Footing is an essential part of any structure as it is the key element transferring load safely from the structure to the ground.
What is Eccentricity in Civil Engineering?
Eccentric in civil engineering means the centres of two elements are not aligned. Here, the centre of the column is not aligned with the centre of the respective footing. Hence it is called eccentric footing. Thus, eccentricity in civil engineering means the distance between these two centres, and it is denoted by ‘e’.
To dig more about what is eccentric footing let’s start with the basics of structure.
According to Dr.B.C.Punmia, Ashok Kumar Jain and Arun Kumar Jain (1984) (Authors of “A textbook of Building Construction”), every building consists of two basic components: the super-structure and the substructure or foundations. The super-sturcture is usually that part of the building which is above the ground, and which serves the purpose of its intended use.
The substructure or foundations is the lower portion of the building, usually located below ground level, which transmits the load of the super-structure to the sub-soil. Footing is a part of the substructure just above the soil. It is basically the bottommost part of a foundation that is in direct contact with the soil. The main function of footing is to transfer the load from the column uniformly into the soil. The load is transferred uniformly when the centre of gravity of the column and its footing are in the same line.
Types of Footing
Footings are of two types concentric and eccentric based on the alignment of the centre of gravity (CG) of the column and its respective footing. The Centre of gravity is the point where the mass of the body is concentrated.
- What is Concentric footing – a footing is called concentric when the centre of gravity (CG) of the column is in alignment with the centre of gravity (CG) of the footing. Hence, the load coming from the column is distributed uniformly downward to the soil. This generates uniform upward pressure below the footing by the soil. The foundation provided here is concentric foundation. (When the load is applied upon an area, pressure is generated and here upward pressure is generated by the soil below footing).
- What is Eccentric footing – a footing is called eccentric when the centre of gravity (CG) of the column is NOT in alignment with the centre of gravity (CG) of the footing and the load from the column is not transferred uniformly to the soil. The foundation provided here is called eccentric foundation. Due to this, linear upward pressure is generated by the soil below. This can happen in two cases. The two cases have been given by N C Sinha and S K Roy (1991) (Authors of “Fundamentals of Reinforced Concrete”).
- In the first case, the centre of gravity (CG) of eccentric load coincides with the centre of gravity (CG) of the footing as a result of which the centre of gravity (CG) of footing is unsymmetrical with the centre of gravity (CG) of column by a distance e=M/P. In the second case, the centre of gravity (CG) of footing coincides with the centre of gravity (CG) of column, but the load is eccentric with respect to the centre of gravity (CG) of the footing. In case of unsymmetrical footing, the soil pressure distribution is uniform, whereas in case of a symmetrical footing the distribution is trapezoidal. Eccentric load is the load that is not applied at the centre of the element; here column.
For the time being let us leave the concentric footing aside and lay our emphasis on eccentric footing. But before that let us first know what is eccentric loading? Eccentric load is the load that is not applied at the centre of the element; here column.
Moving further, we will discuss everything about eccentric footing, beginning with what is eccentric footing.
What is Eccentric Footing?
It is a footing in which the column is not concentric with the footing slab; it is shifted from the centre towards the side of the boundary wall. The amount of shift from the centre to one side or two (in case of corner column) is termed eccentricity and so the footing provided here is called eccentric footing or eccentrically loaded footing. As the column cage is fixed on one side of the footing reinforcement it resembles the shape of a shoe, thus it is also called shoe footing. Foundation here is called eccentric foundation.
The pressure generated by the soil below eccentrically loaded footing is linear, with maximum pressure right below the column and linearly decreasing away from the column that is towards the edges. Below fig shows the linear pressure under the eccentrically loaded footing. In the figure, it is clearly seen that the column is eccentric towards the right side of the footing.
P – Load from column
e – Eccentricity (distance between centre of footing and centre of column)
B – Width of the footing
L – Length of the column
Why is There a Need For Eccentric Column Footing?
Most of the time an attempt is made to provide concentric column footing where there is an alignment in the centre of gravity (CG) of the column and the centre of gravity (CG) of the footing, but this requires a great deal of space which is possible in the columns that are in the interior columns and exterior columns only if there is the provision of extra space for footing projection. In the cases where the exterior columns lay on the boundary walls and there is restriction of space for footing width, in such cases eccentric column footing is provided.
What is Eccentricity in a Column Footing?
The column is erected from one side of the footing slab and not from the centre of the footing slab, and due to this distance, the centre of gravity (CG) of the column does not coincide with the centre of the footing slab. So, there is a considerable distance between the centre of the column and centre of the footing. This distance is called the eccentricity in a column footing.
Eccentricity in a column could be in one direction or in two. When the column is located in the line of wall and property line the eccentricity is only in one direction. When the column is at the corner of the boundary wall, then the eccentricity is from both the sides that make the corner. In both the cases, eccentrically loaded footing is provided.
Basic Calculation Regarding Design of Eccentric Footing
A footing is called eccentric when load on column is eccentrically loaded or column centre of gravity (CG) does not align with footing centre of gravity (CG). In design of eccentric footing, certain factors are to be kept in mind so that the footing does not overturn due to uplift pressure from the soil below.
