Recalibration of LRFR Permit Live
Load Factors in the AASHTO
Manual for Bridge Evaluation
NCHRP 20-07/Task 285
Bala Sivakumar, HNTB Corp
Michel Ghosn, City College, NY
Objectives of this study
(March 2011 – June 2012)
Determine the appropriate target β for
permit trucks aligned with current
practice
Calibrate permit load factors for βTarget
for use with LRFD distribution analysis
and with refined analysis methods
Research Team
Bala Sivakumar
HNTB Corp
Principal Investigator
Michel Ghosn
City University of New York
Co-Principal Investigator
Permit Classifications
Routine Permits: unrestricted crossings; taken as up
to 100 crossings a day of a bridge
Single-trip Special Permits: The permit truck is
allowed to cross a bridge one time only but can mix
with regular truck traffic; and
Escorted Single-trip Special Permits: permit truck
crosses the bridge with no other vehicles allowed on
the bridge.
Multiple-trip Special Permits: less than 100
crossings of a bridge within the permit validity period;
LRFR Permits: Reliability Levels
Routine permits:
=
2.5
Special permits – single trip escorted:
= 2.5
Special permits – single & multiple trips
allowed to mix with traffic:
=
3.5
LRFR load Factors & Permit Load
Distribution
Routine Permits
Use LRFD two-lane distribution factors
Special Permits
Use LRFD one-lane distribution factor
excluding multiple presence factor of 1.2
LRFR Permits
ROUTINE
PERMIT
ROUTINE
PERMIT
2 Lane
DF
Random
Legal
SPECIAL
PERMIT
1 Lane
DF
SPECIAL
PERMIT
1 Lane
DF
9-7
LRFR Routine Permit
Load Factors
Distribution ADTT
Factor
(1-Way)
Two or More
Lanes
9-8
≥ 5000
= 1000
≤ 100
Load Factor by
Permit Weight
≤ 100 kips
≥ 150 kips
1.80
1.30
1.60
1.40
1.20
1.10
LRFR Special Permit
Load Factors
Trips
Other
Traffic
DF
Single Trip
Escorted,
no other vehicles
1 Lane
Single Trip
Mix with traffic
(other vehicles
may be on the
bridge)
Multiple
Trips
(< 100)
Mix with traffic
(other vehicles
may be on the
bridge)
1 Lane
1 Lane
Load
ADT
Fact
T
or
N/A
1.15
≥ 5000
1.50
= 1000
1.40
≤ 100
1.35
≥ 5000
1.85
= 1000
1.75
≤ 100
1.55
Reliability levels
in
Current LFR Ratings
at
the Operating Level
Average β for legal Trucks at LFR
Operating Level
Using Recent National WIM Data from Interstate Sites with Heavy
Trucks
ADTT=5000
ADTT=1000
ADTT=100
Moment
Shear
Moment
Shear
Moment
Shear
T-BEAM
1.291
0.978
1.855
1.180
1.855
1.558
P/S
1.083
0.995
1.329
1.204
1.783
1.595
NC STEEL
Comp.
Steel
1.124
0.809
1.361
1.067
1.799
1.547
1.113
0.793
1.351
1.052
1.790
1.533
Average Operating Level Reliability = 1.35
Average β for LFR Routine Permit Ratings
LFR Operating Rating, Two Lane DF
ADTT=5000
ADTT=1000
Mome
nt
Shear
Moment
Shear
Moment Shear
T-BEAM
2.71
2.58
2.75
2.61
2.83
2.68
P/S Conc
2.99
2.66
3.05
2.69
3.25
2.76
NC STEEL
3.01
3.10
2.99
3.14
3.10
3.23
2.93
3.09
2.98
3.13
3.10
3.23
Comp. Steel
Average β = 2.94
ADTT=100
Composite Steel Bridges
LFR Permit Ratings
Average b Index vs Span Length
COMP. STEEL, ADTT = 5000, MOMENT LOAD EFFECT, ALL SPACINGS
S = 4 FT
S = 6 FT
S = 8 FT
S = 10 FT
S = 12 FT
6.0
Reliability Index,
b
5.0
4.0
3.0
2.0
1.0
0.0
0
50
100
150
Span Length [ft]
200
250
Average β for LFR Routine Permit Ratings
LFR Operating Rating
Overall average reliability index for Routine
Permits is on the order of 2.94.
considerably higher than β for legal load rating
Average reliability index for LFR legal load
Operating ratings is less than 2.0.
