UFC 3-260-02

30 June 2001

deep enough to weaken the subgrade soil. When this is the case, the reduced subgrade strength

method shall be used for design. The procedures to determine the PCC thickness using the elastic

layered model are then applied in the same manner as in the first part of this example; the only input

parameter change is the (reduced) subgrade elastic modulus. To check for a lesser thickness

requirement with limited frost penetration procedures, the criteria in Chapter 20 should be used.

11. DESIGN EXAMPLE 2, MIXED AIRCRAFT TRAFFIC. This design example is for type A traffic areas.

The airfield has traffic of 12,500 passes of C-141 (156,500 kilograms (345,000 pounds)), 100 passes of

B-52 (181,450 kilograms (400,000 pounds)), and 25,000 passes of F-15 (30,850 kilograms

(68,000 pounds)), and 12,500 passes of C-17 (1,179,400 kilograms (260,000 pounds)). The

characteristics of the design aircraft are presented in Table 19-5. A 250-millimeter (10-inch) thick

stabilized base layer is used in this example. An SCI = 80 is desired at the end of the design life.

a. Computations of Critical Stresses and Damage. A number of trial concrete slab thicknesses were

selected for design. The maximum tensile stress in each concrete slab under each aircraft loading was

computed using the elastic layered model. Equations 19-1, 19-2, and 19-3 were used to calculate the

allowable coverages based on the calculated stresses and the flexural strength of 4.48 MPa (650 psi) for

the PCC following the same procedure outlined in example 1. The amount of damage is the ratio of the

design or applied passes to the allowable passes. The computed damage for different PCC

PCC

Base

Maximum

Design

Allowable

Passes1

Thickness, in.

Thickness, in.

Stress, psi

Passes

Damage

(1)

(2)

(3)

(4)

(5)

(6) = (4)/(5)

13

6

343

200,000

18,691

10.7

14

6

309

200,000

64,299

3.11

15

6

280

200,000

232,481

0.86

16

6

255

200,000

894,250

0.22

13

18

336

200,000

23,888

8.37

14

18

303

200,000

80,842

2.47

15

18

275

200,000

290,452

0.69

16

18

251

200,000

1,111,962

0.18

1

Allowable passes are computed using equations 19-1, 19-2, and 19-3 based on the computed

maximum stress shown in column (3), on a selected value of R, and the pass-to-coverage ratio for the

C-130 aircraft.

Conversion Factors: Millimeters = 25.4 inches, Megapascals = 0.006894 psi

19-13

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