Session 04 What is the Fatigue Truck[1]

8/4/2019 Session 04 What is the Fatigue Truck[1]

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Session 4ession 4

Loads for Fatigue Designoads for Fatigue Design

orr

What is the AASHTOhat is the AASHTO

Fatigue Truck?atigue Truck?



Must Distinguish between Loads forust Distinguish between Loads for

Fatigue vs. Other Limitatigue vs. Other Limit-statestates

Limit State:Limit State:

“ “ Condition beyond which the bridge or Condition beyond which the bridge or

component ceases to satisfy the provisions component ceases to satisfy the provisions

for which it was designed for which it was designed ” ”

(from AASHTO LRFD)(from AASHTO LRFD)



Common Limitommon Limit-statestates

Service limitService limit

--statestate

Relates to stress, deformation, cracking,Relates to stress, deformation, cracking,

relies somewhat on experiencerelies somewhat on experience

Strength limitStrength limit--statestate Relates strength & stabilityRelates strength & stability

Extensive distress & structural damageExtensive distress & structural damage

Structural integrity is maintainedStructural integrity is maintained



Common Limitommon Limit-statestates

Extreme eventExtreme event

Unique occurrenceUnique occurrence

collision, flood, earthquakecollision, flood, earthquake

Ensure survival of structureEnsure survival of structure

Fatigue and fracture limitFatigue and fracture limit--statestate

Limit cracking under repeated loadsLimit cracking under repeated loads

Prevent fracturePrevent fracture



Clearly, structures are subjected toClearly, structures are subjected to

many different types of loads for many different types of loads for

each limiteach limit--statestate

This limit-state vs. …….



This limit-state !



But itut it ’s still standing!!still standing!!



Fatigue Limitatigue Limit-statetate

Considers stresses induced byConsiders stresses induced byrepeated loadsrepeated loads……millions of cycles millions of cycles

Maximum loading conditions notMaximum loading conditions not

appropriateappropriate e.g., permit loads in all lanese.g., permit loads in all lanes

However, fatigue damageHowever, fatigue damage isis the resultthe result

of the entire variable load spectrumof the entire variable load spectrum



Design Loadsesign Loadsfor any Limitor any Limit-statetate

Desirable to use a minimumDesirable to use a minimum

number of different trucksnumber of different trucks Design vehicles should produceDesign vehicles should produce

reasonably similar effects asreasonably similar effects as

real trucksreal trucks



Load foroad for

Fatigue Limitatigue Limit-statetate

Desirable to use one vehicleDesirable to use one vehicle

This results in constant amplitude fatigueThis results in constant amplitude fatigue

Easiest to work withEasiest to work with

Vehicle must produce the sameVehicle must produce the same

equivalent fatigue damage as entireequivalent fatigue damage as entire

variable live load spectrumvariable live load spectrum



Development ofevelopment of

“Fatigue Truckatigue Truck”

Developed based on:Developed based on:

WIM studiesWIM studies

FHWA loadometer dataFHWA loadometer data Field testing of inField testing of in--service bridgesservice bridges

Led to truck proposed in NCHRP Report 299Led to truck proposed in NCHRP Report 299

Developed for longitudinal and transverse membersDeveloped for longitudinal and transverse membersof girder bridgesof girder bridges



Objective of the Fatigue Truck is Representbjective of the Fatigue Truck is Represent

Equivalent Cumulative Damagequivalent Cumulative Damage?



AASHTO Fatigue TruckASHTO Fatigue Truck

Truck which produces equivalentTruck which produces equivalentfatigue damage as variable amplitudefatigue damage as variable amplitude

spectrumspectrum

Does not included panel, pickDoes not included panel, pick--up, & other up, & other

22--axle 4axle 4--wheel vehicleswheel vehicles

GVW > 20 kipsGVW > 20 kips

These contribute little to fatigue damageThese contribute little to fatigue damage



AASHTOASHTO

“Fatigue Truckatigue Truck”(Equivalent to HS15 w/ fixed trailer length)Equivalent to HS15 w/ fixed trailer length)



AASHTO LRFDASHTO LRFD

LoadingLoading

Damage produced byDamage produced by ““Fatigue Truck Fatigue Truck ””

