06 pre stressed elements

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

Prestressed Elements6

INV-T BEAM INV-T BEAM

CIP SLAB

MASTICBOND

BREAKER

PIER6”

Inverted Tees

Prestressed Elements7

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

68 @ 5.6

68@5.6

68@5.6

68@5.6

68@5.6

68@5.6

68@5.6

68@5.6

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

68 @ 5.6

68@5.6

68@5.6

68@5.6

68@5.6

68@5.6

68@5.6

68@5.6

68@5.644@5.1

44@5.1

44@5.1

44@5.1

44@5.1120

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

Prestressed Elements23

• 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

Prestressed Elements26

• 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

Prestressed Elements33

6” MIN.

Curved Bridge Design –Layout Considerations

Prestressed Elements34

CHECK MAX OVERHANG

PARALLEL

PARALLEL

6” MIN.CHECKMAX

OVERHANG

Curved Bridge Design –Layout Considerations

Prestressed Elements35

Curved Bridge Design –Layout Considerations

Prestressed Elements36

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