These factors include load from column, eccentricity, moment, pressure exerted by the soil, bearing capacity of soil below, self-weight of footing, length, breadth, depth and thickness of footing slab, amount of steel reinforcement, dimensions of steel reinforcement and grade of concrete. In eccentric column footing, the column transfers load along with bending moment (due to eccentricity) at the connection of the footing and the column. The pressure ‘q’ that is developed by the soil will not be uniform. It will be maximum right below the column and decreasing outwardly. This pressure is evaluated from the following equation:
| qmax = | Q/BL + 6M/B²L |
| qmin = | Q/BL – 6M/ B²L |
Where,
Q – Vertical load
M – Moment on the foundation
B – Width of the foundation
L – Length of the foundation
For the soil, the magnitude of qmax should not exceed the safe bearing pressure, and also, qmin should be positive so that the footing may remain in contact with the ground.
In the extreme case of eccentrically loaded footing, qmin = 0, when e = B/6, and this gives the maximum value of eccentricity.
The tension will be developed if ‘e’ is greater than B/6, and the end of the footing (one that is not below the column) will have loose contact with the soil in that case.
Another important calculation in eccentric footing is volume of concrete required for eccentric column footing. It is called centric footing volume formula.
Eccentric footing volume formula (V) = Area of footing slab (A) x Height of footing slab (h)
What are the Impacts of Eccentric Loading on Footing?
We already defined what is eccentric loading, so now here we see its impacts. Eccentric loading causes the eccentricity between the centre of the column and centre of the footing. Due to this, linear pressure is formed below the footing. Subsequently there is a decrease in contact pressure linearly from toe to heel of the footing. So, design of eccentric footing takes consideration of maximum and minimum pressure. When minimum pressure is negative, there is an uplift of footing slab from the soil due to soil uplift pressure.
In order to deal with this and provide stability against soil uplift pressure, an eccentric column footing needs to be connected with an internal concentric column footing. This will prevent the uplift of the eccentric column footing slab. There are two ways of connecting an eccentric column footing with a concentric column footing namely combined footing and strap beam.
(i) Combined footing (H4) – Here a common footing slab is shared with a concentric column footing and an eccentric column footing. The Combined footing is usually rectangular or trapezoidal. Providing combined footing will resist the uplift of the eccentrically loaded footing slab and give more contact pressure.
(ii) Strap Beam (H4)- Here a strap beam is provided to connect the eccentric footing with a concentric interior footing. The strap beam functions to keep the eccentrically loaded footing slab grounded and there is no uplift of the footing slab due to soil uplift pressure. This is an economical option compared to the combined footing.
Why Choose Isolated Eccentric Footing Over Combined Footing?
A combined footing is a great solution when it comes to providing stability to an eccentric footing, but combined footing gets quite expensive when the distance between an eccentric column and a concentric column is considerable. In such cases, strap footing may not be so effective and thus it is advisable to select isolated eccentric footing and not combined footing.
To get some perspective on isolated footing, click the link below:
How Safe is Eccentric Footing?
Eccentric footing is a safe and practical solution when a column is to be erected on the boundary walls and where the column footing width is restricted due to limitations of the boundary. Design of eccentric footing with no negative pressure is quite safe against the uplift.
A well-designed eccentric column footing design will have a positive value of ‘q’ (upward pressure exerted by soil on footing). This ensures that footing does not fail by uplifting of footing slab from one end. Uplifting of footing slab takes place when there is negative pressure, that is the magnitude of ‘q’ is less than zero. In such case, soil exerts pressure on the footing slab causing it to uplift.
What are the Possible Drawbacks of Eccentric Footing?
There are a couple of drawbacks, firstly, when the eccentricity- that is the distance between the centre of the column and the centre of the footing slab is larger and when the pressure falls below zero, then there are chances of an uplift. Secondly, it requires more depth and more steel, which makes it expensive.
Eccentric Footing Details of Reinforcement
Eccentric column footing design as per Indian Standard code SP34
Cover
The concrete cover is provided to safeguard the reinforcement steel. As the footing is in contact with the earth, the minimum thickness of the cover to the main reinforcement steel should not be less than 50 mm. For the externally exposed faces, the minimum concrete cover should not be less than 40mm. In the cases where concrete is in direct contact with the soil, for instance, when lean concrete is not used to level the ground surface at the bottom of the footing, then concrete cover of 75 mm should be provided.
Minimum Reinforcement and Bar Diameter
Mild Steel Reinforcement
Minimum reinforcement of not less than 0.15 percent of the gross sectional area should be provided.
High Strength Deformed Bars
Minimum reinforcement of not less than 0.12 percent of the gross sectional area should be provided.
Bar Diameter – Main reinforcing bars should not be less than 10mm.
If you are looking for information on steel reinforcement, then Gharpedia gives you complete information on the same here.
Reinforcement Requirements
Total tensile reinforcement is to be distributed across the resisting section. Reinforcement extending in both directions should be distributed uniformly throughout the width of the footing.
Vertical Reinforcement or Dowels
Vertical reinforcement from the column is extended, also known as dowels, for at least 0.5 percent of the cross-sectional area of the supported column or pedestal. Minimum 4 bars of 12mm diameter should be provided. The diameter of dowel bars should not exceed the diameter of column bars by more than 3mm.
To economize, footings are sloped or stepped towards the edge, fulfilling all the requirements of bending and punching shear. The thickness at the edges shall not be less than 15 cm for footings on the soil base.
Conditions in Which Eccentric Footing is Used?
Usually in cases where the adjacent land owner does not allow for footing projection or where there already exists footing projection of the adjacent owner and in the cases where cantilever balcony is projected because here bending moment is induced along with the axial load, hence, eccentric footing is provided.
Conclusion
This article covered all questions regarding what is eccentric footing. Considering today’s scenario where buildings and houses are closely spaced due to high population density, the question of enough available footing space always exists. To provide a safe column foundation for structure, eccentric loaded footing is the best option. It helps to erect a column without disturbing the adjacent property.
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