Design and Construction

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Design

*What is Lean-on-Bracing?

*Benefits of Lean-on-Bracing

*How to design a Lean-on-Bracing System

Construction & Research Results

*US 82 Underpass @ 9th Street

*US 82 Underpass @ 19th Street EB & WB

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Research Report 1772-1

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*Fewer cross frames –

Significant Cost Savings

*Improved Fatigue Performance

*Reduced Construction Timeline

*Simplifies Future Inspections

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βt = Torsional system brace stiffness

βb = Brace stiffness

βsec = Cross Section stiffness (web

distortional stiffness)

βg = In-plane girder stiffness

1

𝛽𝑏+

1

𝛽𝑔 +

1

𝛽𝑠𝑒𝑐=

1

𝛽𝑡

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Ideal Total Stiffness

Required System Stiffness

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Braces @ Mid-span

Braces @ supports b1

E S2

hb2

Ab

ng Ld3

S3

ng 1 2

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Mbr = Fbr hb = βt Фo

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Fs ng 1 F

Nc

Fs

ng

Nc 2

F

Force in Diagonal

Force in Struts @ Supports

Force in Struts @ Mid-Span

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*Changes in Strain

*Girder Rotations

*Girder Deflections

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Actual

Predicted

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Load Test X2-DT (kips)

Location

(ft.) X2-DB (kips)

Location

(ft.)

Staggered Ahead 0.7 350 -27.8 100

Staggered Back 0.45 220 8.26 220

Side-by-Side South 0.6 350 6.8 240

Side-by-Side North 0.6 140 8.9 120

End to End South 0.5 140 -25.7 120

End to End Central -0.28 220 10.3 100

Load Test X3-DT (kips)

Location

(ft.) X3-DB (kips)

Location

(ft.)

Staggered Ahead 18.5 140 -11.2 140

Staggered Back 11.7 150 -3.6 150

Side-by-Side South 11.4 140 -3.4 140

Side-by-Side North -15.3 140 13.8 140

End to End South 5.5 190 -5.6 180

End to End Central -14 140 36.1 140

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*Design of Lean-on-Bracing is not difficult

*Improves fatigue performance

*Significant cost savings

*Reduces construction time

*It is a conservative method of torsional bracing that works

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*Will Barnett, P.E.

*Todd Helwig, Ph.D., P.E.

*Anthony Battistini, M.S.E.