Overview of New Practices & Policy Skewed Bridges.

Overview of New Overview of New Practices & PolicyPractices & Policy

Skewed BridgesSkewed Bridges

What does all this mean?What does all this mean?

Preliminary Design ConsiderationsPreliminary Design Considerations

• What potential skewed solutions should be considered?

• What about non-skewed

solutions?

Potential Skewed SolutionsPotential Skewed Solutions

1.1. Erect Girders PlumbErect Girders Plumb

2.2. Erect Girders Out-of-PlumbErect Girders Out-of-Plumb

3.3. Stiffen SuperstructureStiffen Superstructure

4.4. ““Lean-on” BracingLean-on” Bracing

Potential Solutions – Solution 1Potential Solutions – Solution 1Erect Girders PlumbErect Girders Plumb

Erect girders plumb → Girders rotate Erect girders plumb → Girders rotate out of plumb during deck placement out of plumb during deck placement (girders will be permanently twisted)(girders will be permanently twisted)

How much twist is too much ?How much twist is too much ?

ODOT Limit:

≤ 0.6°

Or

1/8” per ft.

x

1

For Line Girder Analysis:For Line Girder Analysis:If δ ≤ S/100 then ≤ 0.6°

S

δ

Potential Solutions – Solution 2Potential Solutions – Solution 2Erect Girders out of PlumbErect Girders out of Plumb

Erect girders out of plumb (twisted) → Erect girders out of plumb (twisted) → Girders rotate to plumb during deck Girders rotate to plumb during deck placementplacement

Potential Solutions – Solution 3Potential Solutions – Solution 3Stiffen SuperstructureStiffen Superstructure

Add steel Add steel – Bigger flangesBigger flanges– Deeper websDeeper webs– Add girder line(s)Add girder line(s)

Use shored constructionUse shored construction

Use only upon approval from Use only upon approval from OSEOSE

Potential Solutions – Solution 4Potential Solutions – Solution 4 Lean-on Bracing Lean-on Bracing

Internal Lean on Bracing (Solution “4I”)Internal Lean on Bracing (Solution “4I”)

External Lean on Bracing (Solution “4E”)External Lean on Bracing (Solution “4E”)

Skewed Bridge Design Process (Steel Beams/Girders)

Check That DesignRates Using PC-BARS

Skew > 45°

Girder Twist < 1/8”/ft?


No


No

Implement External Lean-on Bracing*

Implement Internal Lean-on Bracing with

Refined Analysis*

No

Finish DesignUsing Refined Analysis:Erect Girders VerticalAnd Allow To Rotate


Yes

Yes

DifferentialDeflections

< S/100

30° < Skew ≤ 45°

Perform Line Girder Analysis

Stiffen Design:0% to ± 25%

Additional Steel


< S/100

No No

Design Using LineGirder Analysis

Yes

Yes

Yes


Additional Steel

Perform Refined Analysis*


< S/100

30° < Skew ≤ 45°




Yes

Skewed Bridge Design Process

(Branch #1)


< S/100

30° < Skew ≤ 45°




Yes

30° < Skew ≤ 45°


(Branch #1)


< S/100

30° < Skew ≤ 45°




Yes

Perform Line Girder Analysis for “Optimized” Design.

Remember:Main girder elements act independently and effect of crossframes are ignored.


(Branch #1)


< S/100

30° < Skew ≤ 45°




Yes


(Branch #1)

S

δ


< S/100

30° < Skew ≤ 45°




Yes Complete Design Using Line Girder Analysis for “Optimized” Design


(Branch #1)

Potential Skew Solution 1: Erect Girders Plumb


< S/100

30° < Skew ≤ 45°




Yes

Check Design Rating Using PC-BARS

• Rating for New bridges must equal or exceed HS-25

• Rating assumes no future wearing surface

• Should be considered during preliminary element sizing


(Branch #1)


< S/100

30° < Skew ≤ 45°




Yes


(Branch #1)

Required for Preliminary Design


< S/100

30° < Skew ≤ 45°





Additional Steel


< S/100

Yes

No


(Branch #2)


< S/100

30° < Skew ≤ 45°





Additional Steel


< S/100

Yes

No


(Branch #2)

Stiffen design: Increase “Optimized” steel design 0% to ±25% (By Weight)• Increase depth• Increase flange sizes• Add girder(s)

Potential Skew Solution 3: Stiffen Superstructure


< S/100

30° < Skew ≤ 45°





Additional Steel


< S/100

Yes

No


(Branch #2)

S

δ


< S/100

30° < Skew ≤ 45°





Additional Steel


< S/100

Yes

No


(Branch #2)

Complete Design Using Line Girder Analysis for Stiffened DesignPotential Skew Solution 1: Erect Girders Plumb


< S/100

30° < Skew ≤ 45°





Additional Steel


< S/100

Yes

No


(Branch #2)



< S/100

30° < Skew ≤ 45°



Additional Steel





< S/100

NoNo

Yes



(Branch #3)


< S/100

30° < Skew ≤ 45°



Additional Steel





< S/100

No

Yes



(Branch #3)

Perform Refined Analysis* to account for crossframe-girder interaction.

