CE 671 Lect 23 and 24

7/30/2019 CE 671 Lect 23 and 24

http://slidepdf.com/reader/full/ce-671-lect-23-and-24 1/15

AASHTO Provisions for

Fatigue Design of Highway Bridges

CE671 – Lecture 23 & 24

Specifications Considered

Standard Specifications

LRFD Bridge Design Specifications

Commonalities to all

Specifications:

Consider fatigue as a limit-state

Compare calculated stress range to anallowable

Utilize same resistance curves (Cat F*)

Implicitly consider secondary stresses

Include provisions for finite & infinite life

* Cat. F removed from LRFD

7/30/2019 CE 671 Lect 23 and 24


Major Differences are

Related to:

Load models (i.e., trucks)

Load distribution models

How redundancy is addressed

AASHTO StandardSpecifications

AASHTO Standard Spec.

Loading

– ASD approach for fatigue

– Truck loading HS20

– Stress ranges for fatigue identical to thoseused for strength design

– Generally assumes all lanes loaded

For Case I highways, infinite life check also req’d.single truck loading

7/30/2019 CE 671 Lect 23 and 24


HS20 Design Truck

AASHTO Standard Spec.

Design life defined by “Cases”

– Case I – ADTT >2,500

– Case II – ADTT <2,500

– Case III – Not defined with ADTT

Minimum required life

Design

StressCycles

Why the difference?

7/30/2019 CE 671 Lect 23 and 24


AASHTO Standard Spec.(Design Life Cont’d)

Design life function of member type

Distinguished between– Longitudinal vs. transverse

– Main members vs. those subjected towheel loads

Recognized that #of applied cyclesdiffers for different members

Redundancy 1st Addressedin 1977 Standard Spec.

Objective to address catastrophicfailure due to fracture in non-redundant bridges

Ideally, should increase fracturetoughness of material

But required toughness could not bemet by steel producers at the time

Alternate to Increasing Toughness Requirements

Recognized that almost all fractureswere result of initial fatigue cracking

Hence decreasing probability of fatiguecracking would decrease potential forfracture

Arbitrarily modified allowable fatiguestresses for nonredundant structures

7/30/2019 CE 671 Lect 23 and 24


7/30/2019 CE 671 Lect 23 and 24


242436243660 A

9

N/A

8

10

13

18

500k

81291215F

N/AN/A5.89.416E’

512.5812.521E

716101627D

1019131932C

1627.51827.545B

2,000k100k2,000k500k100k

NonredundantRedundantLife

Cat

Allowable Fatigue Stresses(Redundant vs. Nonredundant)

Load Distribution(Standard Specifications)

Load distribution affects calculated stress rangein the girder

– Hence, an integral part of approach

Utilized standard ‘S over’ type distributionfactors

Generally result in conservative estimates of stress

– Especially for single loaded lane

Summary Standard Specifications

Design life unrealistically low

However, loading was overly conservative

– All lanes loaded The conservative and unconservativeassumptions seemed to result in generallyconservative designs

– Conservative load distribution model helped

Overall, infinite fatigue life of bridges designed toStand. Spec. can be expected if design for CAFL

– Exception of out-of-plane cracking

– Not necessarily true for some deck elements

7/30/2019 CE 671 Lect 23 and 24


AASHTO LRFD(up to 2005 Interims)


Appeared in 1994 with 1st Ed. of LRFD

– Based on Guide Specifications and NCHRPReport 299

Major modifications to design lifeestimates (i.e., N for design)

Major modifications to load model

Major modifications to load distributionfactors


Loading

– Damage produced by “Fatigue Truck”

– Equivalent to HS15 (54 kip GVW)

Constant axle spacing

– Produces the “Effective Stress Range”

– Developed from WIM Data

NCHRP 299

7/30/2019 CE 671 Lect 23 and 24


LRFD “Fatigue Truck”(HS15)

Lehigh

University Field

Testing

Two Approaches to Design

Finite life if:

– Sr from HS15 >CAFL / 2.0

– Defined number of cycles or years

Infinite life if:

