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Fundamentals of Earthquake Engineering
Part 1. Seismic Forces
 Many factors that influence a structure's ability to resist seismic forces are decided before a structural engineer becomes involved in a project. For that reason, it is valuable for other design professionals to have a basic understanding of how structures behave under seismic loading.
During a seismic event, the ground shaking imparts both horizontal and vertical forces on the structure due to its mass acceleration in accordance with Newton's second law of motion. Therefore, structures must be designed to withstand the force of their own mass acceleration. Hence, structural engineers refer to a building's mass accelerating due to an earthquake as a "seismic force".
We have no control over the ground's acceleration in an earthquake. However, we do have some control over the mass of our structures, where the mass is located, how far that mass is located from the ground (building height), and how those forces can be resisted.
Naturally the building type will dictate its own mass. But with careful planning, we can avoid excessive seismic forces by locating the mass in strategic locations. Employing the following basic characteristics improves the structural performance under seismic loads and reduces construction cost.
- Symmetrical layout
- Lightweight top floors
- Evenly distributed masses
- Single diaphragm per story
With a basic understanding of seismic forces, the above guidelines should seem intuitive. Unsymmetrical buildings and clustered concentrated masses cause torque on a structure. Tall buildings and heavy top floors result in greater forces since the force is applied farther from the base supporting ground, think of a cantilevered beam. Stories with multiple diaphragms instead of a single diaphragm typically result in additional costly seismic resisting systems. Every diaphragm has its own mass and must have a seismic resisting system. It is often the case that one diaphragm's seismic resisting system cannot be connected to another's, even if separated by just a few feet.
Functionality commonly dictates how to orient mass and layout of a structure. Consequently, seismic resistance can add substantial costs to a building. Just being aware of what affects these seismic forces can help us make better and more cost efficient designs.
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