LFR ratings may be imposing unnecessary
conservatism on truck permits.
β for Special Permits
For special permits, the reliability index
values will be higher than those of the
routine permits.
Special permits will have: a) no
uncertainties in their weights and b)
reduced probability of running
alongside a heavy random truck.
Recalibration of LRFR Permit
Load Factors
Criteria for Permit Recalibrations
The target reliability index set for
the calibration is βtarget=2.5
Reliability index values for all
conditions that remain above a
minimum βmin=1.50.
Live load factor:
Min gL=1.10
Recalibration Approach
Use recent Weigh-In-Motion data from a
number of representative U.S. sites
Use representative samples of Routine and
Special Permits from several states (NCHRP
12-78)
Use side-by-side probabilities from WIM data
Compare LRFD distribution factors to refined
analysis / measured values, include bias and
COV in reliability calculations
Bridge Types
The analysis covered the bending
and shear loading effects
Simple spans, 20 ft to 200 ft
T-beams, prestressed I-beams,
composite and noncomposite steel
I-beams.
Traffic Data for Recalibration
• Truck WIM data from Representative
National Interstate Sites
• Typical Permits configurations from national
survey.
• Special permit weights are taken to be
accurately known.
• Routine permit weights have some
uncertainty in the GVW.
Recent Traffic Data from Six
National WIM Sites
NY Site 9121 --- I-81
CA Site 0001
--- I -5
FL site 9926
--- I-75
IN Site 9512
--- I-74
MS Site 2606
--- I -55
TX Site 0526 --- I -20
Truck Multiple Presence Statistics
from WIM Data
Permit Recalibration
Distribution Analysis
AASHTO LRFD Distribution Factors
Has limitations for checking permits
Two unequal trucks side-by-side
Non-standard gage widths
Refined Analysis Methods
LRFR permit load factors were not calibrated
for use with refined analysis
MBE lacks guidance on load factors for
permit and adjacent truck
LRFR Permit Load Factors for use
with Refined Methods of Analysis
•Refined analysis use is more widespread
•Refined analysis will lower the COV
• LRFD DF has a conservative bias
• Calibrated load factors are required for
the permit load and for the alongside
random truck for use with refined methods
of analysis
Comparison of AASHTO LRFD
Distribution Factors to Refined Analysis
MOMENT
Ratio of AASHTO
LRFD / Rigorous
Methods
INTERIOR
SHEAR
EXTERIOR
INTERIOR
EXTERIOR
1 LANE
2 LANES 1 LANE
2 LANES
1 LANE
2 LANES
1 LANE
2 LANES
Steel
I
Girder
AVERAGE
1.294
1.113
1.369
1.191
1.388
1.216
1.358
1.436
COV
0.072
0.115
0.131
0.119
0.115
0.162
0.132
0.197
Conc.
T-Beam
AVERAGE
1.264
1.074
1.415
1.241
1.312
1.134
1.415
1.337
COV
0.133
0.125
0.147
0.160
0.091
0.157
0.204
0.250
AVERAGE
1.292
1.109
1.348
1.208
1.296
1.134
1.305
1.307
COV
0.083
0.104
0.132
0.221
0.072
0.136
0.132
0.239
P/S
I
Girder
Comparison of AASHTO LRFD
Distribution Factors to Refined Analysis
Interior beams under two lanes of loading
give the closest results compared to those
of the refined analysis but are still about
11% more conservative with a COV =13%.
Shear of exterior beams under two lanes of
loading, show that the AASHTO LRFD may
be up to 44% more conservative than
observed from the refined analysis with a
COV that may reach up to 25%.
Reliability Indices with Refined
Analyses
NCHRP 592 (2007) found that the LRFD DF can be
significantly more conservative when compared to
those obtained from refined analysis and field
measurements
The reliability indices when the Permit check is
performed using refined analysis will be lower when
compared to the values obtained if the Permit
checking uses the AASHTO LRFD DF.
Permit Recalibration
Load Cases
Recalibration reflects four load cases:
Case I: Special permit vehicle alone
Case II: Routine permit vehicle
alongside another permit
Case III: Routine permit vehicle
alongside a random vehicle.
Case IV: Special permit vehicle
alongside a random vehicle
Case I -- Special Permit Alone
Refined Analysis & LRFD 1 Lane DF
SPECIAL PERMIT
RIGOROUS
T-beam bending
T-beam shear
Ps/C bending
Ps/C shear
Noncomp. Bending
Noncomp. Shear
Comp. bending
Comp. shear
ALL BRIDGE TYPES
Min
2.804
2.805
2.586
2.272
2.658
2.681
2.657
2.679
gL=1.10
Max
3.129
3.009
2.902
2.366
2.886
2.949
2.884
2.948
Ave.