– Sr from HS15 <CAFL / 2.0

– Unlimited cycles required

Finite Life

Design for specific number of cycles

Must forecast expected spectrum– Stress and #cycles

– HS15 truck yields effective stress range

– Use it just like a constant amplitudestress range with the S-N curve fornumber of cycles desired

– Finite-life only useful if ADTT less than550 (gives 10 million cycles in 50 years)

7/30/2019 CE 671 Lect 23 and 24


Infinite Life

Essentially all stress range cycles lessthan CAFL

Specification approach:

– Design so that fatigue limit-state stressrange is below the constant-amplitudefatigue limit (CAFL)

Cracks will not propagate significantly inthe life of the structure

Fatigue Limit-State-Load(AASHTO LRFD)

HS15 results in effective stress range (i.e.,equivalent cumulative damage)

Recognized that many trucks in thespectrum are heavier than HS15 and canproduce damage

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

sufficient to produce fatigue damage

– Expressed as X x HS15 =HSXX

– X >1.0

Development of theFatigue Limit-State Load

Based on testing (NCHRP Report 354)

– 0.05% exceeded CAFL, cracks observed– 0.001% exceeded the CAFL, no cracks

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

Fatigue limit-state stress range is stress rangewith 0.01% exceedence

‘X’ can now be defined

7/30/2019 CE 671 Lect 23 and 24


GVW

% O c c u r r e n c e

How does this relate tolive load spectrum?

Sreff Srmax

0.01% prob. of

exceedence

Magnitude of Fatigue Limit-State Load

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

– 3.0 x HS15 or HS45 (162 kips)

-However-

AASHTO fatigue limit-state load range is:

– 2 x HS15 or HS30 (108 kips)

GVW

% O c c u r r e n c e

How does the AASHTO LRFDfatigue limit-state load relate to

live load spectrum?

Sreff Srmax

0.01% prob. of

exceedence

Srmax

(LRFD)

7/30/2019 CE 671 Lect 23 and 24


Why the discrepancy?

Actual stress ranges in primary membersare often much less than predicted usingconservative design equations

LRFD calibrated to experience andobserved cracking

AASHTO LRFD Specification(Infinite life check)

Compares calculated effective stressrange to CAFL / 2– Factor of 2 accounts for Srmax/Sreff (i.e., X =2.0)

– Intended to account for increase in load or loadeffects (i.e., stress range)

– Implies that the resistance is reduced

– Uses fatigue load factor of 0.75 on HS200.75 x HS20 =HS15

– Impact factor of 15%

AASHTO LRFD Specification(Finite life check)

Compares calculated effective stressrange to S-N curve

– Fatigue load factor of 0.75 on HS20

0.75 x HS20 =HS15

– Impact factor of 15%

7/30/2019 CE 671 Lect 23 and 24


AASHTO LRFD Specification(Finite life check)

Equation for Fatigue Design

THnF

N

AF )()( Δ≥⎟

⎠

⎞⎜⎝

⎛ =Δ

2

13

1

Existing Eq.

Load Distribution(AASHTO LRFD)

LRFD contains most refined equationsof any previous specification

Recognizes fatigue is result of individualtrucks

– Use distribution factors for one loaded lane

7/30/2019 CE 671 Lect 23 and 24


Redundancy(AASHTO LRFD)

Redundancy penalty removed in 1994

– New steels had superior toughness– Improvements in NDT techniques– Improvements in Q/A and fabrication

Specifications required:Increased toughness AND

Reduced allowable fatigue stresses

– Essentially a double penalty

Committee felt this was too harsh andprovision removed

Fracture vs. Failure Critical

Failure Critical

– Elements or members whose failure areexpected to cause collapse of the bridge

Fracture Critical

– Steel bridge component in loaded tension orwith a tension element, whose failure wouldlikely cause a portion of or the entire bridge tocollapse

– Fracture traditionally applies to steel

Fracture vs. Failure Critical

Fracture Critical members are failure

critical

Failure Critical members are notnecessarily Fracture Critical

7/30/2019 CE 671 Lect 23 and 24


7/30/2019 CE 671 Lect 23 and 24


Questions ?

Documents

CE 671 Lect 23 and 24