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 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  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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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  

 

1-9 are 9 Strain Rosettes

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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  

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Questions?

         

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Thank You!