The following table shows the boring log at a site where a multistory shopping center would be constructed. Soil classification and the standard penetration number, N60, are provided in the boring log. All columns of the building are supported by square footings that must be placed at a depth of 1.5 m. Additionally, the settlement (elastic) of each footing must be restricted to 20 mm. Since the column loads at different location can vary, a design chart is helpful for quick estimation of footing size required to support a given load.
a. Prepare a chart by plotting the variation of maximum allowable column loads with footing sizes, B = 1 m, 1.5 m, 2 m, and 3 m. Use a factor of safety of 3.
b. If the gross column load from the structure is 250 kN, how would you use this chart to select a footing size?
c. For the footing size determined in Part (b), what would be the net allowable column load if you use Terzaghi's bearing-capacity equation? For the well-graded sand, assume that ( = 33° and y = 17 kN/m3. Use Fs = 3.
d. Compare and discuss the differences in footing sizes obtained in parts (b) and (c).

Soll type (m) Depth (m) N Well-graded sand (SW) 12 Sandy silts (ML) 4 6 Gravelly sands (SP) 19 10