Structural requirements

Figure 5.2(a) illustrates a simple building where seismic forces are resisted by four shear walls, two acting in each orthogonal direction. Inertia forces are transferred into the walls by diaphragms at each level. Shear forces and bending moments in a wall increase with the distance from the roof and reach their maximum values at the foundations (see Fig. 5.3) . A structurally adequate wall possesses sufficient strength to resist both shear forces and bending moments. Shear force is resisted by the web area of a wall; that is, its length times its thickness. The shear strength of reinforced concrete walls is provided by a combination of the concrete strength and horizontal reinforcing bars that act as shear reinforcement. Penetrations for windows and doors can reduce the shear strength of a wall significantly. For that reason, where a wall is highly stressed, particularly near its base, such large penetrations might not be possible (Fig. 5.8).

Bending moments cause tension and compression forces at each end of a wall. Some walls are provided with ' chords ' or thickenings at their ends specifically to provide the cross-sectional area necessary to accommodate the vertical tension reinforcement and also to prevent the ends of walls buckling in compression. For the lowest storey of ductile shear walls where plastic hinges form, the ratio of the clear interstorey height to chord or wall thickness, whichever is greater, should be less than or equal to 16.' Where the maximum bending moment is low with respect to wall length, chords may not be necessary for concrete or masonry construction. Often sufficient bending strength can be achieved with adequate vertical reinforcing placed within the wall and provided that buckling is suppressed by provision of sufficient wall thickness.

Shear forces and bending moments, as well as gravity compression forces acting on a wall, are ultimately resisted by the foundations. Potential wall sliding due to shear force is avoided by friction between the base of the foundations and the ground, and horizontally induced soil pressure. Wall over-turning or toppling is counteracted by one or more of the following mechanisms: the inherent stability of a wall due to its length and the gravity force acting upon it including its own weight, a foundation beam with a footing to increase the lever-arm between the line of gravity force through the wall and the centre of soil-pressure, and tension piles (Fig. 5.9). On some sites rock or ground anchors, well protected from corrosion, provide the necessary tension forces to resist overturning.

Inertia force

Tension

Inertia force

Tension

Gravity force Shear

Compression I

Gravity force Shear

Compression I

fl I

Wall elevation: Forces from a shear wall acting on the foundations

Friction force between foundation and soil

Soil strength and weight prevents pile uplift

Wall

Horizontal soil pressure

Belled tension-pile

Vertical soil pressure

Wall stability provided by tension piles and horizontal soil pressure

Vertical soil pressure Wall stability provided by a foundation beam with footing

Lever-arm between centre of gravity and the centre of vertical soil pressure

Wall

Foundation beam Horizontal soil pressure

▲ 5.9 How foundations resist overturning moments and shear forces from a vertical structure, in this case a shear wall.

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