LRFR Ratings on Existing Bridge Inventories – A Case StudyDaniel Whittemore, PE, LEED AP – AI Engineers, Inc.

Prasad Nallapaneni, PE, Virginia Department of Transportation

VDOT Load Rating for Existing Bridges

• AI was retained by the Virginia Department of Transportation to perform LRFR Load Ratings using AASHTOWARE’s Virtis Program in 2008.

• In past 3 years, AI has rated over 1,500 bridges throughout the Commonwealthbridges throughout the Commonwealth

Conc. Slab15%

Conc T Beam14%

Rolled Steel23%

Steel Plate16%

PS Bulb 16%

PS I Beam16%

Bridges by Type

Bridge Data Set

• In came to our attention that we had, with 1,500 bridges to choose from, a large dataset to analyze

• VDOT gave AI permission to work with the data to draw conclusions for other bridge owners.bridge owners.

LFR Versus LRFR

• Using Virtis Software, bridges are easily able to be switched from LFR ratings to LRFR ratings from same data input

• Virtis software version 6.2 has introduced a new analysis engine of their own designing – The “Virtis Engine”designing – The “Virtis Engine”

• It was decided to use the data set to draw conclusions between LFR and LRFR directly applicable for bridge owners

AASHTO Legal Loads

Why AASHTO Legal Loads?

• Apples to apples comparison – live loads are identical

• Widely understood by all AASHTO aware bridge owners

• Directly related to posting, which for a bridge owner is:bridge owner is:

– Money

– Time

– Safety

– Inconvenience

• Limited Scope / $

Bridge Types Selected – 10 Each Category

1. Concrete Slab- Simple Spans

2. Concrete Slab- Continuous Spans

3. Concrete T-Beams – Simple Spans

4. Rolled Shape – Simple span

5. Rolled Shape – Continuous Spans

6. Plate Girder – Simple Span

7. Plate Girder – Continuous Spans

8. Prestressed Bulbs Tees

9. Prestressed AASHTO-I Beams (Simple Spans)

Project Assumptions

• Virtis Engine is accurate and complete

• Virginia bridges are similar to yours

• As-Inspected conditions were ignored

• All loads factors, distribution factors, etc. were calculated as the load rating method dictatedmethod dictated

• The lowest rated structural member controlled the load rating for each method – even if they were located on different structural members or resulted from different limit states.

