193 Lateral Force Resisting Systems

The parts of the structure that connect the structure's mass to the ground and resist or otherwise accommodate these displacements or equivalent forces are termed the lateral force resisting system (LFRS). An LFRS is usually capable of resisting only forces that result from ground motions parallel to them. However, the combined action of LFRS along the width and length of a building can typically resist earthquake motion from any direction. LFRS differ from building to building because the type of system is controlled to some extent by the basic layout and structural elements of the building. Figure 19.2 illustrates the basic elements that may be used in LFRS, which consist of axial (tension and/or compression bracing) elements, shear (wall) elements, and/or bending resistant (frame) elements. Horizontal load distributing elements are termed diaphragms and are most often floor or roof slabs, but can be horizontally braced (i.e., truss) elements. Few buildings would use all of these LFRS elements, but most buildings would use one or more.

The earthquake resisting systems in modern steel buildings take many forms, Figure 19.3. Many types of bracing configurations have been used (diagonal, "X," "V," "K," etc.). Moment resisting steel frames

FIGURE 19.2 Types of elements comprising an LFRS.

Bracing

Moment frame

Base isolation

Bracing types "K" "V" Chevron eccentric

"X" diagonal

Braced frame

Damped frame

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