Recalibration of LRFR Permit Live Load Factors in the AASHTO Manual for Bridge Evaluation

NCHRP 20-07/Task 285

Bala Sivakumar, HNTB CorpMichel 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 PermitsUse LRFD one-lane distribution factor

excluding multiple presence factor of 1.2

LRFR Permits

9-9-77

ROUTINE PERMIT

ROUTINE PERMIT

SPECIAL PERMIT

Random Legal

SPECIAL PERMIT

2 Lane DF

1 Lane DF

1 Lane DF

LRFR Routine PermitLoad Factors

9-9-88

Distribution Factor

ADTT(1-Way)

Load Factor by Permit Weight

≤ 100 kips

≥ 150 kips

Two or More Lanes

≥ 5000 1.80 1.30

= 1000 1.60 1.20

≤ 100 1.40 1.10

LRFR Special PermitLoad Factors

TripsOther Traffic

DFADT

T

Load Factor

Single TripEscorted,

no other vehicles1 Lane N/A 1.151.15

Single Trip

Mix with traffic (other vehicles may be on the

bridge)

1 Lane

≥ 5000 1.501.50

= 1000 1.401.40

≤ 100 1.351.35

Multiple Trips (< 100)

Mix with traffic (other vehicles may be on the

bridge)

1 Lane

≥ 5000 1.851.85

= 1000 1.751.75

≤ 100 1.551.55

Reliability levels in

Current LFR Ratings at

the Operating Level

Average β for legal Trucks at LFR Operating Level

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 1.124 0.809 1.361 1.067 1.799 1.547 Comp. Steel 1.113 0.793 1.351 1.052 1.790 1.533

Average Operating Level Reliability = 1.35

Using Recent National WIM Data from Interstate Sites with Heavy Trucks

Average β for LFR Routine Permit Ratings LFR Operating Rating, Two Lane DF

 ADTT=5000 

ADTT=1000 

ADTT=100 

 Moment 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

Comp. Steel2.93 3.09 2.98 3.13 3.10 3.23

Average β = 2.94

Composite Steel BridgesLFR Permit Ratings

Average Index vs Span LengthCOMP. STEEL, ADTT = 5000, MOMENT LOAD EFFECT, ALL SPACINGS

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 50 100 150 200 250

Span Length [ft]

Relia

bilit

y In

dex,

S = 4 FT S = 6 FT S = 8 FT S = 10 FT S = 12 FT

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 L=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 RecalibrationDistribution 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

Ratio of AASHTO LRFD / Rigorous Methods

MOMENT SHEAR

INTERIOR EXTERIOR INTERIOR EXTERIOR

1 LANE 2 LANES 1 LANE 2 LANES 1 LANE 2 LANES 1 LANE 2 LANES

SteelIGirder

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

P/SIGirder

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

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 RecalibrationLoad Cases

Recalibration reflects four load cases:

Case I: Special permit vehicle aloneCase II: Routine permit vehicle

alongside another permitCase III: Routine permit vehicle

alongside a random vehicle.Case IV: Special permit vehicle

alongside a random vehicle

Case I -- Special Permit AloneRefined Analysis & LRFD 1 Lane DF

L=1.10 SPECIAL PERMIT RIGOROUS Min  Max  Ave. 

T-beam bending  2.804  3.129  3.002 T-beam shear  2.805  3.009  2.930 Ps/C bending  2.586  2.902  2.825 Ps/C shear  2.272  2.366  2.344 

Noncomp. Bending  2.658  2.886  2.833 Noncomp. Shear  2.681  2.949  2.879 Comp. bending  2.657  2.884  2.831 Comp. shear  2.679  2.948  2.877 

ALL BRIDGE TYPES  2.272  3.129  2.815 

SPECIAL PERMIT AASHTO LRFD

L=1.10 Min  Max  Ave. 

T-beam bending  3.007  3.660  3.413 T-beam shear  3.148  3.901  3.594 Ps/C bending  2.933  3.847  3.544 Ps/C shear  2.558  3.346  3.025 Noncomp. Bending  2.999  3.882  3.553 

Noncomp. Shear  3.305  4.036  3.883 Comp. bending  3.000  3.882  3.555 Comp. shear  3.308  4.036  3.885 ALL BRIDGE TYPES  2.558  4.036  3.557 

Case IV – Special Permit Mixed with Random Truck (1 Lane DF)

SPECIAL PERMIT AASHTO LRFD 1 LANE DF

L=1.10 L=1.40

Min  Max  Ave.  Min  Max  Ave 

T-beam bending  1.18  2.74  2.28  1.98  3.53  2.89 

T-beam  shear 

1.36  3.11  2.55  2.07  4.00  3.22 

Ps/C  bending  0.56  2.67  1.98  1.54  3.58  2.75 

Ps/C  shear  0.82  2.64  2.06  1.57  3.58  2.75 

Noncomp. Bending  0.83  2.69  2.02  1.72  3.47  2.75 

Noncomp. Shear 

1.11  3.43  2.56  1.94  4.38  3.43 

Comp. bending 

0.82  2.68  2.01  1.72  3.47  2.75 

Comp.  shear 

1.11  3.43  2.56  1.94  4.38  3.43 

ALL BRIDGE TYPES  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 L =1 .4 with D.F. from LRFD Tables.

