Seismic Design of Tunnels - Parsons Brinckerhoff

10. Calculate the allowable shear strength of concrete during the MDE:
fV c = 0.85x2
fc A shear
where f = shear strength reduction factor (0.85)
f c = yield strength of concrete (4000 psi)
A shear = effective shear area = A c /2
Note: Use of f = 0.85 for earthquake design may be very conservative.
A shear
fi fVc = 0.85x2 4000 x 62.8
2 x 144
1000
= 486 kips
11. Compare the induced maximum shear force with the allowable shear resistance:
V max = 625 kips > fVc
= 486 kips
Although calculations indicate that the induced maximum shear force exceeds the
available shear resistance provided by the plain concrete, this problem may not be of
major concern in actual design because:
• The nominal reinforcements generally required for other purposes may provide
additional shear resistance during earthquakes.
• The ground displacement amplitudes, D, used in this example are very conservative.
Generally the spatial variations of ground displacements along a horizontal axis are
much smaller than those used in this example, provided that there is no abrupt
change in subsurface profiles.
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