Page 380 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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LATERAL-FORCE DESIGN

8.34 CHAPTER EIGHT

unequal, shear may be apportioned to each column in proportion to the lengths of the girders it sup-
ports. When bays are equal, the axial load in interior columns due to lateral load is zero.
Inflection points (points of zero moment) are placed at midheight of the columns and midspan of
beams. This approximates the deflected shapes and moment diagrams of those members under lateral
loads. The location of the inflection points may be adjusted for special cases, such as fixed or pinned
base columns, or roof beams and top-story columns, or other special situations. On the basis of the pre-
ceding assumptions, member forces and bending moments can be determined entirely from the equa-
tions of equilibrium. As an example, Fig. 8.16 indicates the geometry and loading of an eight-story
moment-resisting frame, and Fig. 8.17 illustrates the use of the portal method on the upper stories of

FIGURE 8.16 Eight-story moment-resisting frame subjected to static lateral loading.

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