Physics
posted by Ray .
A tunnel of length L = 141 m, height H = 6.9 m high, and width 6.0 m (with a flat roof) is to be constructed at distance d = 60 m beneath the ground. The tunnel roof is to be supported entirely by square steel columns, each with a crosssectional area of 960 cm^2. The mass of 1.0 cm^3 of the ground material is 2.8 g.
What is the total weight of the ground material the columns must support?

totalweight= densityEarth*volume
= density*141*60*6.0 m^3
and the density=2.8g/cm^3 which is 2800kg/m^3 
How many columns are needed to provide a safety factor of two against rupture.
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