– B303 Sole Plate
– B310 Curved Plate Bearing Assembly – Fixed
– B311 Curved Plate Bearing Assembly – Expansion
– B412 Steel Intermediate Bolted Diaphragm (All MWPrestressed Beams)
– B814 Concrete End Diaphragm – Parapet Abutment
Prestressed Elements25
Archiving M Shapes
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• Archiving 45M through 81M beams
• Similar depth MN and MW shapes more efficient
• 27M and 36M still available
40
60
80
100
120
140
160
180
200
220
4 5 6 7 8 9 10 11 12 13 14
SP
AN
LE
NG
TH
(FE
ET
)
BEAM SPACING (FEET)
PRESTRESSED CONCRETE BEAM CHART
81M
72M
63M
54M
45M
36M
27M
40
60
80
100
120
140
160
180
200
220
4 5 6 7 8 9 10 11 12 13 14
SP
AN
LE
NG
TH
(FE
ET
)
BEAM SPACING (FEET)
PRESTRESSED CONCRETE BEAM CHART
81M
72M
63M
54M
45M
36M
27M
MN45
MN54
MN63
40
60
80
100
120
140
160
180
200
220
4 5 6 7 8 9 10 11 12 13 14
SP
AN
LE
NG
TH
(FE
ET
)
BEAM SPACING (FEET)
PRESTRESSED CONCRETE BEAM CHART
96MW
82MW
81M
72M
63M
54M
45M
36M
27M
MN45
MN54
MN63
40
60
80
100
120
140
160
180
200
220
4 5 6 7 8 9 10 11 12 13 14
SP
AN
LE
NG
TH
(FE
ET
)
BEAM SPACING (FEET)
PRESTRESSED CONCRETE BEAM CHART
96MW
82MW
81M
72M
63M
54M
45M
36M
27M
MN45
MN54
MN63
Camber Study - Background
• Estimation of camber at erection:
– PCI: 1.85 for self-weight, 1.80 for prestress
• Girders arriving at bridge site with cambers muchlower than predicted
– MnDOT: 1.50 for self-weight and prestress based onlimited internal study
• Study by University of Minnesota to investigateMnDOT’s factors
Prestressed Elements27
DPRESTRESS DSELF
CAMBER- =
Camber Study – Methodology
• Historical camber data
– Fabricator records for 1,067 girders from 2006-2010
– Erection records for 768 of 1,067 girders
• Instrumentation/monitoring of 14 girders
• Measurement of compressive strength/elasticmodulus of samples from two precasting plants
• Parametric study to investigate time-dependenteffects using PBEAM
Prestressed Elements28
Camber Study – Girder FabricationRecommendations
• Pouring Schedule/Management
• Strand Tensioning and Temperature Corrections
• Bunking/Storage Conditions
Prestressed Elements29
Camber Study – Release CamberPrediction Considerations
• Increase f’ci by multiplying by a specified factorfor camber calculations
• Use a different equation to calculate concretemodulus of elasticity
• Reduce the stress in the strands at release forcamber calculations
Prestressed Elements30
Camber Study – Long-Term (Erection)Camber Prediction Suggested Changes
Prestressed Elements31
NO CHANGE TO RELEASECAMBER ESTIMATION
CHANGE RELEASECAMBER ESTIMATION
NO OTHER CHANGES
• MnDOT is currently in the process of decidingwhich multipliers will be used
Curved Bridge Design
Prestressed Elements32
EDGE OF DECK
Curved Bridge Design –Layout Considerations
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6” MIN.
Curved Bridge Design –Layout Considerations
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CHECK MAX OVERHANG
PARALLEL
PARALLEL
6” MIN.CHECKMAX
OVERHANG
Curved Bridge Design –Layout Considerations
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Curved Bridge Design –Layout Considerations
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4’-0” MIN.
PREFERABLY ONLY 1 FLARED SPACE
Curved Bridge Design –Design Considerations
Prestressed Elements37
1/3POINT
1/3POINT
1/3POINT
1/3POINT
Curved Bridge Design –Design Considerations
Prestressed Elements38
2/3 POINT(LOADS)
1/3 POINT(PROPERTIES)
Curved Bridge Fascia Design –Design Considerations
• Stool
– Should take into account horizontal curve
– For straight bridges, typically use stool thickness of2.5” for initial load calculations and 1.5” forproperties.
– For curved bridges, consider using stool thickness ofsomething larger than 2.5” for initial loadcalculations to account for horizontal curve andincreased stool heights. Use 1.5” for properties.
Prestressed Elements39
Summary
• Inverted Tees
• MW-Shapes
• Archiving M-Shapes
• Camber Study
• Curved Bridges
Prestressed Elements40
Questions and Discussion
Prestressed Elements41
Inverted Tees
MW-Shapes
M-Shapes
Camber Study
Curved Bridges