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CULVERT LOAD RATING & LOAD TESTING
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* The Office of Structures and Bridges Iowa Department of Transportation ** Bridge Engineering Center Iowa State University [email protected] 515-239-1290
Ping Lu*, Scott Neubauer*, Brent Phares**, Ahmad Abu-Hawash*
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Acknowledgement
n Appreciate all the coauthors for their contribu2ons to this work
n Iowa DOT for funding the load tes2ng program through Federal SPR funding
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What is Load Rating
n Determina2on of the live load carrying capacity based on current condi2on q Using as-‐built plans q Using latest field inspec2on data
n Results are expressed as Ra2ng Factor (RF) or tonnage for a par2cular vehicle
n General equa2on for load ra2ng
𝑅𝐹= 𝐶−𝐷𝐿/𝐿𝐿 In which, C = Capacity of bridge element
DL = Dead load effect LL = Live load effect
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Why Do We Need Load Rating n Ensure public safety and serviceability of structures over 2me
n Evaluate pos2ng needs
n Processing overload permits
n Maintenance priori2za2on
n Comply with federal regula2ons : q required by NBIS (Na2onal Bridge Inspec2on Standards)
q need to be reported every year
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Guidelines for Culvert Load Rating
n AASHTO Manual for Bridge Evalua2on (MBE) q Sec2on 6A.5.12, AASHTO MBE, 2nd Edi2on, with 2013
Interim Revisions
n AASHTO LRFD/Standard Bridge Design Specifica2ons
n Iowa DOT Bridge Ra2ng Manual n Iowa DOT Bridge Design Manual
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Problems in Rating Older Culverts n Rated with LRFR, many exis2ng culverts that were designed to
LFD or ASD specifica2ons are seen as deficient; they cannot even support the dead load
n Most of these in-‐service culverts have performed sa2sfactorily for decades
n This is a common problem for older culverts q A na2onwide survey was conducted by Iowa DOT in 2013 through
AASHTO SCOBS (Sub-‐Commi_ee of Bridges & Structures)
q Replies from more than 21 agencies were received; and 10 indicated they have the same problem
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Reasons – Changes in Design Guidelines n AASHTO Standard Specifica2ons evolved
q Ver2cal earth pressure (EV) q Horizontal earth pressure (EH)
n Compare to Standard Specifica2ons, LRFD recommends, in general, higher q Live load distribu2on through the earth fill q Live load surcharge q Dynamic load impact factor
n Factors contribute the most to the ra2ng problems in Iowa q Live load distribu2on and impact q EH
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Dynamic Impact
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 1 2 3 4 5 6 7 8 9
Impa
ct
Fill Depth - H (ft)
LRFD
Standard
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Live Load Distribution n Fill depth < 2c
Standard LRFD
Perpendicular to the span E E
Parallel to the span concentrate load 𝐿↓𝑇 +𝐿𝐿𝐷𝐹(𝐻)
𝐸=96+1.44𝑆
E in inch and S in ft
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Live Load Distribution (Con’t) n Fill depth >= 2c
Standard LRFD
Spread w (Perpendicular) 1.75(𝐻) W+𝐿𝐿𝐷𝐹(𝐻)
Spread l (Parallel) 1.75(𝐻) L+𝐿𝐿𝐷𝐹(𝐻)
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Live Load Distribution (Con’t) n Fill depth >= 2c (P=16k)
0
200
400
600
800
1000
1200
1400
1600
1800
0 5 10 15 20 25
Whe
el P
ress
ure
(psf
)
Fill Depth - H (ft)
LRFD Design Truck
Standard Design Truck
305.4psi
6ft
145.1psi
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Lateral Earth Pressure (EH)
Standard Specifications LRFD & MBE
Before 1983 After 1983
EH
𝐸𝐻= 𝑘↓𝑎 𝛾↓𝑠 𝑧≥(30𝑝𝑐𝑓)𝑧 𝑘↓𝑎 = 1−𝑠𝑖𝑛∅/1+𝑠𝑖𝑛∅
60/30pcf 60/30pcf
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Earth Pressure used in Iowa Culvert Standards
n 1987 standards (single and twin boxes): office earth pressure
n 1994/1995 standards (metric units) q Triple boxes were added q AASHTO loading was adopted
n Early 2000’s (US units) q Single/twin boxes – Office loading (87 standards were used) q Triple boxes – AASHTO loading (Metric standards were converted to US unit)
n 2012 – LRFD
Office loading: EV = 140pcf EH = 36pcf
AASHTO loading: EV = 120pcf EH = 60pcf
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Live Load Testing n Six culverts tested
FHWA No. County
Earth Fill Sizes
Year Built
1 605880 Boone <1ft 2X12X6X70 1989 2 15351 Boone 11 2X12X10X124 1979 3 15190 Boone 2 2X10X8X160 1962 4 15371 Boone 15 2X12X12X142 1979 5 26600 Hamilton 5 2X12X14X135 1968 6 27111 Hardin 3 2X10X10X86 1976
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Live Load Testing - Results n FHWA #15190 (fill depth = 2c)
• 10 by 8 Twin box • built in 1962 • carries US 30 over Noah
Creek in Boone County • ADT = 7900, 9% truck
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Live Load Testing - Results n FHWA #15190 (fill depth = 2c)
-‐2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0.0 20.0 40.0 60.0 80.0 100.0 120.0
Strain (μ
ε)
Loca/on (0)
R4: Max R4: X-‐axis R4: Y-‐axis
Max Strain = 13.9µƐ Max Stress = 43.53psi
-‐10.0
0.0
10.0
20.0
30.0
40.0
50.0
0.0 20.0 40.0 60.0 80.0 100.0 120.0
Stress (p
si)
Loca/on (0)
R4: Max R4: X-‐axis R4: Y-‐axis
Max Moment = 6.1k-in
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Results Discussion Load Testing
(k-in) Standard
(k-in) LRFD (k-in)
Max M+ of Slab 6.1 47.5 49.9
Standard LRFD
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Conclusions n LRFD/LRFR for concrete box culvert might be overly
conserva2ve in es2ma2ng LL and EH effects
n Current AASTO guideline for concrete box culvert ra2ng is not adequate
n More research is required