Page 318 - Planning and Design of Airports

276 Airp o r t D e sign

B–757
Contact area = 168.35 sq. in. Annual departures
Dual spacing = 34.00 in.
1,200 6,000 25,000
Tandem spacing = 45.00 in. 3,000 15,000
900 22 23 24 26 26
250,000 lb 21 22 23 25 25
850 500 300 200 100 K = 50 pci 225,000 200,000 150,000 20 21 22 24 24
23
Concrete flexural strength, psi 750 175,000 125,000 17 18 19 20 20 Slab thickness, in
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800
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20
18
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700
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650
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600
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550 11 11 12 12 13
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8 9 9
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7 8
500 6 7 7
FIGURE 7-8 Example design curve for estimating the slab thickness of rigid
pavement using Westergaard’s analysis.
these assumptions do not satisfy theory in a strict sense, the results
compared reasonably with observations. The Westergaard analysis
was used to evaluate stress in a pavement, as well as the deflection of
the slab. For airports, Westergaard developed formulas for stresses
and deflections in the interior of a slab and at an edge of a slab.
The U.S. Army Corps of Engineers applied Westergaard’s formu-
las toward the creation of approximation charts and design curves.
An example is illustrated in Fig. 7-8.
The arrow found in Fig. 7-8 illustrates an example rigid pavement
design analysis, considering the use of a PCC mixture of flexural strength
2
3
of 660 lb/in , a subgrade with k value 100 lb/in , for a Boeing 757 design
aircraft with maximum gross weight of 175,000 lb and 6000 annual
equivalent departures, resulting in a design slab thickness of approx-
imately 12 in.
Finite Element Theory
Similar to flexible pavement design, FAARFIELD processes the user
defined fleet mix and E value of the subgrade to determine the thick-
ness requirements of the PCC surface. FAARFIELD recommends
default subbase layers at 6 in thickness. Multiple subbase layers are
recommended for certain subgrades with low E values. Figure 7-9
illustrates an example rigid pavement structure output from FAAR-
FIELD.
FAARFIELD applies finite element theory to estimate the thick-
ness of the PCC surface and any necessary subbase courses. Three-
dimensional finite element design theory (3D-FE) is similar to layered