CULVERT LOAD RATING & LOAD TESTING

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CULVERT LOAD RATING & LOAD

TESTING

* The Office of Structures and Bridges Iowa Department of Transportation ** Bridge Engineering Center

Iowa State University

Ping.lu@dot.iowa.gov

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

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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

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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)

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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

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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

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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 Im p ac t 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

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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 W h ee l Pr es su re (p sf) Fill Depth - H (ft) LRFD Design Truck Standard Design Truck

305.4psi

6ft

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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)

2012 – LRFD

Office loading: EV = 140pcf EH = 36pcf

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Culvert Load Testing

n  Live load distribu2on through earth ﬁll

n  Lateral Earth Pressure (EH)

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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 (Con’t)

n 

Sensor deployment

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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

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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 Str ain ( με ) Loca/on (0) R4: Max R4: X-‐axis R4: Y-‐axis Max Strain = 13.9µƐ

Max Stress = 43.53psi

10.0 20.0 30.0 40.0 50.0 Str ess (psi) R4: Max R4: X-‐axis R4: Y-‐axis

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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

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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

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Questions?

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