midas skew presentation - 5-14-12
DESCRIPTION
Skew steel girder with concrete decking - composite bridge Analysis.Force comparisons for grillage and FEM model.TRANSCRIPT
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Practical Design Methods for
Skewed Bridges
Travis Butz, PE
Burgess & Niple, Inc.
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• Recurring constructability problems during deck pours
• Predicted deflections disagree with field results
• Decks with exposed rebar, poor finish, inconsistent
thickness
• Excessive girder twist – in one case, capacity of the
structure was compromised
• A study was commissioned to identify causes and to
recommend solutions
- Why is this happening?
- What analysis methods are appropriate?
- How can we prevent these problems?
Ohio’s Skew Problems:
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Skewed Bridge Behavior
• Out-of-plane effects occur in skewed bridges that cannot
be predicted by line girder analysis methods (neglecting
crossframe effects).
• AASHTO/NSBA “Guidelines for Design for
Constructability” identifies two separate issues:
Intermediate Crossframe Effects
End Crossframe Effects
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FRAMING PLAN
TRANSVERSE SECTION
Test Case, Intermediate Crossframe Effects:
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Line Girder
Analysis Results
Crossframe Effects
Ignored
0
1
2
3
4
5
6
0.00 50.00 100.00 150.00 200.00
Length (ft)
De
fle
cti
on
(in
)
G2 G1 G3 G4 G5
Crossframe
Locations
Test Structure, Deflection Due to Deck Weight
Results Show:
• Large differential
deflections between
interior and exterior
girders
• Abrupt changes in
differential
deflection across
the width of the
bridge
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D
D
Section D-D
Differential Deflection (in)
Girder Deflection (in)
Deflections Exaggerated x 12
Framing Plan
Line Girder
Analysis Results
Crossframe Effects
Ignored
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• Problem: If the girders are assumed to stay
vertical, the crossframes will not permit
differential deflections of this magnitude.
• Conclusion: Crossframe interaction needs to be
included to accurately model structure behavior.
Line Girder Analysis Results
Crossframe Effects Ignored
Section D-D
Differential Deflection (in)
Girder Deflection (in)
Deflections
Exaggerated x 12
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• Differential vertical deflection causes crossframes to
deform if the girders do not twist.
• Large forces are needed to create axial deformations
in the crossframe members, so resistance to this type
of deflection is very high.
Lengthened
Shortened
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• Twisting of the girders allows differential deflection to
occur without deforming the crossframe.
• The torsional stiffness of the girders is low compared
to the stiffness of the crossframes, so this behavior
is dominant.
Undeformed
Undeformed
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Refined Analysis
Results
Intermediate
Crossframe Effects
Included
Results Show:
• More uniform
differential deflection
across the width of
the bridge at
crossframe locations
(compared to line
girder analysis)
Test Structure, Deflection Due to Deck Weight
0
1
2
3
4
5
6
0.00 50.00 100.00 150.00 200.00
Length (ft)
Defl
ecti
on
(in
)
G2 G1G3G4G5
Crossframe
Locations
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Refined Analysis
Results
Intermediate
Crossframe Effects
Included
Section D-D
Differential Deflection (in)
Girder Deflection (in)
Deflections
Exaggerated x 12
Section D-D
Differential Deflection (in)
Girder Deflection (in)
Line Girder
Analysis Results
Crossframe Effects
Ignored
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Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
A
A
Deflections
Exaggerated x 12
Section A-A
Differential
Vertical
Deflection
(inches)
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B
B
Deflections
Exaggerated x 12
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Section B-B
Differential
Vertical
Deflection
(inches)
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Deflections
Exaggerated x 12
Section C-C
C
C
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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D
D
Deflections
Exaggerated x 12
Section D-D
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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E
E
Deflections
Exaggerated x 12 Section E-E
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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F
F
Deflections
Exaggerated x 12
Section F-F
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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Deflections
Exaggerated x 12
Section G-G
G
G
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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H
H
Deflections
Exaggerated x 12
Section H-H
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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J
J
Deflections
Exaggerated x 12
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Section J-J
Differential
Vertical
Deflection
(inches)
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K
K
Deflections
Exaggerated x 12
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Section K-K
Differential
Vertical
Deflection
(inches)
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L
L
Deflections
Exaggerated x 12
Section L-L
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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M
M
Deflections
Exaggerated x 12
Section M-M
Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):
Differential
Vertical
Deflection
(inches)
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Support Reactions Due to Wet Concrete Weight, Refined Analysis:
Rear Bearings
(Fixed) Forward Bearings
(Exp.)
