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# Footings and Boussinesq Stress Contour Chart

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I'm going through Lindeburg's Civil PE Sample Examination. Question#12 of the morning session group has me stumped. The beginning of the solution solves for the footing width based on the column load and foundation bearing pressure: width = (N/p)^1/2. I cannot seem to find this formula anywhere in my references. Here's the question with the possible answers:

A square foundation supports a column load of 800 kN. The soil beneath the footing is generally homogeneous. If the foundation bearing pressure from this load is reduced from 400 kPa to 100 kPa (the column load remaining constant), the change in stress at a depth of 3 m below the foundation center will be most nearly:

A. a decrease in stress of 20 kPa

B. a decrease in stress of 10 kPa

C. an increase in stress of 10 kPa

D. an increase in stress of 20 kPa

One of my old concrete design books references ACI-15.2.2 as a possible reference for sizing footings. Any help in locating a document that explains that formula would be appreciated. Thanks.

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from FESR Handbook stress=Cr(P/z^2)

Soln

100-(800/3^2)= 11 Kpa, increase of stress

good luck.

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The beginning of the solution solves for the footing width based on the column load and foundation bearing pressure: width = (N/p)^1/2. I cannot seem to find this formula anywhere in my references.

Load N applied to a square footing BxB. Pressure directly under the footing p = N/B2. Therefore B = (N/p)^1/2