No

* Begin Refined Analysis by sizing primary members using member sizes from “Optimized” Line Girder Analysis.


< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No

Yes



(Branch #3)No

1/8”

1’



< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No

Yes



(Branch #3)No

Complete Design Using Refined AnalysisPotential Skew Solution 1: Erect Girders Plumb



< S/100

30° < Skew ≤ 45°



Additional Steel





< S/100

No

Yes



(Branch #3)No



< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No



(Branch #4)No


No

Yes


Additional Steel



< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No



(Branch #4)No


No

Yes


Additional Steel

Potential Skew Solution 3: Stiffen Super.

Increase “Optimized” steel design 0% to ± 25% (by weight) then recheck twist.



< S/100

30° < Skew ≤ 45°



Additional Steel





< S/100

No



(Branch #4)No


No

Yes


Additional Steel



< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No



(Branch #5)No


NoStiffen Design:0% to ± 25%

Additional Steel



Refined Analysis*

No

Yes



NoStiffen Design:0% to ± 25%

Additional Steel

No


< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No



(Branch #5)


Refined Analysis*


Yes

NoPotential Skew Solution 4:

Implement Internal Lean-on Bracing using Refined Analysis *


* Begin Refined Analysis for Lean-on bracing by sizing primary members using member sizes from “Optimized” Line Girder

Analysis.

Check GirderTwist


< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No



(Branch #5)


Refined Analysis*


Yes

No Perform Refined Analysis*



Additional Steel

NoPotential Skew Solution 3:

If “No” increase “Optimized” steel design 0 to ±25% (by weight) and recheck twist.


< S/100

30° < Skew ≤ 45°



Additional Steel





< S/100

No



(Branch #5)


Refined Analysis*


Yes

No



Additional Steel

No

Required for Preliminary

Design


< S/100

30° < Skew ≤ 45°



Additional Steel



< S/100

No


(Branch #6)


Refined Analysis*




No

No Perform Refined Analysis*



Additional Steel

No


< S/100

30° < Skew ≤ 45°



Additional Steel



< S/100

No


(Branch #6)


Refined Analysis*




No

No

* Size primary members using “Optimized”

steel design from Line Girder analysis.




Additional Steel

No

Potential Skew Solution 4:

Implement External Lean-on Bracing with Refined Analysis*


(Branch #6)DifferentialDeflections

< S/100

30° < Skew ≤ 45°



Additional Steel




< S/100

No


Refined Analysis*


No



No



Additional Steel

No


Design

Skew > 45°



Yes



(Branch #7)Perform Refined Analysis*

Skew > 45°



Yes



(Branch #7)Skew > 45°


Skew > 45°



Yes



(Branch #7)

* Begin Refined Analysis by sizing primary members using member sizes from “Optimized” Line Girder Analysis.


Skew > 45°




Yes



(Branch #7)


Skew > 45°




Yes


(Branch #8)

NoGirder Twist < 1/8”/ft?


Additional Steel


Skew > 45°



No



Yes


(Branch #9)


Refined Analysis*

No Girder Twist < 1/8”/ft?



Additional Steel

Skew > 45°



No



Yes


(Branch #9)


Refined Analysis*




Additional Steel


Design


Skew > 45°



No


(Branch #10)

No



Refined Analysis*




Additional Steel


No


(Branch #10)


Skew > 45°


No



Refined Analysis*




Additional Steel


Design

Skewed Bridge Design Process (Steel Beams/Girders)


Skew > 45°



No

No



Refined Analysis*

No



Yes

Yes


< S/100

30° < Skew ≤ 45°



Additional Steel


< S/100

No No


Yes

Yes

Yes




Additional Steel

What about skewed PrestressedWhat about skewed PrestressedI-Beams Superstructures?I-Beams Superstructures?


< S/100




Stiffen DesignDifferentialDeflections

< S/100

YesYes

NoNo

Alternate Superstructure

Type


Prestressed Beams/Girders: Skew > 30°


< S/100




Yes


(Branch #1)


< S/100




Yes

Perform Line Girder Analysis for “Optimized” Design.


