Design of Shallow Foundation (Isolated Footings)

Aim:-

Design a square footing for a column size of 400x400. The compression axial load for the load combination of (1.5 DL + 1.5 LL) is 2000 KN. The safe soil bearing capacity is 150 KN/m2 at a depth of 2 meters below E.G.L. Participants are free to go for either a tapered or stepped footing. Besides the total axial load, also account for the self-weight of the footing and soil above it. Assume M25 concrete grade. Do the following checks

introduction:-

• Footings that are provided under each column independently are called Isolated footings.
• They are usually square, rectangular or circular in section.
• Footing is laid on PCC. Before laying PCC, termite control liquid is sprayed on the top face of the PCC to restrict the termites to damage the footing.
• Isolated footings are provided where the soil bearing capacity is generally high and it comprises a thick slab that may be flat or stepped or sloped.
• This type of footing is the most economical when compared with the other kind of footing.

Drawing based on the above calculation as shown below:-

Critical section for Bending moment

Critical section for two shear

Critical section for one-way shear

RESULT:-

Using the given details the square footing has been designed successfully and detained drawing refer as give below.

 Critical section for Bending moment

Critical section for two shear

Critical section for one-way shear

Isolated footing is a footing that is provided beneath a single column. It is used to transfer a superstructure load through a reinforced concrete column to the soil underneath without exceeding the bearing capacity of the soil and excessive settlement. Because of having reinforcement only in the bottom layer, the isolated footing is one of the most cost-effective options.

An isolated footing is used to support a single column's load. Its plan is usually square or rectangular. It is the simplest, most cost-effective, and most widely used type of footing. Square footings are used to reduce bending moments and shearing forces at critical sections. Isolated footings are used when the column loads are light, the columns are not closely spaced, and the soil is homogeneous. The footing bends with concavity upwards as a result of upward soil pressure and hence two layers of reinforcement are recommended. 

Although providing isolated footing is an economical option, there are only a few situations when we can employ an isolated footing, like: 

• When the safe bearing capacity of the soil underneath is very high.
• When the load coming from the superstructure is low.
• Where columns are not tightly spaced, otherwise, overlapping of adjacent column footing occurs. 
• Isolated footing must be located within property lines which are decided based on the cadastral surveys.

Types of Isolated Footings

There are three types of isolated footing: 

1. Isolated spread footing or Isolated pad footing
2. Isolated sloped footing
3. Isolated stepped footing 

Isolated Spread Footing

It is constructed independently under each column and is typically square, rectangular, or circular in shape. This type of footing has a uniform thickness. Isolated spread footing reduces bending moments and shearing forces at critical sections. Concrete is added with or without reinforcement, i.e., RCC or PCC respectively to increase its ultimate load-bearing capacity. Isolated spread flooring is also known as isolated flat, pad footings, etc.

Fig.1. Flat Isolated Footing

Isolated Sloped Footing

It is also called isolated trapezoidal footing. It is designed with ultimate attention to maintain an angle of 45 degrees from all sides. As compared to PCC and RCC footing, the requirement of concrete and steel is lesser, and hence this type of footing is rather a cost-effective option. 

Fig.2. Slope Isolated Footing

Isolated Stepped Footing

In the early days of civil engineering, this type of footing was very popular. But with the advancement of technology, the application of stepped footing has declined. This is primarily because of economic concerns. Isolated stepped footing is commonly used in residential projects. Usually 3 concrete cross-sections are stacked upon each other to give it the shape of a step.

Fig.3. Stepped Isolated Footing

Different Shapes of Isolated Footings

There are mainly 3 shapes which are very common. They are picturized below: 

1. Square
2. Rectangular
3. Circular

Fig.4.Different shapes of Isolated Footing

Design of Isolated Footing 

The objective behind designing isolated footing is 

• To find out the dimensions of footing including length, width, and thickness
• Calculation of reinforcement required in terms of area of steel (Ast)
• Calculation of development length
• Calculation of bearing stresses 

Design Requirements: 

• Safe bearing capacity of soil underneath
• Grade of concrete, for e.g., M20, M30, etc.
• Grade of steel, for e.g., Fe415, Fe500, etc.
• Column load 
• Column size

Design Steps as per IS Code

As per IS 456:2000, the following are the steps involved in designing an isolated footing: 

1. Area of footing is calculated based on the factored column load
2. Net soil pressure is then calculated, and net factored soil pressure is found.
3. The effective depth of footing is calculated
4. Critical section found.
5. One-way and two-way shear checks are done
6. Calculation of bending moment is done
7. Area of steel, both main and distribution, is calculated 
8. Development length and bearing stress check is applied.

Reinforcement Detailing of Isolated Footing

The detailing of reinforcement used to strengthen the isolated footing comprises of following parameters: 

1. Reinforcement bars
2. Distribution of reinforcement
3. Concrete covers as per specifications
4. Dowel action 
5. Lap splices

1.Diameter of Reinforcement Bars

The minimum diameter of reinforcement to be used should be more than 10 mm. The total reinforcement should not be less than 0.12 % of the total cross-sectional area of the footing in the case of HYSD (High Yield Strength Deformed) bars and 0.15 % in the case of mild steel.

2.Distribution of Reinforcement

The reinforcement in one-way RCC footing is distributed uniformly across the entire width of the footing. The reinforcement extending in both directions is distributed uniformly across the entire width of the footing in two-way square footings. However, reinforcement in two-way rectangular footings is distributed across the entire width of the footing in a long direction. Reinforcement in the short direction is distributed in the central band according to the calculations below.

 Reinforcement in central band  Total reinforcement in short direction =2(yx)+1 Reinforcement in central band  Total reinforcement in short direction =2(��)+1

where, “y” is the long side of the footing and “x” is the shorter side.

3.Concrete Covers

According to IS 456:2000, the concrete cover to isolated footing is provided as follows:

• More than 50 mm if the footing is in direct contact with the earth's surface
• 40 mm in case of PCC levelled surface
• 75 mm in case of the uneven surface of excavation.

4.Dowel Bars

Dowel bars are used to transfer loads coming from the columns to footings and finally to the soil stratum. When concrete alone is not strong enough to withstand the loads, dowel bars come into the picture. The dowel bars should extend into the column to a length equal to the development length required. 

Fig.5. Dowel

5.Lap Splices 

Splice length of dowel and column reinforcement shall be clearly shown. Anchorage of both flexural and dowel reinforcement lengths shall be checked to prevent bond failure of the dowels in the footing and to prevent failure of the lap splices between the dowels and the column bars.

Fig.6. Lap splice

Advantages and Disadvantages of Isolated Footings

Let’s look at the advantages and disadvantages of isolated footings. 

Advantages

• Isolate footings are economical.
• Construction is rather easy 
• Its construction does not require skillful persons
• It is one of the shallow footings and hence needs lesser depths, and ultimately lesser excavation is required.

Disadvantages

• Uniform surface of the earth, i.e. same strength at every location is needed.
• It is mainly designed for lower loads. If the load coming from the superstructures increases, a large-sized isolated footing will be required to construct.
• Because of the separate foundations, this design is vulnerable to a differential settlement that could harm the structure.