Nonparallel systems

Figure 8.21 illustrates two non-parallel systems. In each case the directions of strength of the vertical structures are angled with respect to any sets of orthogonal axes. The ability of each configuration to resist horizontal forces and torsion is understood by considering the length of each vertical system as a strength vector. A vector can be resolved

y - inertia force x y - inertia force x

▲ 8.22 A non-parallel system showing the orthogonal force components of each wall and secondary diaphragm stresses for a y direction force.

Mexico Earthquake 1985 Glass Damage

▲ 8.23 Pounding damage during the 1985 Mexico City earthquake.

(Reproduced with permission from David C. Hopkins).

▲ 8.23 Pounding damage during the 1985 Mexico City earthquake.

(Reproduced with permission from David C. Hopkins).

into components parallel to, and normal to, a set of axes (Fig. 8.22). But what is less apparent is that when these systems resist horizontal force their skewed orientation leads to unexpected secondary forces that are required to maintain equilibrium. In this symmetrically configured building, as the shear walls resist y direction forces, the diaphragms must provide tension and compression forces to keep the system stable. When the configuration of non-parallel systems is asymmetrical the distribution of these internal forces becomes far more complex. For this reason codes insist that structural engineers model non-parallel systems in 3-D in order to capture these effects and design for them.

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