Introduction

Post and beam structure

Load-bearing wall Stable under vertical load

Out-of-plane loads

Unstable under horizontal load

▲ 5.1 Instability of two gravity force resisting structures against horizontal forces.

Chapter 4 explained the importance of horizontal structure in the form of floor and roof diaphragms, bond beams and collector or tie members. The role of all these members is to resist inertia forces and then transfer them horizontally into vertical structure. Vertical structure resists those forces and, in next stage of the force path, transfers them downwards through the strength of its members be they walls, columns or diagonal braces and into the foundations.

The vertical structure required for seismic resistance is often very different from that resisting gravity forces. In its two most simple forms, a gravity resisting structure consists of post-and-beam or load-bearing wall construction (Fig. 5.1). The vertical elements of both systems support their load by compression alone. They require sufficient cross-sectional dimensions only to prevent buckling. Consequently, gravity frameworks usually offer little or no resistance to horizontal forces. Post-and-beam structures with pins top and bottom of the columns are completely unstable. Frames designed to resist gravity forces only, where joints between columns and beams are rigid enough to form moment frames, may be more stable against horizontal forces depending on the slender-ness of their columns. As for load-bearing walls, they are usually weak at their base with respect to out-of-plane forces, overturning easily when loaded in that direction. Their length does offer bracing potential when they function as shear walls to resist in-plane forces, but this inherent capacity is realized only by intentional structural design and detailing.

(a) Shear walls

(b) Braced frames

(a) Shear walls

(b) Braced frames

(c) Moment frames

▲ 5.2 The three basic seismic force resisting systems.

(c) Moment frames

▲ 5.2 The three basic seismic force resisting systems.

Forces acting on wall from diaphragm

Bending moment diagram

Wall elevation

Shear force diagram

Bending moment diagram

Tension stress

Shear force diagram

Tension stress

Sliding resistance from foundation

Compression from foundation

Tension tie-down force from foundation

▲ 5.3 Forces acting on and within a shear wall.

Total force from diaphragm

Compression stress

Sliding resistance from foundation

Compression from foundation

Tension tie-down force from foundation

▲ 5.3 Forces acting on and within a shear wall.

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