Lecture05-BC-under-Eccentric-Loads

Lecture05-BC-under-Eccentric-Loads - Foundation Engineering...

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Foundation Engineering Lecture #05 Bearing Capacity of Soils on Eccentrically Loaded Footings Luis A. Prieto-Portar, 2009
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Foundations with a One-Way Eccentricity. In most instances, foundations are subjected to moments in addition to the vertical load as shown below. In such cases the distribution of pressure by the foundation upon the soil is not uniform. Q M B B B X L e The effective width is now, B’ = B - 2e F o r e < B /6 q m in q m a x F o r e > B /6 q m a x e (b ) (a ) Figure 6. An eccentrically loaded footing. whereas the effective length is Still, L’ = L
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The distribution of the nominal pressure is : where Q is the total vertical load and M is the moment on the footing in one axis. Figure 6b shows a force system equivalent to that in Figure 6a. The distance e is the eccentricity of the load, or max 2 min 2 6 (14) and 6 q (15) Q M q BL B L Q M BL B L = + = - Substituting equation 16 in equations 14 and 15 yields: M e (16) Q = max min 6 1 (17 a) 6 q 1 (17 b) Q e q BL B and Q e BL B ° ± = + ² ³ ´ µ ° ± = - ² ³ ´ µ
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Note that in these equations, when the eccentricity e becomes B/6, q min is zero. For e > B/6, q min will be negative, which means that tension will develop. Because soils can sustain very little tension, there will be a separation between the footing and the soil under it. Also note that the eccentricity tends to decrease the load bearing capacity of a foundation. In such cases, placing foundation column off-center, as shown in Figure 7 is probably advantageous. Doing so in effect, produces a centrally loaded foundation with a uniformly distributed pressure. M e Q Figure 7. A footing with the column off-center to preserve a uniform pressure on the soil.
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The general bearing capacity equation is therefore modified to, q u = c N c F cs
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