ECCENTRICALLY LOADED BRICK WALL

In general, walls in buildings are commonly loaded with some eccentricity. Eccentricity may be caused due to one reason or another. Thus, there is a little possibility of establishing an exact relationship between factors which may cause eccentricity.

Some of the factors which contribute for eccentricity on brick walls are:

1. Long floor edges
2. Magnitude of loads
3. Relative stiffness (of slab or beam and the wall)
4. Flexibility of the support
5. Geometry of the support
6. Unequal spans

Thus a designer has to use his judgment to assess the degree of eccentricity based on the situation. However, I.S. Code (IS: 1905, 1987) provides certain guidelines for determination of eccentricity which are discussed below.

17.11.1 Exterior Walls

1. When a span of concrete floor or roof is more than 30 times the thickness of the wall, then all eccentricity may be anticipated due to sagging. The eccentricity is given as one-sixth of the bearing width.
2. When the roofs or floors do not bear on the entire width of the wall, then there is a possibility for eccentricity even for normal span. In such cases, the eccentricity is taken equal to 1/12^th the thickness of the wall.
3. For timber and other light weight floors, eccentricity is assumed one-sixth the thickness even for full-width bearings.

Interior Walls

1. Eccentricity is caused by unequal span of roof or floor. In such cases a net bending moment is induced (Fig. 17.14). This bending moment is due to an eccentric load.
2. The load is considered axial if the difference between the two loads is within 15%. Otherwise, each floor load is assumed to act at a distance equal to one-sixth the thickness of the wall and then the overall eccentricity is computed.
3. In general, eccentricity of loading increases with the increase in the fixity of slabs/beams at the supports.

Figure 17.14 Eccentricity due to unequal span in interior wall

Stress Distribution Under Eccentric Loads

In an eccentrically loaded wall, there is an axial load and a bending moment. These two may be combined into a single resultant load acting at a distance. This is known as equivalent eccentricity (Fig. 17.15).

Figure 17.15 Equivalent eccentricity

The stress distribution due to axial load and the bending moment are combined to get the stress distribution due to the resultant load. The stress distributions for various eccentricities are shown in Fig. 17.16.

Figure 17.16 Variation of stress distribution (Source: IS: 1905, 1987)

It can be observed that with an increase in eccentricity, the net compressive stress in the tension face decreases. That is, the tensile stress due to bending moment decreases.