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Bridge Deck Overhang Design 2/7/2018
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Bridge Deck Overhang Design
Robert J. Frosch, PhD, PE, FACI
Professor of Civil EngineeringSenior Associate Dean of Facilities & Operations
Overhang Design
• Case 1
• Case 2
• Case 3
Faxle
Fv
Rw
Bridge Deck Overhang Design 2/7/2018
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0
5
10
15
20
25
30
2 4 6 8 10
Mom
ent
(ftꞏ
kip
/ft)
Overhang Length (ft)
Overhang Design for TL-4 Barrier
Case 1
Case 2
Case 3
Yield Line Analysis
+‐ ‐
Mc
Mw
· ∆
nominalrailingresistance
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Barrier Design Forces
Design Forces and Designations Railing Test Levels
TL-1 TL-2 TL-3 TL-4 TL-5 TL-6
Ft Transverse (kips) 13.5 27.0 54.0 54.0 124.0 175.0
FL Longitudinal (kips) 4.5 9.0 18.0 18.0 41.0 58.0
Fv Vertical (kips) Down 4.5 4.5 4.5 18.0 80.0 80.0
Lt and LL (ft) 4.0 4.0 4.0 3.5 8.0 8.0
Lv (ft) 18.0 18.0 18.0 18.0 40.0 40.0
He (min) (in.) 18.0 20.0 24.0 32.0 42.0 56.0
Minimum H Height of Rail (in.) 27.0 27.0 27.0 32.0 42.0 90.0
Barrier Resistance
Rw = 150 kips >> Ft = 54 kips
• Overhang must resist larger moments and axial forces
• Barrier overdesign • more reinforcement
required in overhangs
INDOT Type FC TL-4 Barrier
33”
8”
#5, typ.
9”
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Reducing Collision Forces
AASHTO procedure requires more reinforcement (~3 x Ft) Design for barrier capacity
Past practice required less reinforcement Lower design forces
INDOT and other states reduce collision force (1.25 x Ft) Design for transverse force
What is the appropriate design procedure?
Research Objective
Investigate appropriate design procedure
Determine failure mechanism in overhang specimens
Improve design assumptions
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Schematic Design
15ft Specimen Design
A
A
#3, typ.
5”
16”
9”
4”
4.5”
Section A-ASpecimen #1 Specimen #2
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Dial Gage
Experimental Setup
Strain Gages
Horizontal Pot.
Vertical Pot.
Jack w/ Load Cell
Dial Gage
Specimen #1 Cracking
Barrier Cracking Pattern
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Specimen #2 Cracking
Barrier Cracking Pattern
Vertical Displacement
Specimen #1 Specimen #2
1.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 13.50
0.1
0.2
0.3
0.4
0.5
Potentiometer Location (ft)
Dis
plac
emen
t (in
.)
26.8k
25.1k
24.2k
22.4k
20.1k
1.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 13.50
0.1
0.2
0.3
0.4
0.5
Potentiometer Location (ft)
Dis
plac
emen
t (in
.) 21.5k
20.1k
18.2k
17.7k
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3’ Strip Specimens
• 5 full scale INDOT Type FC specimens
• 2 half scale specimens3’1’ TO BARRIER FACE
Full-Scale Specimen Design
33”
8”
#5
11”
Test Variables:
• Top Transverse Bar Size• #5 and #6
• End Condition• Straight and Hook
• Bar Coating• Epoxy coat and black
Varies
#4
Flat Face
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Half-Scale Specimen Design
A
A
#3 hooked (Strip Sp. #6)
5”
16”
4”
4.5”
Section A-A
Specimen #1 and #2
#3 straight (Strip Sp. #7)
Slab Cast
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Experimental Setup
Vertical Pot.
Jack w/ Load Cell Horizontal Pot.
Strain Gages
Full Scale Half Scale
#5 Epoxy @ 3.5” Straight14 kips
Full-Scale Cracking Behavior
#6 Epoxy @ 4.75” Straight14 kips
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#5 Epoxy @ 3.5” Hooked15 kips
Full-Scale Cracking Behavior
#5 Black @ 3.5” Straight15 kips
#6 Black @ 4.75” Straight14 kips
Full-Scale Cracking Behavior
Typical Specimen Failure Plane
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#3 @4.5” Hooked8 kips
Half-Scale Cracking Behavior
#3 @ 4.5” Straight6 kips
27ft Specimens
• Form Barrier Mechanism
Objectives
• Refine Distribution Length
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27ft Specimen Design
A
A
hooked #3, typ.
5”
16”
9”
4”
4.5”
Section A-ASpecimen #1 Specimen #2
Experimental Setup
Specimen #2L LoadingSpecimen #1L Loading
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Specimen #1L at 29.2 kips
Behavior at Ultimate Load
Punching Shear Failure
Specimen #2L at 23.7 kips
Behavior at Ultimate Load
Punching Shear Failure
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Vertical Displacement
Specimen #1L Specimen #2L
2.5 6.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 20.5 24.50
0.1
0.2
0.3
0.4
0.5
Potentiometer Location (ft)
Dis
plac
emen
t (in
.)
26.2k
24.2k
28.1k
29.2k
22.6k
2.5 6.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 20.5 24.50
0.1
0.2
0.3
0.4
0.5
Potentiometer Location (ft)
Dis
plac
emen
t (in
.)
20.4k
16.0k
22.4k
23.7k
18.2k
Distribution Length
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Lower Punching Shear
Upper Punching Shear
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Nonsymmetrical Failure
Conclusions
• Diagonal tension joint failure (15ft and 3ft strip specimens)• Concrete failure rather than reinforcement anchorage in overhang• Only applicable for short bridge lengths (< 30 ft)
• Punching shear failure (27ft specimens)• Punching controls prior to forming barrier mechanism (110 kips vs 150 kips)• Consistent with actual failures
• Research suggests overhang reinforcement not critical• Punching shear governs design• Large distribution length (10Lt)• Reinforcement can be reduced in overhang
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Recommendations
• Design overhang for vertical loads (Case 2 and 3)• If lateral force considered:
• Design for lesser of:• Punching Shear Strength
• Yield Line Strength
• Consider distribution length as 10Lt
Questions?