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Alpha Factor Determination for 6-Wheel Gears. Gordon Hayhoe, AAR-410, FAA William J. Hughes Technical Center, Atlantic City, New Jersey, U.S.A. Need for evaluation Full-scale test structures and results Procedure for calculating alpha factors Alpha factor Proposals for consideration by ICAO

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Alpha factor determination for 6 wheel gears
Alpha Factor Determination for 6-Wheel Gears

  • Gordon Hayhoe, AAR-410, FAA William J. Hughes Technical Center, Atlantic City, New Jersey, U.S.A.

    • Need for evaluation

    • Full-scale test structures and results

    • Procedure for calculating alpha factors

    • Alpha factor Proposals for consideration by ICAO

    • Implications for thickness design

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B 777 six wheel acns
B-777 Six-Wheel ACNs

  • For flexible pavements, the ACNs initially computed for B-777 6-wheel gears appeared to be unreasonably high.

  • The FAA had similar concerns about the existing CBR method for 6-wheel gears.

  • A380 also has 6-wheel body gears.

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Interim 6 wheel alpha factor at 10 000 coverages
Interim 6-Wheel Alpha Factor at 10,000 Coverages

  • 4-Wheel alpha = 0.825

  • “Original” 6-wheel alpha = 0.788 (inception to 1995)

  • “Interim” 6-wheel alpha = 0.72 (1995 to present)

  • Current 12-wheel alpha = 0.722

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Alpha Factors – MWHGL Data

C5-A as two 6-wheel gears

C5-A as one 12-wheel gear

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National airport pavement test facility naptf for 6 wheel tests
National Airport Pavement Test Facility (NAPTF) for 6-Wheel Tests

  • Joint FAA and Boeing.

  • Testing is funded and conducted entirely by the FAA.

  • Tests run on flexible test items to compare 4-wheel and 6-wheel gears.

  • Construction cycles CC1 etc.

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Naptf construction cycles
NAPTF Construction Cycles Tests

  • CC1 = original construction.

    • Conventional and stabilized base flexible on low-strength subgrade (LFC and LFS).

    • Conventional and stabilized base flexible on medium-strength subgrade (MFC and MFS).

  • CC2 = rigid pavements, trafficking completed.

  • CC3 = flexible pavement reconstruction with four conventional test items, trafficking and posttraffic testing completed.

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Cc3 test pavements profile
CC3 Test Pavements - Profile Tests

Direction of Traffic

8


North, 6-Wheel Track Tests

LFC4

LFC3

LFC2

LFC1

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Computation of alpha factor
Computation of Alpha Factor Tests

  • Pass/Coverage ratios calculated from surface coverages in test wander pattern:

    • 4-Wheel = 2.36 for CC3 and 2.06 for CC1

    • 6-Wheel = 1.57

  • Subgrade CBR = trench measurements.

  • Total structure thicknesses are known.

  • Contact area = 265 square inches.

  • Compute Alpha using COMFAA.

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Cbr equations
CBR Equations Tests

Pre-MWHGL equation:

t = Total Thickness

P =ESWL

Post-MWHGL equation:

t =  (Ac)0.5 [-0.0481 – 1.1562 (log CBR/P)

– 0.6414 (log CBR/P)2 – 0.473 (log CBR/P)3]

Solve the Post-MWHGL equation for 

OR

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Mwhgl subgrade cbr measurements
MWHGL Subgrade CBR Measurements to the test structure thickness.

  • The CBR of the subgrade for each MWHGL test item was calculated from all available measurements:

    • After construction, before traffic.

    • Trench and pit after traffic at surface, 12-inch, and 24-inch depth.

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Summary of naptf flexible pavement full scale test results
Summary of NAPTF Flexible Pavement Full-Scale Test Results to the test structure thickness.

* Extrapolated from rut depth curve

Bold = corrected values

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Naptf and mwhgl alpha factor results no conversion of naptf to mwhgl structures
NAPTF and MWHGL Alpha Factor Results to the test structure thickness.(No conversion of NAPTF to MWHGL structures)

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Ledfaa 1 3 flexible failure model
LEDFAA 1.3 Flexible Failure Model to the test structure thickness.

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Naptf versus mwhgl test results
NAPTF versus MWHGL Test Results to the test structure thickness.

  • NAPTF pavements tended to last longer than MWHGL pavements. Possible reasons for this are:

    • Indoor NAPTF operation means lower asphalt temperatures.

    • NAPTF asphalt and base layers are thicker.

    • NAPTF subbase material is of higher quality (strength – screenings versus uncrushed aggregate).

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Procedure for converting naptf structures to equivalent mwhgl structures example
Procedure for Converting NAPTF Structures to Equivalent MWHGL Structures (Example)

Steps:

(a) real structure, 29.0 in.

