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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
- Implications for thickness design

1

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.

2

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

4

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.

6

NAPTF Construction Cycles

- 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.

7

Computation of Alpha Factor

- 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.

11

CBR Equations

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

12

Change the Input Alpha until the design thickness is equal to the test structure thickness.

13

MWHGL Subgrade CBR Measurements

- 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.

14

Summary of NAPTF Flexible Pavement Full-Scale Test Results

* Extrapolated from rut depth curve

Bold = corrected values

15

NAPTF and MWHGL Alpha Factor Results(No conversion of NAPTF to MWHGL structures)

16

NAPTF versus MWHGL Test Results

- 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).

18

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.

19

NAPTF and MWHGL Alpha Factor Results(With conversion of NAPTF to MWHGL structures)

21

NAPTF and MWHGL Alpha Factor Results

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

22

4- and 6-Wheel Alpha Factors forBase-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

23

4- and 6-Wheel Alpha Factors forBase-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

24

Subbase Equivalency 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

BLS stabilized layers replaced by MWHGL equivalent thicknesses

26

Subbase Equivalencies for 12-Wheel Traffic

BLS stabilized layers replaced by MWHGL equivalent thicknesses

27

Alpha Factor Results - Discussion

- 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.

28

MWHGL Designs versus Current FAA CBR Designs

- 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

29

Alpha Factor Results - Discussion

- 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).

30

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

Pass No = 0

Pass No = 66

Pass No = 132

Pass No = 198

Pass No = 264

Pass No = 330

34

CC3-LFC1 Traffic Results Summary

- 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.”

36

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