Earthquake Load has a very different nature than the other types of loads discussed above. While the other loads are external, the earthquake load is generated by the movements of ground and it is internal.
To better understand the action of earthquakes on man-made structures we first need to learn about the origin of earthquakes. As we know, the center of the earth (core) is made of molten materials which are very hot. This causes a heat exchange between the core and earth surface, which have resulted in the earth surface (crust) to become fractured into several plates. This is called “Plate Tectonics Theory”. The boundaries of these plates are called "faults". If these plates can move with respect to each other, no seismic activities will occur. However, if the plates are restrained by each other, then stresses will gradually accumulate along the faults. Once the stresses become larger than the strength of the restraint, the plates suddenly rupture and release a large amount of energy which results in the movement of earth.
The location that the rupture occurs is called ‘hypocenter’ or ‘focus’ of the earthquake. The location on the earth surface directly above the hypocenter is called the ‘epicenter’.
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Various Plates and Fault Lines
Due to the sudden action of earthquakes, they generally result in the rapid movement or vibration of structures. These movements turn into earthquake loads when the ground acceleration (change of speed with time) is multiplied by the mass of the structure (F=ma). This is also called the ‘Newton’s Second Law’ (after Isaac Newton, the British Scientist who first discovered it). The earthquake load is a ‘dynamic load’, which means it brings about the vibration of structures. Other loads such as dead, live, and snow loads generally act on structures very slowly and do not cause any rapid movement or vibration. These are called ‘static loads’. Depending on the type of structure, wind loads can be static or dynamic.
Therefore, earthquake loads depend on the mass (m) or weight of the structure and the ground acceleration (a), which, itself, depends on the building location (close or far from an active fault).
There are also other parameters involved in calculating earthquake design loads, such as the type of structural system and the building period, T (the time that it takes for the structure to have one complete back and forth movement). The building period is a function of the structural stiffness and mass (decreases with stiffness and increases with mass). Soil type and building importance are also factors.
During an earthquake, the building moves in all directions. However, since it is generally weaker horizontally. in most cases structures are designed to resist the horizontal component of earthquake loads. Also, since earthquake loads act on the structural mass, their resultant is at the center of mass.
Earthquake Loads Acting at the Center of Mass of Each Floor Level
