How to design Pad Foundation

How to design Pad Foundation with example

Pad foundation is the most common of all the reinforced concrete footings. It supports columns and transmits the loads to the soil evenly. If the areas of pad supporting two or more columns overlap, the pad should be designed as a combined footing. The footing may be square in plan, but where there is large moment acting about one axis, it may be more economical to have a rectangular base or where there is restriction on one side of the base due to the presence of boundary walls, pit or other unavoidable structures or facilities.

Below is the example of a pad foundation:

how to design foundaton

A 225mm by 450mm column supports an ultimate load, including its own self weight, of 504KN. The soil bearing capacity is estimated to be in the region of 150KN/m2. Design a simple pad footing for the column using a grade 25 concrete and high yield high tensile bars with Fy = 410N/mm2.

The following parameters can be deducted:

Column size = 450mm x 225mm

Column load = 504KN

Concrete Stress Fcu = 25N/mm2

Steel stress Fy = 410N/mm2

Soil bearing pressure = 150KN/m2

Assume h = 400mm and d = 400-50-10 = 340mm (assumed 20mm bars will be used)

The base area required:

A = (504×1.1)/(1.46×150)

= 2.532m2

Provide 1600mm x 1600mm x 400mm square base (i.e. 2.56m2 base)

Net pressure:

Fnet = [(504×1.1)/(1.6×1.6)] – 0.4(24)1.4KN/m2

= 203.123KN/m2

Calculate for steel area

X-X Axis:

Span = 575mm

Moment, M = 0.5×0.5752x203.123KNm

= 33.579KNm

K = 0.012, La = 0.95

As = (33.579×106)/(410×0.87×0.95×334)

= 268mm2

Minimum steel area required = 0.15%bh = 600mm2

Provide Y12mm @ 150mm c/c BTM (754mm2)

 

Y-Y Axis:

Span = 687.5mm

Moment, M = 0.5×0.6882x203.123KNm

= 48.074KNm

K = 0.017, La = 0.95

As = (48.074×106)/(410×0.87×0.95×334)

= 389mm2

Provide minimum steel

Provide Y12mm @ 175mm c/c Near BTM (646mm2)

 

Design summary

Provide 1600mm x 1600mm x 400mm Base

Provide Y12mm @ 150mm c/c BTM

Provide Y12mm @ 175mm c/c Near BTM

 

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