Introduction to LRFR

1-1

Learning Outcomes

Identify the primary purposes of load rating highway bridges

Understand the benefits of LRFR

Identify key differences between LRFR and LFR

1-2

What is a Load Rating?

Vehicular live load capacity of a bridge• Using as-built bridge plans

• Using latest field inspection (NBIS)

Expressed as a Rating Factor (RF) or in tonnage for a particular vehicle

1-3

Purposes of Load Rating

Ensure bridge safety

Rehabilitation or replacement needs

Submit data to the National Bridge Inventory - Comply with NBI Standards

Posting needs

Processing of overload permits

1-4

LRFR Rating Process

Design Load Rating NBI data(HL-93)

Legal Load Rating Posting(AASHTO and State Legal Loads)

Permit Load Rating Permits(Overweight Trucks)

3-5

NBIS – 23 CFR 650 Subpart C

650.313 Inspection Procedures• (c) ‘Rate each bridge as to its safe load

carrying capacity in accordance with the AASHTO Manual. Post or restrict the bridge in accordance with the AASHTO Manual when unrestricted legal loads or State routine permit loads exceed….’

1-6

When Should a LoadRating be Performed?

Design stage

Initial inventory inspection

Change in the live loading

Change in the dead load on the structure

Physical change in any structural member of the bridge

1-7

Elements of a Bridgeto be Load Rated (cont.)

Dead Load Considerations

Typical Stringer Bridge

Interior Stringer

Exterior Stringer

1-8

Elements of a Bridgeto be Load Rated (cont.)

Truss Member

Floorbeam

Truss Connection

Steel Pier Bent

Deck Truss Bridge

Stringer

1-9

LRFR Limit States andReliability Index

Strength Limit State has been calibrated to achieve uniform safety using structural reliability methods

Reliability Index ‘’ provides a new measure of safety that is statistically based

Service Limit State not calibrated. Several works underway to move to calibrated limit states

2-10

LRFR Serviceability Considerations

SERVICE I – Permanent deformation of reinforcing steel in R/C & P/Sconcrete members for permit loads

SERVICE II – Permanent deformation of steelmembers

SERVICE III – Cracking of P/S concrete members

FATIGUE – Fatigue of steel members

6-11

2-12

Q

Q

Probabilistic Designand Evaluation

LOAD, Q

R,QPf

RESISTANCE, R

RESISTANCE MARGIN

LOAD MARGIN

CENTRAL SAFETY MARGIN

RRD

2-13

As-built-Q

Probabilistic Designand Evaluation (cont.)

As-built-R

Increase over time-QDecrease over time-R

Reliability Index, ‘’

Design Level Reliability: = 3.5 or 1 in 5,000 notional probability of exceedence

Minimum for Evaluation: = 2.5or 1 in 1,200 notional probability of exceedence

2-14

Reliability Indices

LFD LRFD

β ~ 3.5

β ~ 2.5

LRFR (OP)

Bet

a

LRFR (INV)

2-15

Minimum Reliability for LRFR = 2.5

Comparable to average reliability inherent in load factor ratings at Operating Level

Shown to be an acceptable minimumlevel of safety for bridge evaluation

Exposure period for evaluation is 2-5 years vs. 75 years for design

2-16

Reliability Index vs.LRFR and LFR

2-17

2.5

DESIGN LOAD OPERATING RATING FACTOR

RE

LIA

BIL

ITY

IN

DE

X,

1.25 1.5 1.75 2.0 2.25

3.0

2.0

1.0

0

LRFR

LFR

1.00.5

0.5

3.5

2.5

1.5

4.0

4.5

0.75

Study’s Key Findings

LRFR ratings correlate well with limit state exceedence rates

LFR ratings did not correlate well

2-18

Uniform Reliability Predictors

Live load model

Distribution factors

Multiple presence of live loads

Resistance formulations (LRFD)

2-19

Live Load Effect on Reliability

Uniform reliability requires uniform bias for load effects across all span lengths

Force effects from HS20 load model to “exclusion vehicles” does not provide a uniform bias

New live load model needed to achieve uniform reliability

2-20

SPAN IN FEET

Actual vs. Design Load Moments

2-21

MOMEN

T RATIO

0 20 40 60 80 100 120 140 1600.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

EXCLUSION VEHICLESAASHTO HS‐20

EXCLUSION VEHICLESLRFD Load Model

What are Exclusion Loads?

Trucks exempted from Federal weight laws

Comply with state vehicle weight regulations

Allowed to operate on non-interstate highways

2-22

Michigan “Exclusion Truck”Load: 3-S3-5

Total weight = 77 ton Total length = 72.4 ft.

2-23

What makes this an “Exclusion Truck?”

