4. The ground floor of a building serves as parking space. The main part of the building, above the parking space, rests on 10 columns that are fixed in the

ground. The mass of the main part is 40 000 kg. The cross section of each column is 10 cm x 10 cm. The building is hit by an earthquake. The ground tremors of the aftershock of the earthquake may be approximated as a sinusoidal wave whose maximum horizontal displacement was 7 cm, and whose period was 6 s. The building, whose natural period is 2.5 s did not enter into resonance. As aiming rigid structure, what was the maximum shear stress at the top of the columns during the aftershock?