(b) convert 2 in. AC to 3.2 in. CA (E.F. 1.6)

(c) add 3.2 in. CA to exist. 8 in. CA = 11.2 in. CA

(d) convert 5.2 in. CA to 8.3 in. SQS (E.F. 1.6)

(e) convert 16 in. HQS to 19.2 in. SQS (E.F. 1.2)

(f) equivalent MWHGL structure, 36.5 in.

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Naptf and mwhgl alpha factor results with conversion of naptf to mwhgl structures
NAPTF and MWHGL Alpha Factor Results Factors(With conversion of NAPTF to MWHGL structures)

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Naptf and mwhgl alpha factor results
NAPTF and MWHGL Alpha Factor Results Factors

NAPTF structures converted to equivalent MWHGL structures (SQS = 1.6 x CA) and C5-A as two 6-wheel gears

No structure conversions and C5-A as two 6-wheel gears

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4 and 6 wheel alpha factors for base to subbase equivalency 1 4
4- and 6-Wheel Alpha Factors for FactorsBase-to-Subbase Equivalency = 1.4

Alpha factor quadratic curve fit intercepts at 10,000 coverages:

4-wheel  = 0.806

6-wheel  = 0.7178

From MWHGL report:

4-wheel  = 0.825

6-wheel  = 0.788

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4 and 6 wheel alpha factors for base to subbase equivalency 1 6
4- and 6-Wheel Alpha Factors for FactorsBase-to-Subbase Equivalency = 1.6

Alpha factor quadratic curve fit intercepts at 10,000 coverages:

4-wheel  = 0.832

6-wheel  = 0.7295

From MWHGL report:

4-wheel  = 0.825

6-wheel  = 0.788

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Subbase equivalency factors
Subbase Equivalency Factors Factors

  • Burns, C.D., R.H. Ledbetter, and R.W. Grau.

  • “Study of Behavior of Bituminous-Stabilized Pavement Layers,” Miscellaneous Paper S-73-4, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississipi, March 1973.

  • Bituminous stabilized base, asphalt base, bituminous stabilized subbase.

25


Subbase equivalencies for 12 wheel traffic
Subbase Equivalencies for 12-Wheel Traffic Factors

BLS stabilized layers replaced by MWHGL equivalent thicknesses

26


Subbase equivalencies for 12 wheel traffic1
Subbase Equivalencies for 12-Wheel Traffic Factors

BLS stabilized layers replaced by MWHGL equivalent thicknesses

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Alpha factor results discussion
Alpha Factor Results - Discussion Factors

  • Conversion of NAPTF structures gives better agreement with MWHGL test results.

  • This indicates that extra conservatism for subgrade protection has been built into the design procedure by increasing minimum thickness requirements for surface (5 in versus 3 in) and base (8 in versus 6 in) without reducing total thickness.

  • If 150/5320-6D is used to calibrate LEDFAA then LEDFAA is also conservative.

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Mwhgl designs versus current faa cbr designs
MWHGL Designs versus Current FAA CBR Designs Factors

  • The MWHGL alpha factor curves give design thicknesses for structures with 3-in asphalt and 6-in base, and for material properties the same as the MWHGL test materials.

  • Thickness designs for other layer thicknesses and properties must be converted to MWHGL compatible structures to give the same level of subgrade protection.

1.15 x 28.7 in

0.87 x 33 in

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Alpha factor results discussion1
Alpha Factor Results - Discussion Factors

  • But, overconservative thicknesses for subgrade protection may provide other benefits for operation with heavy aircraft loads.

    • Safety factor for structural failure.

    • Compaction rutting in base and subbase materials.

    • Fatigue cracking of stabilized layers.

  • LEDFAA and FEDFAA are therefore being calibrated against -6D designs (5 and 8+ in), not MWHGL designs (3 and 6 in).

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North, 6-Wheel Track Factors

LFC4

LFC3

LFC2

Subgrade CBR = 3.3

LFC1

32


LFC1 Center Line, 6-Wheel Track Factors

LFC1

CBR = 4.3

33


CC-3 PHASE-2: LFC-1 CL TRAFFIC TESTS Factors

Pass No = 0

Pass No = 66

Pass No = 132

Pass No = 198

Pass No = 264

Pass No = 330

34



Cc3 lfc1 traffic results summary
CC3-LFC1 Traffic Results Summary Factors

  • A relatively small change in subgrade CBR can produce a very significant change in the magnitude and character of flexible pavement structural performance.

  • Very large deformations can occur at, say, 5 passes, even when the life to the failure criterion is as large as 100 passes.

  • This is the basis for the 240 coverage requirement in Engineering Brief No. 65, “Minimum Requirements to Widen Existing 150‑Foot Wide Runways for Airbus A380 Operations.”

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