Rear
Bearings
(Fixed)
Forward
Bearings
(Exp.)
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End Crossframe Effects
• To illustrate end crossframe behavior, we will examine a 2-
girder structure with end crossframes only (no intermediate
bracing).
• This illustration is adapted from Beckmann & Medlock, 2005
PLAN VIEW
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• The end diaphragm can be thought of as a pair of rigid
links connecting the top flange of one girder to the
bottom flange of the adjacent girder.
2-Girder Structure:
ISOMETRIC VIEW (PARTIAL)
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Deflection of a Cambered Girder:
• When a girder deflects, the top flange moves
longitudinally relative to the bottom flange at the
beam ends. We will define this distance as ∆.
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• The end crossframe of a skewed structure restrains
the longitudinal translation of the top flange.
2-Girder Structure:
PLAN VIEW (PARTIAL)
Dx
GIRDER A
GIRDER B
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• The end crossframe forces the top flange to move
radially about the adjacent bearing point. The
resulting motion produces twist in the girders.
2-Girder Structure:
PLAN VIEW (PARTIAL)
Dx Dy
GIRDER A
GIRDER B
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• The movement of the top flange is approximately
perpendicular to the centerline of bearings.
2-Girder Structure:
PLAN VIEW (PARTIAL)
Dx Dy
Dx Dy
GIRDER B
GIRDER A
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Test Structure, Girder End Twist
End Crossframes Only:
Intermediate Crossframes Only:
Sign Convention: (+ Clockwise, Looking Forward - Counterclockwise, Looking Forward)
Forward
Sign Convention: (+ Clockwise, Looking Forward - Counterclockwise, Looking Forward)
Forward
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Combined Effects: Girder End Twist
End Crossframes Only / Intermediate Crossframes Only:
Combined effects:
Sign Convention: (+ Clockwise, Looking Forward - Counterclockwise, Looking Forward)
Forward
Sign Convention: (+ Clockwise, Looking Forward - Counterclockwise, Looking Forward)
Forward
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Evaluation of Analysis Methods
Parametric Study:
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Mz
Line Girder Analysis
Girder modeled using beam elements
Parametric Study, Analysis Methods:
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Parametric Study, Analysis Methods:
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Study Conclusions:
• When the effects of intermediate crossframes are considered,
significant redistribution of shear and moment occurs across
the width of the structure.
• For the structures studied, line girder analysis can be used to
conservatively calculate member forces for skews up to 45°.
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2D Grid Analysis vs. 2D Grid Analysis w/ Truss Crossframes:
• Little variation was observed between the girder and intermediate crossframe forces
obtained from 2D grid analysis with truss crossframes.
• The use of 2D grid analysis was shown to be generally accurate in the calculation of
moments and shears for the cases investigated.
• Note that higher levels of analysis provide more precise results, and are
recommended when higher precision is needed, or with more complex structures
(variable skews, partial length girders, etc.).
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2D Grid Analysis w/ Truss Crossframes vs. 3D FEM:
• The moment and shear results obtained from 3D FEM analysis show general
agreement with the results obtained from 2D grid analysis with truss crossframes..
• Although the 2D grid was found to be generally accurate for calculating moments and
shears for the structures investigated, 3D FEM analysis does provide more precise
results.
• 3D FEM is recommended for more complex structures.
