Flexible Pavement Design

3/21/12
CE 563
Airport Design
  VERY important pavement design factor

  Consideration MUST be given to:
  Load magnitude
  Load configuration
  Total load repetitions (volume)
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3/21/12
  Three procedures for considering traffic

effects for pavement design
  Fixed vehicle
  Fixed traffic
  Variable vehicle and traffic
  Pavement thickness is governed by the number

of repetitions of a standard vehicle or axle Load
  Usually the 18kip (80kN) axle load
  All other axle loads are converted to 18kip axle
loads by Equivalent Axle Load Factors (EALF) or
(LEF)
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  Summation of the equivalent effects of all axle loads

over the design period (life) gives the Equivalent
Single Axle Load (ESAL) used in design
  Used in AASHTO pavement design methods
  Pavement thickness governed by

single wheel load (ESWL)
  Number of load repetitions not
considered
  Used in airport and industrial
design
▪  WWII, B-‐29 bombers had dual
gear and methods were based
on single wheel loads at that
time, thus ESWL
  ESWL = worst case load
  Considers both traffic (number of repetitions)

and load uniquely
  σ, ε, δ resulting from each load is considered in
design
  Used in mechanistic design procedures
  Must use computers to solve problems
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3/21/12
  Series of design curves based on the CBR design

method
  Design curves depend on gear configuration
  Curves provide total pavement thickness
  Curves based on 20 year design life
  Load
  Pavement design based on maximum anticipated
takeoff weight
  Assumes 95% of the load on main gear

  Landing gear type and geometry

  Single
  Dual
  Dual tandem
  Wide body aircraft (B-‐747)
  Tire pressure (75-‐200 psi)
  Traffic volume (annual departures by aircraft type)
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3/21/12
  Subgrade CBR value

  Subbase CBR value
  Design aircraft gross weight
  Number of annual departures of the design aircraft
  One standard deviation below the mean

  Minimum Design CBR=3
  If less, use stabilization, etc.
  Can be determined in the laboratory

  FAA P-‐154 is subbase specification
  Assumes a CBR=20
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3/21/12
  Forecast annual departures by aircraft type

  Determine required pavement thickness for each
aircraft type
  Aircraft requiring the thickest pavement is the
design aircraft
  Will not necessarily be heaviest aircraft
Ave. Annual Max. Takeoff

Aircraft Gear Type Departures Weight (lbs)
727-‐100 Dual 3,760 160,000
727-‐200 Dual 9,080 190,500
707-‐320B Dual tandem 3,050 327,000
DC-‐9-‐30 Dual 5,800 108,000
CV-‐800 Dual tandem 400 184,500
737-‐200 Dual 2,650 115,500
L1011-‐100 Dual tandem 1,710 450,000
747-‐100 2-‐dual tandem 85 700,000
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  For given data, assuming a CBR=6

  Design aircraft is the 727-‐200
  Group all aircraft traffic into gear configuration of

design aircraft
  Convert all departures into equivalent annual
departures of the design aircraft (see table)
  Assume a load of 300,000 lbs and dual tandem for
all wide body aircraft
Convert From To Multiply By

Single wheel Dual wheel 0.8
Single wheel Dual tandem 0.5
Dual wheel Single wheel 1.3
Dual wheel Dual tandem 0.6
Dual tandem Single wheel 2.0
Dual tandem Dual wheel 1.7
2-‐dual tandem Dual tandem 1.0
2-‐dual tandem Dual wheel 1.7
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  To convert CV-‐800 departures into 727-‐200

(design aircraft) departures
Eq. Dual Gear Departures = 400 !1.7

= 680
Eq. Annual
Eq. Dual Gear Wheel Departures Design
Aircraft Departures Load (lbs) Aircraft
727-‐100 3,760 38,000 1,891
727-‐200 9,080 45,240 9,080
707-‐320B 5,185 38,830 2,764
DC-‐9-‐30 5,800 25,650 682
CV-‐800 680 21,910 94
737-‐200 2,650 27,430 463
L1011-‐100 2,907 35,625 1,184
747-‐100 145 35,625 83
  Aircraft wheel loads
190,500 × 0.95 × 0.5