• Use L =1 .0 for Permit and L =1.1 for alongside AASHTO Legal with refined analysis.

Recommended LRFR Load FactorsSpecial Permits

Load Factor by Permit Weight Ratio

Permit Type

Frequency

DF

ADTT GVW / AL < 2.0

(kip/ft) 2.0 < GVW/AL < 3.0

(kip/ft) GVW/AL > 3.0

(kip/ft) >5000 1.40 1.35 1.30

=1000 1.35 1.25 1.20 Two or

more lanes <100 1.30 1.20 1.15 Routine or

Annual Unlimited Crossings

For refined analysis

Add 0.10 to above load factors

Load Factor for All Weights

One lane All ADTTs 1.10 Single-Trip (escorted)

For refined analysis

L=1.10 (for crawl speed with refined analysis use IM=1.05)

One lane All ADTTs 1.40

Special or Limited Crossing

Single-Trip or less than 100 trips For refined analysis L=1.0 for Permit with L=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 FactorsOld vs New

TripsOther Traffic

ADTTLoad

Factor

Single TripEscorted, no other vehicles

N/A 1.15 / 1.15 / 1.101.10

Single Trip Mix with traffic

≥ 5000 1.50 / 1.50 / 1.401.40

= 1000 1.40 / 1.40 / 1.401.40

≤ 100 1.35 / 1.35 / 1.401.40

Multiple Trips

Mix with traffic

≥ 5000 1.85 / 1.85 / 1.401.40

= 1000 1.75 / 1.75 / 1.401.40

≤ 100 1.55 / 1.55 / 1.401.40

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

L=1.10  L=1.25  L=1.40 

Min  Max  Ave.  Min  Max  Ave  Min  Max  Ave 

T-beam bending  1.22  2.75  2.25  1.67  3.19  2.57  2.08  3.62  2.88 

T-beam  shear  1.51  2.90  2.40  1.92  3.26  2.73  2.29  3.67  3.05 PS/C  bending  0.68  2.69  1.95  1.21  3.10  2.36  1.70  3.67  2.75 PS/C  shear  1.01  2.40  1.91  1.43  2.79  2.25  1.81  3.23  2.57 Noncomp. Bending  0.93  2.71  1.99  1.42  3.06  2.38  1.87  3.59  2.75 Noncomp. Shear  1.33  3.11  2.37  1.80  3.52  2.80  2.23  3.96  3.20 Comp. bending  0.93  2.70  1.99  1.42  3.06  2.38  1.87  3.59  2.75 Comp.  shear  1.33  3.11  2.36  1.80  3.52  2.80  2.23  3.96  3.20 ALL BRIDGE TYPES  0.68  3.11  2.15  1.21  3.52  2.53  1.70  3.96  2.89                         

Recommended LRFR Load FactorsRoutine Permits

Load Factor by Permit Weight Ratio

Permit Type

Frequency

DF

ADTT GVW / AL < 2.0

(kip/ft) 2.0 < GVW/AL < 3.0

(kip/ft) GVW/AL > 3.0

(kip/ft) >5000 1.40 1.35 1.30

=1000 1.35 1.25 1.20 Two or

more lanes <100 1.30 1.20 1.15 Routine or

Annual Unlimited Crossings

For refined analysis

Add 0.10 to above load factors

Load Factor for All Weights

One lane All ADTTs 1.10 Single-Trip (escorted)

For refined analysis

L=1.10 (for crawl speed with refined analysis use IM=1.05)

One lane All ADTTs 1.40

Special or Limited Crossing

Single-Trip or less than 100 trips For refined analysis L=1.0 for Permit with L=1.1 for governing AASHTO legal

GVW = Gross Vehicle Weight

AL = Front axle to rear axle length Use only axles on the bridge

RecommendationsRoutine 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 FactorsOld vs. New

9-9-3838

Distribution Factor

ADTTLoad Factor

Lighter Heavier

Two or More Lanes

≥ 5000 1.80 / 1.40 1.30 / 1.30

= 1000 1.60 / 1.35 1.20 / 1.20

≤ 100 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 Frequency Loading Condition DFa ADTT (one direction)

Load Factor by Permit Weight Ratiob

GVW / AL < 2.0 (kip/ft)

2.0 < GVW/AL < 3.0 (kip/ft)

GVW/AL > 3.0 (kip/ft)

Routine or Annual

Unlimited Crossings

Mix with traffic (other vehicles may be on the bridge)

Two or more lanes

>5000 1.40 1.35 1.30

=1000 1.35 1.25 1.20

<100 1.30 1.20 1.15

All Weights

Special or Limited Crossing

Single-Trip Escorted with no other vehicles on the bridge

One lane N/A 1.10

Single-Trip or Multiple-Trips (less than 100 crossings

Mix with traffic (other vehicles may be on the bridge)

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 L=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