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• Erect girders plumb
• Install crossframes
• Girders rotate out of plumb during deck placement
• Girders will be permanently twisted
Question: How much twist is acceptable?
Detailing Methods
Method 1 – Steel dead load fit – members are detailed to fit with webs
plumb with steel dead load on the structure, but not the deck load
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Detailing Methods
Method 2 – Full dead load fit – members are detailed to fit with webs
plumb with full non-composite dead load of steel and concrete.
• Erect girders out-of-plumb
• Install crossframes
• Girders rotate to vertical during deck placement
• Girders webs will be vertical in the finished structure
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Detailing Methods
Method 2 – Full dead load fit – members are detailed to fit with webs
plumb with full non-composite dead load of steel and concrete.
• This method is generally recommended for skewed bridges by
industry experts.
• ODOT is not comfortable with erecting girders in an out-of-plumb
position. Steel Dead Load fit is required by ODOT policy.
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Detailing Methods
Method 3 – Lean-on Bracing
• Use of an alternative lateral bracing system to minimize or eliminate intermediate crossframe effects.
• Some crossframes are replaced with top and bottom struts only during the deck pour
• Lean-on braces allow differential vertical deflection to occur between girders without inducing twist.
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Lean-on Bracing Two types: Internal and External
In an Internal Lean-on System, bracing is provided by a
crossframe located within the portion of the structure that is
being loaded.
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X X X
X X X
A
A
Section A-A
Lean-on Bracing Internal Lean-on System
In an internal system, crossframe locations can be selected
strategically to minimize twist in the system. Designers
must perform calculations to ensure adequate strength and
stiffness are provided.
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Lean-on Bracing External Lean-on System
In an External Lean-on System, the structure is braced
against an external support or a portion of the structure
that will not be loaded during the deck pour.
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Can girder twist be calculated using line girder results?
• For low skews, girder twist can be
estimated using line girder analysis.
• From AASHTO/NSBA Steel Bridge
Erection Guide Specification, erection
tolerance = 1/8” per foot of web depth
• Data shows this method to be conservative
up to a 45° skew.
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ODOT Policy:
Skewed Bridge Design Process
Check That Design Rates Using PC-BARS
Skew > 45°
Girder Twist < 1/8”/ft?
Girder Twist < 1/8”/ft?
No
Girder Twist < 1/8”/ft?
No
Implement External Lean-on Bracing
Implement Internal Lean-on Bracing with Refined Analysis
No
Finish Design Using Refined Analysis: Erect Girders Vertical And Allow To Rotate
Check That Design Rates Using PC-BARS
Yes
Yes
Differential Deflections < S/100
30° < Skew ≤ 45°
Perform Line Girder Analysis
Stiffen Design: 0% to ± 25% Additional Steel
Differential Deflections < S/100
No No
Design Using Line Girder Analysis
Yes
Yes
Yes
Stiffen Design: 0% to ± 25% Additional Steel
Perform Refined Analysis
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Differential Deflections < S/100
30° < Skew ≤ 45°
Perform Line Girder Analysis
Design Using Line Girder Analysis
Check That Design Rates Using PC-BARS
Yes
S
δ
f
f
ODOT Policy:
Skewed Bridge Design Process
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Differential Deflections < S/100
30° < Skew ≤ 45°
Perform Line Girder Analysis
Design Using Line Girder Analysis
Check That Design Rates Using PC-BARS
Stiffen Design: 0% to ± 25% Additional Steel
Differential Deflections < S/100
Yes
No
Stiffen design: Increase “Optimized” steel design 0% to ± 25% (By Weight) • Increase depth • Increase flange sizes • Add girder(s)
ODOT Policy:
Skewed Bridge Design Process
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Differential Deflections < S/100
30° < Skew ≤ 45°
Perform Line Girder Analysis
Stiffen Design: 0% to ± 25% Additional Steel
Girder Twist < 1/8”/ft?
