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

These same elements are also basic parts of an architect's structural "vocabulary." The choice of the appropriate lateral load resisting system for any particular building is thus highly dependent upon the architectural concept of the building.

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

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.

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.

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 be immediately become greatly increased. This illustrates a very important point: In general, 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.

Braces Frames

Braced frames act in the same manner as shear walls, but they may offer lower resistance depending on their details of their design and construction. Bracing generally takes the form of steel rolled sections, circular bar sections, or tubes. Vibration may cause the bracing to elongate or compress, in which case it will lose its effectiveness and permit large deformations or collapse of the vertical structure. Ducti1ity therefore must be designed into the bracing to create a safe assembly.

Movement Resistant Frames

When seismic resistance is provided by moment resistant frames, lateral forces are resisted primarily by the joints between columns and beams. These joints become highly stressed and the details of their construction are very important. Moment frames use, as a last-resort resistance strategy, the energy absorption obtained by permanent deformation of the structure prior to ultimate failure. For this reason, moment resistant frames generally are steel structures with bolts or welded joints in which the natural ductility of the material is of advantage. However, properly reinforced concrete frames that contain a large amount of steel reinforcing are also effective as ductile frames. They will distort and retain resistance capacity prior to failure and will not fail in a brittle manner.

Architecturally, moment resistant frames offer a certain advantage over shear walls or braced frames because they tend to provide structures that are much more unobstructed internally than shear wall structures, which facilitates the design of accompanying architectural elements such as exterior walls, partitions, and ceilings and the placement of building contents such as furniture and loose equipment. Nevertheless, moment resistant frames require special construction and detailing and, therefore, are more expensive than shear walls or braced frames.

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