Foundations

Types of Footings in Construction and Their Usage

Different Types of Footings

Foundations give the support for structures, transfer their load to layers of soil 

that have sufficient bearing capacity and suitable settlement characteristics. There are very wide range of foundation types for different applications in civil engineering structures, depending on considerations such as:

         1: The nature of the load on soil.

2: Ground or Soil conditions

3: Presence of water in soil

4: Accessibility to the proposed site

5: Sensitivity to seismic vibration

6: Project completion time frames

Broadly, foundations can be categorized as:

1: Shallow foundations

2: Deep foundations

Shallow foundations: are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils.

Deep foundations: are necessary where the bearing capacity of the surface soils is insufficient to support loads imposed and so they are transferred to deeper layers with higher bearing capacity.

Now coming towards the footings Commonly used in daily life construction

activities which are basically shallow foundations:

Shallow Foundations

Wall footings:

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A wall footing or strip footing is a continuous strip of bricks or concrete that serves to distribute the weight of a load-bearing wall across an area of soil. It is the very basic type of a shallow foundation.

Wall Footing Explanation

This type of footing is commonly used in local house construction and light load bearing structures

Spread footing foundations:

A spread footing foundation, which is common in residential as well as local commercial buildings, has a wider bottom portion than the load-bearing foundation walls it supports. This wider part "spreads" the weight of the structure over more area for greater stability.

Spread Footing Illustration

The design and layout of spread footings is controlled by several factors, foremost of which is the weight (load) of the structure it must support, penetration of soft near-surface layers, and penetration through near-surface layers likely to change volume due to frost heave or shrink-swell.

These foundations are common in residential construction that includes a basement, and in many commercial structures. But for high rise buildings they are not sufficient. A spread footing that changes elevation in a series of vertical steps so that it follows the contours of a sloping site or accommodates changes in soil strata, is called a stepped footing.

Stepped or Pedestal Footings:

A concrete pedestal is a compression element provided to carry the loads from supported elements like columns, statues etc. to footing below the ground. It is generally provided below the metal columns. In general pedestal width is greater than its height.

Stepped or Pedestal Footings

Functions of Concrete Pedestal

The main functions of pedestal provision are as follows.

• To avoid contact between soil and metal elements or wood elements.
• To offer support for elements at some elevation
• To allow thinner foundation footings.

Sloped Footing

The type of foundation in which the column over the footing is provided with sloped concrete pedestal for the stability and to prevent the column from punching out of the concrete base is known as Sloped footing.

Sloped Footings

SLOPED FOOTING ISSUES

The sloped footing option may seem appealing because of the simple geometry and apparent ease in formwork construction. It does, however, create the following construction issues:

1: Vertical wall bars above the footing will have different lengths, creating major challenges in the fabrication plant and on the job site. Two of these—managing the inventory and placing the bars in their correct locations— can be eased by detailing the bars with variable lap splice lengths. This will, however, increase the quantity of vertical reinforcement;

2: Horizontal reinforcing bars in the lower portion of the wall will also have different lengths because they are interrupted by the sloped footing. If constant length horizontal bars are used at the wall base, they can be fanned out, but this will create a variable vertical spacing of the reinforcing bars;

3: Sloped footings will require trapezoidal formwork. This will require modifications to standard rectangular formwork;

4: A sloped footing could be unstable, particularly on a very steep slope; and Concrete placement and finishing could be difficult, and a stiff concrete mixture might be required to prevent the concrete from flowing downhill, which may lead to segregation. Alternatively, the top of the form may have to be closed.

Because of these challenges, most engineers and contractors prefer to use stepped footings instead of sloped footings.

Combined Footing

Combined footings are provided when distance between two columns is small and soil bearing capacity of soil is lower and their footings overlap with each other.When two columns are close together and separate isolated footings would overlap, in such case, it is better to provide a combined footing than isolated footing.

Combined Footing Explanation

Following are the reasons when combined footings are preferred,

01. The distance between two columns is small and when soil bearing capacity of soil is lower and their footings overlap with each other.

02. When one column is close to a property line or sewer pipe, the Centre of gravity of column will not coincide with footing. In such cases, it is necessary to provide combined this footing with that of the adjacent internal column.

03. Dimensions of one side of footing are restricted to some lower value so that column footings may be combined.

The combined footing may be rectangular, trapezoidal or Tee-shaped in a plan. The ultimate aim is to get uniform pressure distribution under the entire area of the footings. To achieve this, the centre of gravity of the footing area should coincide with the centre of gravity of the total loads of the two or more columns.

Strap footing

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A strap footing is a component of a building’s foundation. It is a type of combined footing, consisting of two or more column footings connected by a concrete beam. This type of beam is called a strap beam. It is used to help distribute the weight of either heavily or eccentrically loaded column footings to adjacent footings.

Strap Footing

A strap footing is often used in conjunction with columns that are located along a building’s property or lot line. Typically, columns are centered on column footings, but in conditions where columns are located directly adjacent to the property line, the column footings may be offset so that they do not encroach onto the adjacent property. This results in an eccentric load on a portion of the footing, causing it to tilt to one side. The strap beam restrains the tendency of the footing to overturn by connecting it to nearby footings.

Raft or Mat footing

A footing is a concrete support under a foundation that rests in solid ground and is wider than the structure supported. Footings distribute the weight of the structure over the ground.

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Raft foundation is a thick concrete slab reinforced with steel which covers the entire contact area of the structure like a thick floor. Sometimes area covered by raft may be greater than the contact area depending on the bearing capacity of the soil underneath. The reinforcing bars runs normal to each other in both top and bottom layers of steel reinforcement.