Finish Design Using Refined Analysis: Erect Girders Vertical And Allow To Rotate
Differential Deflections < S/100
No
Yes
Check That Design Rates Using PC-BARS
No
1/8”
1’
f
Perform Refined Analysis
ODOT Policy:
Skewed Bridge Design Process
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Differential Deflections < S/100
30° < Skew ≤ 45°
Perform Line Girder Analysis
Stiffen Design: 0% to ± 25% Additional Steel
Girder Twist < 1/8”/ft?
Finish Design Using Refined Analysis: Erect Girders Vertical And Allow To Rotate
Differential Deflections < S/100
No
Check That Design Rates Using PC-BARS
No
Girder Twist < 1/8”/ft?
No Stiffen Design: 0% to ± 25% Additional Steel
Girder Twist < 1/8”/ft?
Implement Internal Lean-on Bracing with Refined Analysis
No
Yes
Perform Refined Analysis
ODOT Policy:
Skewed Bridge Design Process
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Differential Deflections < S/100
30° < Skew ≤ 45°
Perform Line Girder Analysis
Stiffen Design: 0% to ± 25% Additional Steel
Girder Twist < 1/8”/ft?
Differential Deflections < S/100
No
Implement Internal Lean-on Bracing with Refined Analysis
Girder Twist < 1/8”/ft?
Check That Design Rates Using PC-BARS
Implement External Lean-on Bracing
No
No Perform Refined Analysis
Girder Twist < 1/8”/ft?
Stiffen Design: 0% to ± 25% Additional Steel
No
ODOT Policy:
Skewed Bridge Design Process
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Check That Design Rates Using PC-BARS
Skew > 45°
Girder Twist < 1/8”/ft?
Girder Twist < 1/8”/ft?
No
Girder Twist < 1/8”/ft?
No
Implement External Lean-on Bracing
Implement Internal Lean-on Bracing with Refined Analysis
No
Finish Design Using Refined Analysis: Erect Girders Vertical And Allow To Rotate
Check That Design Rates Using PC-BARS
Yes
Yes
Differential Deflections < S/100
30° < Skew ≤ 45°
Perform Line Girder Analysis
Stiffen Design: 0% to ± 25% Additional Steel
Differential Deflections < S/100
No No
Design Using Line Girder Analysis
Yes
Yes
Yes
Stiffen Design: 0% to ± 25% Additional Steel
Perform Refined Analysis
ODOT Policy:
Skewed Bridge Design Process
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Bottom Chord
End Armor
ODOT Policy: End Crossframes
For skews > 30 degrees, do not install end crossframe diagonals
until deck placement in the adjacent span is complete
End Armor
Bottom Chord
Diagonals
Condition at Deck
Placement:
Note that the girder
ends are unbraced.
Temporary bracing
may be required.
Final Condition:
Diagonals installed
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Refined Analysis using Midas Civil:
In midas civil user can model the construction sequence considering
the girder lift, installation and the deck pouring sequence.
The shell elements works well in determination of the girder twist. A
study has been done in midas civil for the determination of the girder
twist during the deck pouring.
The following pouring sequence has been assumed:
Stage 1 Stage 2 Stage 3
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The following Stages were modeled: 1. Stage 1: Steel Girders are installed and self weight of steel is activated.
2. Stage 2: The scaffolding load is activated. The load is activated in the following fashion for the overhangs:
3. Stage 3: The deck dead load is activated for the deck pour 1.
4. Stage 4: The deck dead load is activated for the deck pour 2.
5. Stage 5: The deck dead load is activated for the deck pour 3.
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Deformation Results:
Twisting during deck 1 pouring
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Deformation Results:
Twisting during deck 2 pouring
![Page 59: Midas Skew Presentation - 5-14-12](https://reader031.vdocuments.us/reader031/viewer/2022020519/577cc57f1a28aba7119c9692/html5/thumbnails/59.jpg)
Deformation Results:
Twisting during deck 3 pouring
=> Twisting can be accurately estimated by Midas Civil so that proper measures can be taken