Type 1 - Concrete Simple Spans

LRFR/LFR

Structure Span Length Year Built Type 3 Type 3-3 Type 3S2

1.1 21.25 1927 1.39 1.39 1.38

1.2 21.25 1925 1.51 1.51 1.51

1.3 21.42 1932 1.49 1.49 1.49

1.4 9 1932 1.49 1.49 1.49

1.5 13 1932 1.46 1.46 1.461.5 13 1932 1.46 1.46 1.46

1.6 21 1950 1.56 1.56 1.65

1.7 10.67 1979 1.30 1.30 1.30

1.8 21.25 1939 1.51 1.51 1.51

1.9 21.25 1923 1.53 1.46 1.54

1.10 21.25 1930 1.41 1.41 1.41

AVERAGE 1.46 1.46 1.48

STD DEV 0.08 0.07 0.09

Type 2 – Concrete Continuous Slabs

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

2.1 17.54 1953 1.56 1.56 1.55

2.2 43 1970 0.98 1.08 1.08

2.3 22 2004 1.61 1.27 1.66

2.4 42 2000 1.21 1.23 1.27

2.5 36 1998 1.39 1.33 1.38

2.6 28.25 1994 1.58 2.09 2.21

2.7 40 1969 1.42 1.19 1.42

2.8 32.5 1968 2.26 3.01 2.58

2.9 39.5 1977 1.28 1.28 1.28

2.10 37.5 1974 1.20 1.46 1.33

AVERAGE 1.45 1.55 1.58

STD DEV 0.35 0.58 0.47

Type 3 – Concrete T Beams, Simple Slabs

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

3.1 33.75 1962 3.80 3.93 3.50

3.2 42.33 1964 5.61 6.25 5.80

3.3 31 1969 3.59 3.72 3.25

3.4 26 1935 3.30 3.31 3.133.4 26 1935 3.30 3.31 3.13

3.5 39 1932 5.49 5.76 5.55

3.6 46.5 1935 1.35 1.35 1.35

3.7 41.33 1965 2.29 2.48 2.33

3.8 25.83 1957 4.02 4.03 3.84

3.9 41.42 1940 8.00 8.77 8.25

3.10 36 1970 3.83 4.00 3.66

AVERAGE 4.13 4.36 4.07

STD DEV 1.87 2.10 1.98

Type 4 – Rolled Shapes, Simple Spans

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

4.1 55.25 1966 1.16 1.16 1.16

4.2 74.27 1968 1.18 1.18 1.18

4.3 85.5 1990 1.10 1.04 1.04

4.4 65.17 1978 1.11 1.11 1.114.4 65.17 1978 1.11 1.11 1.11

4.5 63.33 1979 1.46 1.40 1.46

4.6 48.67 1981 1.05 1.05 1.05

4.7 70.97 1975 0.98 0.99 0.98

4.8 69.91 1961 1.22 1.22 1.22

4.9 63.46 1991 1.34 1.34 1.34

4.10 80.58 1991 1.08 1.07 1.08

AVERAGE 1.17 1.16 1.16

STD DEV 0.14 0.13 0.15

Type 5 – Rolled Shapes, Continuous Spans

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

5.1 49.67 1984 1.76 1.81 1.77

5.2 50.25 1984 1.70 1.69 1.66

5.3 65 1940 0.98 0.74 0.88

5.4 100 1980 1.37 1.08 1.175.4 100 1980 1.37 1.08 1.17

5.5 82 1948 0.86 0.79 0.85

5.6 75 1952 1.06 1.05 1.05

5.7 85.5 1954 1.56 1.56 1.09

5.8 41.9 1947 1.91 1.87 1.87

5.9 66 1976 1.62 1.61 1.62

5.10 45.32 1969 0.95 0.95 0.95

AVERAGE 1.38 1.31 1.29

STD DEV 0.39 0.44 0.40

Type 6 – Plate Girder, Simple Spans

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

6.1 92.92 1961 1.11 1.11 1.11

6.2 120.67 1978 1.53 1.37 1.43

6.3 48.25 1970 1.46 1.46 1.46

6.4 125.97 1966 1.44 1.49 1.346.4 125.97 1966 1.44 1.49 1.34

6.5 120.11 1994 1.55 1.10 1.23

6.6 104.74 1996 1.02 1.04 0.96

6.7 84 1996 1.19 1.23 1.19

6.8 105.04 2000 1.23 1.21 1.22

6.9 133.58 2002 1.21 1.19 1.20

6.10 81 2004 1.44 1.44 1.44

AVERAGE 1.32 1.26 1.26

STD DEV 0.19 0.16 0.16

Type 7 – Plate Girder, Continuous Spans

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

7.1 119 1985 0.95 0.98 0.96

7.2 131.83 1976 1.40 1.44 1.35

7.3 120.08 1972 1.48 1.41 1.317.3 120.08 1972 1.48 1.41 1.31

7.4 104.14 2000 1.35 1.34 1.27

7.5 25.768 1999 1.31 1.31 1.31

7.6 183 1998 1.13 1.13 1.05

7.7 125 2003 1.27 1.32 1.19

7.8 72.18 2000 0.94 0.77 0.77

7.9 164.53 2006 1.42 1.46 1.32

7.10 131.23 2003 1.37 1.57 1.41

AVERAGE 1.26 1.27 1.20

STD DEV 0.19 0.24 0.21

Type 8 – Prestressed Bulb Tees

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

8.1 111.2 2004 2.18 2.18 2.18

8.2 111.2 2004 2.18 2.18 2.18

8.3 118 2004 2.51 2.25 2.48

8.4 121.63 2006 1.31 1.31 1.31

8.5 88.09 2001 4.46 5.66 5.63

8.6 92 2003 12.94 13.94 13.88

8.7 102 2001 1.97 1.51 1.72

For the prestressed bulb tee bridges, given the large deviation on structure 8.6, average LRFR/LFR ratios results are given with and without this structure.

8.8 94.8 2001 1.56 1.26 1.25

8.9 94.8 2001 1.58 1.28 1.27

8.10 84 2005 1.52 1.97 1.81

AVERAGE 3.22 3.36 3.37

STD DEV 3.53 3.94 3.91

AVERAGE -8.6 2.14 2.18 2.20

STD DEV - 8.6 0.95 1.37 1.36

Type 9 – Prestressed AASHTO I-Beams, Simple

LRFR/LFR

Structure

Max Span

Length Year Built Type 3 Type 3-3 Type 3S2

9.1 53 1960 2.01 1.72 2.07

9.2 43 1959 1.07 0.98 1.16

9.3 51 1962 2.03 1.81 2.27

9.4 16 1999 1.81 1.78 1.989.4 16 1999 1.81 1.78 1.98

9.5 38 1990 1.28 1.16 1.48

9.6 51 1975 1.69 1.55 1.82

9.7 76 1967 1.30 1.21 1.24

9.8 78.65 1984 2.41 2.41 2.40

9.9 68 1967 1.30 1.17 1.29

9.10 42 1973 1.57 1.50 2.01

AVERAGE 1.65 1.53 1.77

STD DEV 0.42 0.42 0.45

Analysis by Span Length

120

140

160

180

200

Sp

an

Len

gth

(ft

)

Span Length - LRFR/LFR

0

20

40

60

80

100

0.6 0.8 1 1.2 1.4 1.6 1.8 2

Sp

an

Len

gth

(ft

)

LRFR / LFR

Type 3

Type 3-3

Type 3S2

Analysis by Year Built

1970

1980

1990

2000

2010

2020

Year

Bu

ilt

Year Built - LRFR/LFR

1910

1920

1930

1940

1950

1960

1970

0.6 0.8 1 1.2 1.4 1.6 1.8 2

Year

Bu

ilt

LRFR / LFR

Type 3

Type 3-3

Type 3S2

A word about Concrete

• Check you assumptions:

– As-Inspected conditions were ignored

– The lowest rated lowest rated structural member controlled the load rating for each method

• 2011 Manual for Bridge Evaluation • 2011 Manual for Bridge Evaluation (MBE) section 6A.5.8 – Evaluation for Shear states that for legal loads, in service concrete bridges need not be checked for shear unless evidence of shear distress is evident.

• Concrete bridges switched from shear to moment controlling the rating

Analysis Conclusions

• For most standard bridge types, materials, ages within 100 years, and span lengths, at the legal load levels, rating bridges with LRFR results in significant additional load carrying capacity

– 15% Average with steel– 15% Average with steel

– As much as 60%+ with concrete, based on condition per 6A.5.8

– Useful for bridge owners with borderline structures, or critical infrastructure links that posting would impact

• There is little correlation with the results and span length or structure age

Analysis Conclusions (cont.)

• There are no conclusions drawn from this analysis regarding design or permit loads using LRFR or LFR method

– Future Study

– NCHRP 12-78

Questions ?