3.002
2.930
2.825
2.344
2.833
2.879
2.831
2.877
2.272
3.129
2.815
SPECIAL PERMIT
gL=1.15 gL=1.10
AASHTO
Min LRFDMax Min Ave. Max
T-beam
2.843 bending
3.303 3.007
3.109 3.660
T-beam
2.865 shear3.165 3.148
3.048 3.901
Ps/C bending
2.933
3.847
2.744
3.063
2.966
Ps/C shear
2.558
3.346
2.319
2.507
2.462
Noncomp.
2.819
3.029 2.999
2.966 3.882
Bending
2.926
3.085 3.305
3.041 4.036
Noncomp.
Shear
2.818 bending
3.028 3.000
2.965 3.882
Comp.
2.926 shear 3.084 3.308
3.040 4.036
Comp.
ALL
BRIDGE TYPES
2.319
3.303 2.558
2.950 4.036
Ave.
3.413
3.594
3.544
3.025
3.553
3.883
3.555
3.885
3.557
Case IV – Special Permit Mixed
with Random Truck (1 Lane DF)
SPECIAL
PERMIT
AASHTO
LRFD
1 LANE
DF
T-beam
bending
T-beam
shear
Ps/C
bending
Ps/C
shear
Noncomp.
Bending
Noncomp.
Shear
Comp.
bending
Comp.
shear
ALL
BRIDGE
TYPES
gL=1.10
gL=1.40
Min
Max
Ave.
Min
Max
Ave
1.18
2.74
2.28
1.98
3.53
2.89
1.36
3.11
2.55
2.07
4.00
3.22
0.56
2.67
1.98
1.54
3.58
2.75
0.82
2.64
2.06
1.57
3.58
2.75
0.83
2.69
2.02
1.72
3.47
2.75
1.11
3.43
2.56
1.94
4.38
3.43
0.82
2.68
2.01
1.72
3.47
2.75
1.11
3.43
2.56
1.94
4.38
3.43
0.56
3.43
2.25
1.54
4.38
3.00
Case IV -- Special Permit Alongside
Random
• Live Load Model: add effect of Permit to
effect of random truck
• Limited number of crossings (100 total)
• Conservatively assume that a random truck
always crosses alongside Special Permit
• Use gL =1 .4 with D.F. from LRFD Tables.
• Use gL =1 .0 for Permit and gL =1.1 for
alongside AASHTO Legal with refined
analysis.
GVW / AL < 2.0
(kip/ft)
1.40
Routine or
Annual
Permit Weight Ratio
2.0 < GVW/AL < 3.0
(kip/ft)
1.35
GVW/AL > 3.0
(kip/ft)
1.30
>5000
Recommended
LRFR
Load Factors
Two or
=1000
1.35
1.25
more lanes
Unlimited
<100
1.30
1.20
Special Permits
Crossings
For refined analysis
1.20
1.15
Add 0.10 to above load factors
Load Factor for All Weights
Single-Trip
(escorted)
One lane
All ADTTs
For refined analysis
Special or
Limited
Crossing
Single-Trip
or less than
100 trips
One lane
All ADTTs
For refined analysis
1.10
gL=1.10
(for crawl speed with refined analysis use IM=1.05)
1.40
gL=1.0 for Permit with gL=1.1 for governing AASHTO legal
For escorted Special Permits at crawl speed, use
IM=1.05 with refined analysis to maintain a
β min≥ 1.50
LRFR Special Permit load Factors
Old vs New
Trips
Other
Traffic
ADTT
Load
Factor
Single Trip
Escorted,
no other
vehicles
N/A
1.15 / 1.10
≥ 5000
1.50 / 1.40
= 1000
1.40 / 1.40
≤ 100
1.35 / 1.40
≥ 5000
1.85 / 1.40
= 1000
1.75 / 1.40
≤ 100
1.55 / 1.40
Single Trip
Multiple Trips
Mix with traffic
Mix with traffic
Routine Permits
Load Cases
• Case II= Two permits side-by-side
does not govern because of low
probability of side-by-side permits
• Case III = Routine permit alongside
Random Truck governs min=1.5
governs
Case III – Routine Permit Mixed
with Random Truck (2 Lane DF)
ROUTINE
PERMIT
ADTT
=
5000
T-beam
bending
T-beam
shear
PS/C
bending
PS/C
shear
Noncomp.
