Alpha Factor Determination for 6Wheel Gears

1
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

2
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.

3
Alpha Factors MWHGL Data
4
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

5
Alpha Factors MWHGL Data
C5-A as two 6-wheel gears
C5-A as one 12-wheel gear
6
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.

7
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.

8
CC3 Test Pavements - Profile
Direction of Traffic
9
North, 6-Wheel Track
LFC4
LFC3
LFC2
LFC1
10
Trench in LFC2 Flexible
11
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.

12
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
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Change the Input Alpha until the design thickness
is equal to the test structure thickness.
14
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.

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Summary of NAPTF Flexible Pavement Full-Scale
Test Results
Extrapolated from rut depth curve Bold
corrected values
16
NAPTF and MWHGL Alpha Factor Results(No
conversion of NAPTF to MWHGL structures)
17
LEDFAA 1.3 Flexible Failure Model
18
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).

19
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.

20
NAPTF Flexible Pavement Equivalent Thicknesses
and Alpha Factors
21
NAPTF and MWHGL Alpha Factor Results(With
conversion of NAPTF to MWHGL structures)
22
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
23
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
24
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
25
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.

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Subbase Equivalencies for 12-Wheel Traffic
BLS stabilized layers replaced by MWHGL
equivalent thicknesses
27
Subbase Equivalencies for 12-Wheel Traffic
BLS stabilized layers replaced by MWHGL
equivalent thicknesses
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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.

29
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.

0.87 x 33 in
1.15 x 28.7 in
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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).

31
LEDFAA 1.3 Flexible Failure Model
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North, 6-Wheel Track
LFC4
LFC3
LFC2
Subgrade CBR 3.3
LFC1
33
LFC1 Center Line, 6-Wheel Track
LFC1
CBR 4.3
34
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
35
CC-3 PHASE-2 LFC-1 CL TRAFFIC TEST RESULTS
36
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.