1

Eng. Mohammed M. Alkurd

http://site.iugaza.edu.ps/mmkurd/

Lecture 15, 16, 17

Design of concentrically loaded isolated footings

15.1 Introduction

Footings are structural elements that transmit column or wall loads to the underlying soil.

Footings are designed to transmit these loads to the soil without exceeding its safe bearing

capacity.

The permissible pressure on a soil beneath a footing is normally a few tons per square meter.

The compressive stresses in the walls and columns of an ordinary structure may run as high

as a few hundred tons per square meter. It is, therefore, necessary to spread these loads over

sufficient soil areas to permit the soil to support the loads safely.

15.2 Types of footings

Among the several types of reinforced concrete footings in common use are the wall, isolated,

combined, strap raft, and pile-cap types.

2

Eng. Mohammed M. Alkurd

http://site.iugaza.edu.ps/mmkurd/

15.3 Concentrically loaded isolated footings

An isolated or single-column footing, as shown in Figure (b), is used to support the

load of a single column. These are the most commonly used footings, particularly where

the loads are relatively light and the columns are not closely spaced.

If the resultant of the loads acting at the base of the footing coincides with the centroid of

the footing area, the footing is concentrically loaded and a uniform distribution of soil

pressure is assumed in design, as shown in Figure.

3

Eng. Mohammed M. Alkurd

http://site.iugaza.edu.ps/mmkurd/

Isolated Footings could be square, rectangular or circular.

15.3.1 Design of concentrically loaded isolated footings.

1. Select a trial footing depth.

According to ACI Code 13.3.1.2, depth of footing above reinforcement is not to be less

than 15 cm for footings on soil. Noting that 7.5 cm of clear concrete cover is required if

concrete is cast against soil, a practical minimum depth is taken as 25 cm.

2. Calculate the required base area of the footing.

𝑞𝑎𝑙𝑙(𝑛𝑒𝑡) = 𝑞𝑎𝑙𝑙(𝑔𝑟𝑜𝑠𝑠) − 𝛾𝑐(ℎ𝑐) − 𝛾𝑠(𝑑𝑓 − ℎ𝑐)

Where:

𝑞𝑎𝑙𝑙(𝑔𝑟𝑜𝑠𝑠) = 𝐴𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 𝑠𝑜𝑖𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒.

𝑞𝑎𝑙𝑙(𝑛𝑒𝑡) = Allowable net soil pressure.

𝛾𝑐 = 𝐶𝑜𝑛𝑐𝑒𝑡𝑒 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡.

𝛾𝑠 = 𝑆𝑜𝑖𝑙 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡.

𝛾𝑠 = 𝑆𝑜𝑖𝑙 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡.

ℎ𝑐 = 𝐶𝑜𝑛𝑐𝑒𝑡𝑒 ℎ𝑒𝑖𝑔ℎ𝑡.

ℎ𝑐 = 𝐹𝑜𝑡𝑖𝑛𝑔 𝑑𝑒𝑝𝑡ℎ.

4

Eng. Mohammed M. Alkurd

http://site.iugaza.edu.ps/mmkurd/

The required area:

𝐴𝑟𝑒𝑞 = 𝑃𝑠

𝑞𝑎𝑙𝑙(𝑛𝑒𝑡)

3. Calculate the net factored soil pressure.

𝑞𝑢(𝑛𝑒𝑡) =𝑃𝑢

𝐵 × 𝐿

4. Check footing thickness for punching shear.

The critical section for punching shear is located at distance d/2 from column faces and usually

takes the shape of the column. Footing thickness is adequate for resisting punching shear once.

𝑉𝑢 ≤ Ф𝑉𝑐

5

Eng. Mohammed M. Alkurd

http://site.iugaza.edu.ps/mmkurd/

Punching shear force resisted by concrete 𝑉𝑐 is given as the smallest of:

Where:

𝑏𝑜 = perimeter 𝑜𝑓 𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑝𝑢𝑛𝑐ℎ𝑖𝑛𝑔 𝑠ℎ𝑒𝑎𝑟 𝑠𝑒𝑐𝑡𝑖𝑜𝑛.

𝛽 = 𝑙𝑎𝑟𝑔𝑒𝑟 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑐𝑜𝑙𝑢𝑚𝑛

𝑠𝑚𝑎𝑙𝑙𝑒𝑟 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑐𝑜𝑙𝑢𝑚𝑛

5. Check footing thickness for beam shear in each direction.

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Eng. Mohammed M. Alkurd

http://site.iugaza.edu.ps/mmkurd/

The critical section for beam shear is located at distance d from column faces.

If 𝑉𝑢 ≤ Ф𝑉𝑐 thickness will be adequate for resisting beam shear without using shear

reinforcement.

a) In the short direction:

The factored shear force is given by

The factored shearing force resisted by concrete is given by

b) In the long direction:

The factored shear force is given by

The factored shearing force resisted by concrete is given as

6. Compute the area of flexural reinforcement in each direction.

The critical section for bending is located at face of column, or wall, for footings supporting

a concrete column or wall, as specified by ACI Code 13.2.7.1.

a) Reinforcement in the long direction:

𝑀𝑢 = 𝑞𝑢(𝑛𝑒𝑡)𝐵

2(

𝐿 − 𝐶

2)

2

b) Reinforcement in the short direction:

𝑀𝑢 = 𝑞𝑢(𝑛𝑒𝑡)𝐿

2(

𝐵 − 𝐶

2)

2

7

Eng. Mohammed M. Alkurd

http://site.iugaza.edu.ps/mmkurd/

The reinforcement ratio is calculated based on rectangular section design, where the minimum

reinforcement ratio 𝜌𝑚𝑖𝑛 is not to be less than

1. 0.002 for 𝑓𝑦 < 4200 𝑘𝑔/𝑐𝑚2

2. The greater of 0.0018(4200)

𝑓𝑦

and 0.0014 for 𝑓𝑦 ≥ 4200 𝑘𝑔/𝑐𝑚2.