일반물리학 1 — Equilibrium and Elasticity: Problem 48

48 A tunnel of length L = 200 m, height H = 7.2 m, and width 6.3 m (with a flat roof) is to be constructed at distance d = 70 m beneath the ground (Fig. 12-49). The tunnel roof is to be supported entirely by square steel columns, each with a cross-sectional area of 960 cm^2. The mass of 1.0 cm^3 of the ground material is 2.8 g. (a) What is the total weight of the ground material the columns must support? (b) How many columns are needed to keep the compressive stress on each column at one-half its ultimate strength?