How do engineers apply physics in constructing bridges and buildings?

Physics is an integral part of structural engineering. Using the concept of moment and force balance, bending moment and shear stress is determined for every structural member. According to value of bending moment and shear stress further designing of member is done like how much cross section, steel, clear cover etc is required. So basically the force/moment balance is the basic physics that civil engineering apply for determining the stresses/moment at each member.