Basics of Earthquake Resistant Design

Lateral Load Resisting Systems

When designing a building that will be capable of withstanding an earthquake, engineers can choose various structural components, the earthquake resistance of which is now well-understood, and then combine them into what is known as a complete lateral load resisting system. These structural components usually include:

  • shear walls
  • braced frames
  • moment resisting frames
  • diaphragms
  • horizontal trusses

Of course, a building always possesses floors and a roof. But the earthquake resistant characteristics of these basic elements is highly variable. Not only that, the building's horizontal elements can be supported by a wide variety of wall and frame types or wall-frame combinations, the choice of which is usually dictated by considerations other than earthquake resistance. For instance, some buildings such as a warehouse or a parking garage must have a large open floor space--which means that roof and floors of such structures will not be provided with as much vertical support from beneath as they might be otherwise.

The engineer-designer in charge of making a building earthquake resistant must therefore choose a combination of structural elements which will most favorably balance the demands of earthquake resistance, building cost, building use, and architectural design.

Diaphragms

(fig. 1)

Diaphragms are horizontal resistance elements, generally floors and roofs, that transfer the lateral forces between the vertical resistance elements (shear walls or frames). Basically, a diaphragm acts as a horizontal I-beam. That is, the diaphragm itself acts as the web of the beam and its edges act as flanges. (See figure 1)

Shear Walls

Shear walls are vertical walls that are designed to receive lateral forces from diaphragms and transmit them to the ground. The forces in these walls are predominantly shear forces in which the fibers within the wall try to slide past one another.

(fig. 2)

When you build a house of cards, you design a shear wall structure, and you soon learn that sufficient card "walls" must be placed at right angles to one another or the house will collapse. If you were to connect your walls together with tape, it is easy to see that the strength of this house of cards would significantly increase. This illustrates a very important point, in which the earthquake resistance of any building is highly dependent upon the connections joining the building's larger structural members, such as walls, beams, columns and floor-slabs.

Shear walls, in particular, must be strong in themselves and also strongly connected to each other and to the horizontal diaphragms. In a simple building with shear walls at each end, ground motion enters the building and creates inertial forces that move the floor diaphragms. This movement is resisted by the shear walls and the forces are transmitted back down to the foundation.