Equivalent to HS15 (54 kip GVW)Equivalent to HS15 (54 kip GVW) Constant axle spacingConstant axle spacing

Produces theProduces the ““Effective Stress RangeEffective Stress Range””

Developed from WIM DataDeveloped from WIM Data NCHRP 299NCHRP 299



Two Approaches to Designwo Approaches to Design

Finite life if:Finite life if: SSr r from HS15 > CAFL / 2.0from HS15 > CAFL / 2.0

Defined number of cycles or yearsDefined number of cycles or years

Infinite life if:Infinite life if:

SSr r from HS15 < CAFL / 2.0from HS15 < CAFL / 2.0

Unlimited cycles requiredUnlimited cycles required



Finite Lifeinite Life(Design for Specific Number of Cycles)Design for Specific Number of Cycles)

Must forecast expected spectrumMust forecast expected spectrum

Stress and # cyclesStress and # cycles

HS15 truck yields effective stress rangeHS15 truck yields effective stress range Use it just like a constant amplitudeUse it just like a constant amplitude

stress range with the Sstress range with the S--N curve for N curve for

number of cycles desirednumber of cycles desired Finite Finite - - life only useful if ADTT less than life only useful if ADTT less than

550 (gives 10 million cycles in 50 years) 550 (gives 10 million cycles in 50 years)



Infinite Lifenfinite Life(Design Below Threshold)Design Below Threshold)

Design so thatDesign so that fatigue limitfatigue limit--statestate

stress rangestress range is below the constantis below the constant--

amplitude fatigue limit (CAFL)amplitude fatigue limit (CAFL)

Cracks will not propagate significantlyCracks will not propagate significantly

in the life of the structurein the life of the structure



But, it is well known that there are trucks heavierut, it is well known that there are trucks heavier

than the 54 kip fatigue truckhan the 54 kip fatigue truck…...

Note many trucks areheavier than fatigue truck



Fatigue Limitatigue Limit-Statetate-Loadoad(AASHTO LRFD)AASHTO LRFD)

HS15 results inHS15 results in effectiveeffective stress rangestress range(i.e., equivalent cumulative damage)(i.e., equivalent cumulative damage)

Recognized that many trucks in theRecognized that many trucks in the

spectrum are heavier than HS15 andspectrum are heavier than HS15 andcan produce damagecan produce damage

Fatigue limitFatigue limit--state load defined as:state load defined as: Upper bound load with minimum frequencyUpper bound load with minimum frequency

sufficient to produce fatigue damagesufficient to produce fatigue damage Expressed asExpressed as XX x HS15 = HSXXx HS15 = HSXX

XX > 1.0> 1.0



Development of theevelopment of the

Fatigue Limitatigue Limit-State Loadtate Load

Based on testing (NCHRP Report 354)Based on testing (NCHRP Report 354) 0.05% exceeded CAFL, cracks observed0.05% exceeded CAFL, cracks observed

0.001% exceeded the CAFL, no cracks0.001% exceeded the CAFL, no cracks

Concluded infinite life if fewer than 0.01%Concluded infinite life if fewer than 0.01%exceed CAFL (exceed CAFL (1 in 10,000 1 in 10,000 ))

Fatigue limitFatigue limit--state stress range is stress rangestate stress range is stress rangewith 0.01% exceedencewith 0.01% exceedence

‘‘XX’’ can now be definedcan now be defined



GVW

%

O c c u r r e n c

e

How does this relate toow does this relate to

live load spectrum?ive load spectrum?

Sreff Srmax

0.01% prob. ofexceedence



Magnitude ofagnitude ofFatigue Limitatigue Limit-State Loadtate Load

WIM data suggest that the GVW of 0.01%WIM data suggest that the GVW of 0.01%exceedence truck is closer to 150 kipsexceedence truck is closer to 150 kips

3.0 x HS15 or HS45 (162 kips)3.0 x HS15 or HS45 (162 kips)

- - However However - -

AASHTO fatigue limit AASHTO fatigue limit--state load range is:state load range is:

2 x HS15 or HS30 (108 kips)2 x HS15 or HS30 (108 kips)



GVW

%

O c c u r r e n c

e

How does the AASHTO LRFDow does the AASHTO LRFDfatigue limitatigue limit-state load relate totate load relate to

live load spectrum?ive load spectrum?

