Notes on Example 10.2

O The introduction of eccentricity of the applied loads due to construction error needs to recognize that the total action comprises both applied loads and the dead weight of the footing. The dead weight of the footing still acts through the centre of the footing, thus eccentricity of the total action is less than the eccentricity of the applied loads. There is no guidance in the code as to whether the eccentricity should be calculated for the characteristic or design values of the actions, but we consider that it is best to base the calculation on the design actions as shown. For the example given the difference is minimal but as the relative proportions of permanent to variable loads change so the effects may become more apparent.

© The effect of eccentricity is to reduce the effective dimensions of the footing. These reduced dimensions are then used throughout the rest of the calculation to check the adequacy of the footing.

© The introduction of eccentricity into this example results in the foundation being inadequate for DA1. Thus the footing would need to be re-designed in order to satisfy EC7 requirements. This might entail making the footing larger or repositioning the source of the applied loads.

© The footing is just adequate for DA2.

© The footing does not satisfy DA3 and needs to be re-designed.

10.10.3 Strip footing on clay

Example 10.3 considers the design of a strip footing on clay, as shown in Figure 10.11. Groundwater is at a depth dw below ground surface.

This example demonstrates the use of partial factors for undrained and drained parameters. The inclusion of groundwater above the base of the footing illustrates the complications in applying partial factors to water pressures.

Figure 10.11. Strip footing on clay

Example 10.3 Strip footing on clay Verification of strength (limit state GEO)

Design situation

Consider an infinitely long footing of breadth B = 2.5m and depth d = 1.5m, kN

which is required to carry an imposed permanent action Vgk = 250-and an m kN

imposed variable action VQk = 110-. The footing is founded on a medium strength clay© with characteristic undrained strength cuk = 45kPa, angle of shearing resistance ^ = 25°, effective cohesion c'k = 5kPa, and weight kN

density Yk = 21-. The water table is currently at a depth dw = 1m. The m kN

weight density of groundwater is Yw = 9.81-and of reinforced concrete m kN

Design Approach 1

Geometrical parameters

Design depth of water table dwd = 0m©

Actions and effects kN

Characteristic self-weight of footing is Wgk = Yck x B x d = 93.8-

Characteristic pore pressure underneath base uk,b = Yw x(d - dw,d) = 147kPa
0 0

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