2-24

AASHTO Standard Spec. Distribution Factors (DF)

Where:

g = distribution factor, DF (wheel load fraction)

S = average stringer spacing

D = function of # lanes and deck type

5-25

5-26

Influence of DF

S/D Distribution Factorand Reliability Index

Simple Span Moments in Steel Girders

5-27

Influence of DF

Simple Span Shear in Steel Girders

S/D Distribution Factorand Reliability Index

Distribution Factor for moment

Distribution Factor for shear

5-28

LRFD Distribution Factor:Interior Longitudinal Member

5-29

DF for Moment:Interior Beams

One Lane Loaded

Two or More Lanes Loaded

DF for Shear:Interior Beams

5-30

One Lane Loaded

Two or More Lanes Loaded

Vehicular Live Loads

Design load (national)

Legal loads (local)

Permit loads (local)

1-31

LRFR Load Rating Process

3-32

Legal

Design (HL-93)

Permit

• Load Posting

• Strengthening

Post or Restrict For Routine Permits

RF 1

RF < 1RF < 1

NBI reporting

RF 1RF < 1

Federal Weight Limits Single Axle Limit – 20,000 lbs Tandem Axle Limit – 34,000 lbs Gross Vehicle Limit – 80,000 lbs Bridge Formula ‘B’

4-33

Where N = # of axlesL = distance between first and last axle (ft)W = weight (lbs)

LRFD Live Load (HL-93)

Design TruckPLUS

Design Lane

8 kips 32 kips 32 kips

14’ Varies

0.64 kips/foot

25 kips 25 kips

4'

0.64 kips/foot

or

(14’ to 30’)

2-34

Design TandemPLUS

Design Lane

Legal Load Rating

Provides single level load rating

RF ≥ 1.0 Safe for unrestricted indefinite use

RF < 1.0 Need for posting or bridge strengthening

3-35

Routine Commercial TrafficTruck Models

15.5k

11’ 4’22’

15.5k15.5k15.5k10k

4’

Type 3S2 W=72 kips

16’15’ 4’ 15’ 4’12k 12k 12k 15k 14k 14k

Type 3-3 W=80 kips

15’ 4’

17k17k16k

Type 3 W=50 kips

4-36

Routine Commercial Traffic AASHTO Legal Loads

7-37

Specialized Hauling Vehicles (SHV)

Adopted by AASHTO in 2005 to represent new truck models

Trucks comply with Formula ‘B’ – and meet all Federal weight regulations

High axle loads concentrated over shorter distance

Moveable axles – raise/lower as needed for weight

4-38

Examples of SHVs

SHVs withlift axles

4-39

Why SHVs?

AASHTO Type 3

Weight = 50 kips

16k 17k 17k

15’19’

4’

4-40

Example Overstressesfor SHVs

4-41

SU4 TruckGVW = 54 kips

8k12k 17k 17k

10’ 4’4’

17k8k12k 17k

10’ 4’4’4’

8k

SU5 TruckGVW = 62 kips

4-42

AASHTO SHV Posting Loads

SU6 TruckGVW = 69.5 kips

SU7 TruckGVW = 77.5 kips

8k8k8k11.5k 8k17k17k

10’ 4’4’4’4’ 4’

8k8k11.5k 8k17k17k

10’ 4’4’4’4’

AASHTO SHV Posting Loads

4-43

SHVs AASHTO Legal Loads

7-44

Permit Load Rating

Single level rating

Permits only allowed on bridges having RF ≥ 1.0 for legal loads or HL-93

RF ≥ 1.0 safe for permit crossing

Permit rating based on permit type:• Routine / Annual Permits• Special / Single-Trip Permits

3-45

LRFR procedures for permit review

Permit Types:• Routine/Annual Permits ≤ 150 K

• Special Permits > 150 K

4-46

Overload Permits

Example: Special Permit

9-47

Oversized Superloads

9-48

Example: Routine Permit

Oregon 8-Axle Continuous Trip Permit 105.5 k

9-49

LRFR Routine Permit Load Factors: STRENGTH II

7-507-50

Permit Weight Ratio = GVW/AL

GVW = Gross Vehicle Weight (kips)

AL = Front axle to rear axle length (ft) (use only axles on the bridge)

LRFR Special Permit Load Factors: STRENGTH II

7-517-51

Load Posting Signs

8-52

Bridge Posting in LRFR

LRFR posting analysis deviates from past practice

Maintains same reliability for posted and non-posted bridges

Load posted bridges = high overload probabilities

• A more conservative posting is required to maintain the same reliability

8-53

LRFR Posting Analysis

When RF < 0.3 for a vehicle, restrict that vehicle type RF < 0.3 for all posting vehicles, close bridge

to all truck traffic

When 0.3 < RF < 1.0

Posting Load = (W / 0.7) x [(RF) - 0.3]• W= Weight of rating vehicle (tons)• RF= Legal load rating factor (controlling)

Any posting load < 3 tons, close bridge

8-54

Posting Load vs. Rating Factor

8-55

LRFR Loading Rating Equation

3-56

Differences between LRFR and LFR

Design load model, distribution factors, capacity reduction factors, load factors Routine service limit state evaluation LRFR consistently correlates well to

probability of exceedance Single level evaluation for posting and

permitting for LRFR Resistance calculations similar, but

some advances in LRFR

1-57

Summary of Benefits of LRFR

Uniform reliability in bridge analysis More uniform posting levels Guidance for evaluation of overloads Procedures to ensure more consistency and

uniformity in rating (Øc, Øs, DLA) Optimal load factors for lower volume roads Introduces state-of-the-art technologies that

could benefit existing bridges Evaluation of serviceability or service limit

states

1-58

Questions or Discussion?

1-59