Bending
Noncomp.
Shear
Comp.
bending
Comp.
shear
ALL
BRIDGE
TYPES
gL=1.10
gL=1.25
gL=1.40
Min
Max
Ave.
Min
Max
Ave
Min
Max
Ave
1.22
2.75
2.25
1.67
3.19
2.57
2.08
3.62
2.88
1.51
2.90
2.40
1.92
3.26
2.73
2.29
3.67
3.05
0.68
2.69
1.95
1.21
3.10
2.36
1.70
3.67
2.75
1.01
2.40
1.91
1.43
2.79
2.25
1.81
3.23
2.57
0.93
2.71
1.99
1.42
3.06
2.38
1.87
3.59
2.75
1.33
3.11
2.37
1.80
3.52
2.80
2.23
3.96
3.20
0.93
2.70
1.99
1.42
3.06
2.38
1.87
3.59
2.75
1.33
3.11
2.36
1.80
3.52
2.80
2.23
3.96
3.20
0.68
3.11
2.15
1.21
3.52
2.53
1.70
3.96
2.89
Recommended LRFR Load Factors
Routine Permits
Permit Type
Routine or
Annual
Frequency
Unlimited
Crossings
DF
Two or
more lanes
>5000
GVW / AL < 2.0
(kip/ft)
1.40
Load Factor by
Permit Weight Ratio
2.0 < GVW/AL < 3.0
(kip/ft)
1.35
GVW/AL > 3.0
(kip/ft)
1.30
=1000
1.35
1.25
1.20
<100
1.30
1.20
1.15
ADTT
For refined analysis
Add 0.10 to above load factors
Load Factor for All Weights
GVW = Gross Vehicle Weight
One lane
All ADTTs
Single-Trip
(escorted)
AL = Front axle to rear axle length
Special or
Limited
Crossing
1.10
gL=1.10
Use only axles on the bridge (for crawl speed with refined analysis use IM=1.05)
For refined analysis
Single-Trip
One lane
All ADTTs
1.40
Recommendations
Routine Permits
Live load factors for routine permits can be
reduced for the cases where the Permit truck’s
Gross vehicle weight is high
Reflects the lower probability of having a
random truck of equal or higher weight crossing
alongside the Permit truck.
LRFR Routine Permit load Factors
Old vs. New
9-38
Load Factor
Distribution
Factor
ADTT
Two or More
Lanes
≥ 5000
= 1000
≤ 100
Lighter
Heavier
1.80 / 1.40 1.30 / 1.30
1.60 / 1.35 1.20 / 1.20
1.40 / 1.30 1.10 / 1.15
MBE Revisions
Deliverables
Recommended revisions to LRFR
permit rating specifications and
commentary suitable for inclusion in
the AASHTO MBE.
New table of LRFR permit load
factors
Table 6A.4.5.4.2a-1—Permit Load Factors: γL
Permit Type
Routine or
Annual
Special or
Limited
Crossing
Frequency
Unlimited
Crossings
Single-Trip
Single-Trip
or MultipleTrips (less
than 100
crossings
Loading Condition
Mix with traffic
(other vehicles
may be on the
bridge)
Escorted with no
other vehicles on
the bridge
Mix with traffic
(other vehicles
may be on the
bridge)
a
DF
Two or
more
lanes
ADTT (one
direction)
>5000
Load Factor by
b
Permit Weight Ratio
GVW /
2.0 <
GVW/AL
AL < 2.0
GVW/AL
> 3.0
(kip/ft)
< 3.0
(kip/ft)
(kip/ft)
1.40
1.35
1.30
=1000
1.35
1.25
1.20
<100
1.30
1.20
1.15
One lane
N/A
All Weights
1.10
One lane
All ADTTs
1.40
Closing
• The recalibration performed in this
study is based:
• National WIM data for random trucks that may
cross alongside Permits
• Used representative permit configurations
• Reliability levels better aligned with LFR ratings
• Accounts for conservative bias in LRFD
distribution factors compared to FEM
• Calibration criteria: average
target=2.5, min=1.5, min gL=1.1
Closing
Avoids unnecessary conservatism in permit
ratings as MP events are low
Promotes greater use of refined analysis for
permit evaluations
Will be beneficial for LRFR permit ratings
T-18 should consider for adoption in 2012