Wheel Load =
2
= 45,240
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Eq. Annual
727-‐100 3,760 38,000 1,891
727-‐200 9,080 45,240 9,080
707-‐320B 5,185 38,830 2,764
DC-‐9-‐30 5,800 25,650 682
CV-‐800 680 21,910 94
737-‐200 2,650 27,430 463
L1011-‐100 2,907 35,625 1,184
747-‐100 145 35,625 83
  To convert to equivalent annual departures of

the design aircraft
W2
log R1 = log R2
W1
  R1 = equivalent annual departures by the design

aircraft
  R2 = annual departures expressed in design aircraft
landing gear
  W1 = wheel load of the design aircraft
  W2 = wheel load of the aircraft in question
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3/21/12
  Equivalent annual departures, design aircraft
21,910
log R1 = log(680 ) ×
45,240
R1 = 94
Eq. Annual
727-‐100 3,760 38,000 1,891
727-‐200 9,080 45,240 9,080
707-‐320B 5,185 38,830 2,764
DC-‐9-‐30 5,800 25,650 682
CV-‐800 680 21,910 94
737-‐200 2,650 27,430 463
L1011-‐100 2,907 35,625 1,184
747-‐100 145 35,625 83
  Sum to find the total equivalent annual departures

of the design aircraft
  Total = 16,241
  The design should be completed with 16,241
departures of a 190,500 lbs aircraft
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3/21/12
  Complete frost protection

  Limited subgrade frost penetration
  65% of frost penetration depth is composed of
non-‐frost susceptible material
  Reduced subgrade strength
  Increase pavement thickness
  Full-‐depth design thickness required where

departing aircraft use pavement
  Aprons
  Holding areas
  Center of runways and taxiways
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  90% of design thickness required where arriving

aircraft will use pavement
  High speed turnouts
  70% of design thickness required where pavement
use is unlikely
  Outer edges of runways and taxiways
12

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3/21/12

 VERY important pavement design factor

 Three procedures for considering traﬃc

 Pavement thickness is governed by the number

 Summation of the equivalent eﬀects of all axle loads

 Pavement thickness governed by

 Considers both traﬃc (number of repetitions)

 Series of design curves based on the CBR design

 Landing gear type and geometry

 Subgrade CBR value

 One standard deviation below the mean

 Can be determined in the laboratory

 Forecast annual departures by aircraft type

Ave. Annual Max. Takeoﬀ

 For given data, assuming a CBR=6

 Group all aircraft traﬃc into gear conﬁguration of

Convert From To Multiply By

 To convert CV-­‐800 departures into 727-­‐200

Eq. Dual Gear Departures = 400 !1.7

 Aircraft wheel loads

190,500 × 0.95 × 0.5

 To convert to equivalent annual departures of

 R1 = equivalent annual departures by the design

 Equivalent annual departures, design aircraft

 Sum to ﬁnd the total equivalent annual departures

 Complete frost protection

 Full-­‐depth design thickness required where

 90% of design thickness required where arriving

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  VERY important pavement design factor

  Three procedures for considering traﬃc

  Pavement thickness is governed by the number

  Summation of the equivalent eﬀects of all axle loads

  Pavement thickness governed by

  Considers both traﬃc (number of repetitions)

  Series of design curves based on the CBR design

  Landing gear type and geometry

  Subgrade CBR value

  One standard deviation below the mean

  Can be determined in the laboratory

  Forecast annual departures by aircraft type

  For given data, assuming a CBR=6

  Group all aircraft traﬃc into gear conﬁguration of

  To convert CV-‐800 departures into 727-‐200

  Aircraft wheel loads

  To convert to equivalent annual departures of

  R1 = equivalent annual departures by the design

  Equivalent annual departures, design aircraft

  Sum to ﬁnd the total equivalent annual departures

  Complete frost protection

  Full-‐depth design thickness required where

  90% of design thickness required where arriving