(Branch #1)


< S/100




Yes

S

δ


(Branch #1)


< S/100




Yes Complete Design Using Line Girder Analysis for “Optimized” DesignPotential Skew Solution 1: Erect Girders Plumb


(Branch #1)


< S/100




Yes

Check That Design Rates Using PC-BARS


(Branch #1)


< S/100




Yes


(Branch #1)


Design


< S/100





< S/100

YesYes

No


(Branch #2)


< S/100





< S/100

YesYes

No

Stiffen Design:

• Add beam line

• Increase beam size


(Branch #2)


< S/100





< S/100

YesYes

No


(Branch #2)

S

δ


< S/100





< S/100

YesYes

No


(Branch #2)


Design


< S/100



< S/100

NoNo


Type


(Branch #3)


< S/100



< S/100

NoNo


TypeDesign Alternate Structure Type


(Branch #3)

Non-skewed AlternativesNon-skewed Alternatives

Designers should consider:Designers should consider: Longer spans that eliminate or reduce skewLonger spans that eliminate or reduce skew Straddle Bents that span the under-bridge Straddle Bents that span the under-bridge

featurefeature

For alternatives with similar costs:

ODOT will prefer reduced skews over “Lean on” construction.

Detail Design ConsiderationsDetail Design Considerations

• Stakeholder Responsibilities

• Intermediate Crossframes

• End Crossframes

• End Diaphragms

• Bearings

Stakeholder ResponsibilitiesStakeholder Responsibilities

Designer Responsibilities:

• Structural design during deck placement & final condition

• Structural stability during deck placement

Stakeholder ResponsibilitiesStakeholder Responsibilities

Contractor Responsibilities:

• Detailing bracing members to fit under steel dead load with

girders plumb at erection

• Structural stability during erection

• Structural stability during deck placement (if different than plan)

Intermediate CrossframesIntermediate Crossframes2004 ODOT BDM Design Requirements2004 ODOT BDM Design Requirements

“On skewed structures, in order to accommodate differential dead load deflections of adjacent stringers that exceed ½ inch at any intermediate crossframe location, the erection bolt holes in the crossframe members should be detailed as slotted holes. Hole dimensions shall be 1/16” wider than the nominal bolt diameter and ¾” longer than the nominal bolt diameter. Holes in the stiffeners shall be 3/16” larger than the diameter of the erection bolts. Final bolting and welding shall not be completed until after the deck concrete has been placed.“

Intermediate CrossframesIntermediate Crossframes

No Slots in Crossframe members:

Slots cause:• Insufficient bracing • Exaggerated

deflections/rotations


Bracing members should be fully tightened Bracing members should be fully tightened and welded during deck placementand welded during deck placement

Fully Tighten Bolts

Field weld

Type 3 Type 4


Design bracing members for Refined Analysis Loads

• Start with GSD-1-96

• Add beef wherenecessary

End Crossframes: Skews End Crossframes: Skews > 30°DO NOT INSTALL END CROSSFRAME DIAGONALS UNTIL DECK PLACEMENT IN ADJACENT SPAN IS COMPLETE.

End Armor

Bottom Chord

Diagonals

• Eliminates girder end rotation caused by decambering• Simplifies refined analysis model• Reduces locked-in stresses• Temporary bracing should be investigated

Bottom Chord

End Armor

End Crossframes: Skews End Crossframes: Skews > 30° DO NOT INSTALL END CROSSFRAME DIAGONALS UNTIL DECK PLACEMENT IN ADJACENT SPAN IS COMPLETE.

End DiaphragmsEnd DiaphragmsSteel Superstructures (Skew > 30°) ORSteel Superstructures (Skew > 30°) ORConcrete Superstructures (Skew > 10°)Concrete Superstructures (Skew > 10°)

Place diaphragms after deck placement in adjacent span is complete.

Stage 1

Stage 2

End DiaphragmsEnd DiaphragmsSteel Superstructures (Skew > 30°) ORSteel Superstructures (Skew > 30°) ORConcrete Superstructures (Skew > 10°)Concrete Superstructures (Skew > 10°)

Stage 1

Stage 2

• Eliminates girder end rotation caused by decambering• Simplifies refined analysis model• Reduces locked-in stresses in end diaphragm & main members and reduces potential for cracking• Temporary end bracing should be considered

BearingsBearingsRotationsRotations

Design Bearings for the out-of-plane rotations.

• Use results from refined analysis

- OR -

• 1/8” per ft. (max.)

BearingsBearingsLoad PlatesLoad Plates

Do not bevel load plate in transverse direction.

• Forces girder web out-of-plane during erection

• Complicates bearing fabrication

BearingsBearingsSize Bearings for Refined Analysis ReactionsSize Bearings for Refined Analysis Reactions

FixedBearings

ExpansionBearings

Policy ChangesPolicy ChangesBridge Design Manual (BDM)Bridge Design Manual (BDM)

New Appendix SectionNew Appendix Section::ODOT Guide for the Design of Skewed ODOT Guide for the Design of Skewed SuperstructuresSuperstructures

SummarySummary

An itemized summary is provided at the back of the seminar handout and is available for download at our website:

www.dot.state.oh.us/se/skew/skew.htm

Questions ??Questions ??

Email Questions to:

[email protected]

Overview of New Practices & Policy Skewed Bridges.

Documents

Transcript of Overview of New Practices & Policy Skewed Bridges.