Sreff Srmax

0.01% prob. ofexceedence

Srmax

(LRFD)



Why the discrepancy?hy the discrepancy?

Actual stress ranges in primary members Actual stress ranges in primary members are often much less than predicted using are often much less than predicted using

conservative design equations conservative design equations

LRFD calibrated to experience and LRFD calibrated to experience and

observed cracking observed cracking



AASHTO LRFD SpecificationASHTO LRFD Specification(Infinite life checknfinite life check)

Compares calculated effective stressCompares calculated effective stressrange to CAFL / 2range to CAFL / 2

Factor of 2 accounts for SFactor of 2 accounts for Srmaxrmax/S/Sreff reff (i.e., X(i.e., X = 2.0)= 2.0) Intended to account for increase in load or loadIntended to account for increase in load or load

effects (i.e., stress range)effects (i.e., stress range)

Implies that the resistance is reducedImplies that the resistance is reduced

Uses fatigue load factor of 0.75 on HS20Uses fatigue load factor of 0.75 on HS20 0.75 x HS20 = HS150.75 x HS20 = HS15

Impact factor of 15%Impact factor of 15%



AASHTO LRFD SpecificationASHTO LRFD Specification(Finite life checkinite life check)

Compares calculated effective stressCompares calculated effective stress

range to Srange to S--N curveN curve

Fatigue load factor of 0.75 on HS20Fatigue load factor of 0.75 on HS20

0.75 x HS20 = HS150.75 x HS20 = HS15

Impact factor of 15%Impact factor of 15%



AASHTO LRFD SpecificationASHTO LRFD Specification(Finite life checkinite life check)



Equation for Fatigue Designquation for Fatigue Design

TH nF

N

AF )(

2

1)(

3

1

Δ≥⎟ ⎠

⎞⎜⎝

⎛ =Δ

Existing Eq.Existing Eq.



How should the Fatigueow should the FatigueTruck be applied?ruck be applied?

Depends on bridge and elementDepends on bridge and element

Most short to medium span bridges damageMost short to medium span bridges damage

caused by individual truckscaused by individual trucks One primary cycle produced per truckOne primary cycle produced per truck

Use only a single truck for fatigueUse only a single truck for fatigue

Single lane distribution factorsSingle lane distribution factors

Multiple presence can be considered if Multiple presence can be considered if

necessarynecessary



Influence ofnfluence of

Multiple Presenceultiple Presence

Influence is typically small in terms of cumulativeInfluence is typically small in terms of cumulative

fatigue damagefatigue damage

ExampleExample – – two trucks sidetwo trucks side--byby--sideside

Assume ADTT of 10% Assume ADTT of 10% Assume trucks side Assume trucks side--byby--side 10% of ADTTside 10% of ADTT

Assume static stress increases by 80% due to Assume static stress increases by 80% due to

adjacent truckadjacent truck

What is the effect on damage?What is the effect on damage?



Influence of Multiple Trucksnfluence of Multiple Trucks

What is effect on damage?What is effect on damage? Use Miner Use Miner ’’s Rule to calculate equivalents Rule to calculate equivalent

damagedamage

SSreff reff = [(0.1)(1.8S= [(0.1)(1.8Sr r ))33 + 0.9(S+ 0.9(Sr r ))

33]]1/31/3 = 1.14S= 1.14Sr r

SSreff reff increased by less than 15% due toincreased by less than 15% due topresence of second truckpresence of second truck

Two Trucks One Truck



Special Considerationspecial Considerations

for Deck Elementsor Deck Elements

Fatigue truck developed to produceFatigue truck developed to produce

cumulative damage in main memberscumulative damage in main members

Axles of fatigue truck actually represent a Axles of fatigue truck actually represent atandem for simplicitytandem for simplicity

This simplification is not appropriate for This simplification is not appropriate for

deck elementsdeck elements Orthotropic decksOrthotropic decks

Modular bridge expansion jointsModular bridge expansion joints



Five Axle Truck Idealizedive Axle Truck Idealized

as Three Axle Trucks Three Axle Truck

P PR R

R2 P

P PR R

R2 P

Simplified

Actual



Questionsuestions ??

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Session 04 What is the Fatigue Truck[1]