Final answer:
Seismic forces are considered to act in the two horizontal directions of a building to maintain equilibrium. The forces involved can be complex, including longitudinal, transverse, and surface waves.
Step-by-step explanation:
The seismic forces are always considered to act in the 2 horizontal directions parallel to the axes of the building and may act simultaneously. When analyzing seismic forces on a building, engineers use the principle that the net external force and net torque must be zero for the building to be in equilibrium. This is done to ensure the stability of the structure against seismic activities.
Seismic forces result from earthquakes and can have components in multiple directions, including both the longitudinal P-waves, and transverse S-waves, as well as surface waves. In engineering, a free-body diagram might be used to represent the forces acting on a structure. The horizontal seismic forces generally are shown on such diagrams as vectors parallel to the ground surface, pointing towards the potential energy's origin or away from it, represented by positive or negative amplitude, which is critical for an equilibrium analysis.