Midas Skew Presentation - 5-14-12

Practical Design Methods for

Skewed Bridges

Travis Butz, PE

Burgess & Niple, Inc.

• 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:

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

FRAMING PLAN

TRANSVERSE SECTION

Test Case, Intermediate Crossframe Effects:

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

D

D

Section D-D

Differential Deflection (in)

Girder Deflection (in)

Deflections Exaggerated x 12

Framing Plan

Line Girder

Analysis Results

Crossframe Effects

Ignored

• 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



Deflections

Exaggerated x 12

• 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

• 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

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

Refined Analysis

Results

Intermediate

Crossframe Effects

Included

Section D-D



Deflections

Exaggerated x 12

Section D-D



Line Girder

Analysis Results

Crossframe Effects

Ignored

Deflected Shape Due to Intermediate Crossframe Effects (Refined Analysis):

A

A

Deflections

Exaggerated x 12

Section A-A

Differential

Vertical

Deflection

(inches)

B

B

Deflections

Exaggerated x 12


Section B-B

Differential

Vertical

Deflection

(inches)

Deflections

Exaggerated x 12

Section C-C

C

C


Differential

Vertical

Deflection

(inches)

D

D

Deflections

Exaggerated x 12

Section D-D


Differential

Vertical

Deflection

(inches)

E

E

Deflections

Exaggerated x 12 Section E-E


Differential

Vertical

Deflection

(inches)

F

F

Deflections

Exaggerated x 12

Section F-F


Differential

Vertical

Deflection

(inches)

Deflections

Exaggerated x 12

Section G-G

G

G


Differential

Vertical

Deflection

(inches)

H

H

Deflections

Exaggerated x 12

Section H-H


Differential

Vertical

Deflection

(inches)

J

J

Deflections

Exaggerated x 12


Section J-J

Differential

Vertical

Deflection

(inches)

K

K

Deflections

Exaggerated x 12


Section K-K

Differential

Vertical

Deflection

(inches)

L

L

Deflections

Exaggerated x 12

Section L-L


Differential

Vertical

Deflection

(inches)

M

M

Deflections

Exaggerated x 12

Section M-M


Differential

Vertical

Deflection

(inches)

Support Reactions Due to Wet Concrete Weight, Refined Analysis:

Rear Bearings

(Fixed) Forward Bearings

(Exp.)

Rear

Bearings

(Fixed)

Forward

Bearings

(Exp.)

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

• 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)

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 ∆.

• 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

• 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

• 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

Test Structure, Girder End Twist

End Crossframes Only:

Intermediate Crossframes Only:

Sign Convention: (+ Clockwise, Looking Forward - Counterclockwise, Looking Forward)

Forward


Forward

Combined Effects: Girder End Twist

End Crossframes Only / Intermediate Crossframes Only:

Combined effects:


Forward


Forward

Evaluation of Analysis Methods

Parametric Study:

Mz

Line Girder Analysis

Girder modeled using beam elements

Parametric Study, Analysis Methods:

Parametric Study, Analysis Methods:

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°.

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.).

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.

• 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

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

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.

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.

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.

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.

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.

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.

ODOT Policy:

Skewed Bridge Design Process

Check That Design Rates 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 Design Using Refined Analysis: Erect Girders Vertical And Allow To Rotate


Yes

Yes

Differential Deflections < S/100

30° < Skew ≤ 45°

Perform Line Girder Analysis

Stiffen Design: 0% to ± 25% Additional Steel


No No

Design Using Line Girder Analysis

Yes

Yes

Yes


Perform Refined Analysis


30° < Skew ≤ 45°




Yes

S

δ

f

f

ODOT Policy:



30° < Skew ≤ 45°






Yes

No

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

ODOT Policy:



30° < Skew ≤ 45°






No

Yes


No

1/8”

1’

f


ODOT Policy:



30° < Skew ≤ 45°






No


No


No Stiffen Design: 0% to ± 25% Additional Steel



No

Yes


ODOT Policy:



30° < Skew ≤ 45°





No





No

No Perform Refined Analysis



No

ODOT Policy:



Skew > 45°



No


No



No



Yes

Yes


30° < Skew ≤ 45°




No No


Yes

Yes

Yes



ODOT Policy:


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

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

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.



Deformation Results:

Twisting during deck 1 pouring



=> Twisting can be accurately estimated by Midas Civil so that proper measures can be taken

Questions?

[email protected]

Midas Skew Presentation - 5-14-12

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