What Is Spread Footing?
With loads provided within the upward direction, this footing actually acts like an inverted cantilever, and this sort of footing is typically a rigid element & they’re orthogonal just in case of symmetric footing.
This type of footing may be a circular, square, or rectangular slab of uniform thickness and to spread the load over a sizeable area sometimes it’s stepped.
The base of spread footing is slightly wider than load-bearing foundation base. This is also called a stepped spread foundation.
The spread footings are constructed with concrete & steel and, because of their design, they’re less likely to fail than other spot footers.
Types of Spread Foundation:
- Wall Footing.
- Isolated Footing or Column Footing.
- Combined Footing.
- Strap Footing or Cantilever Footing.
- Continuous Footing.
- Inverted Arch Footing.
- Grillage Footing.
- Raft Foundation or Mat Foundation.
1. Wall Footing.
A wall footing or strip footing is a continuous strip of concrete that serves to spread the weight of a load-bearing wall across an area of soil. It is the component of a shallow foundation. Wall footings carrying direct vertical loads might be designed either in plain concrete or in reinforced concrete.
Wall footing are often either simple or steeped. Wall footing may be further categorized into two types:
- Simple wall footing.
- Stepped wall footing.
2. Isolated Footing.
Isolated footing is the single or individual footing which transfers load to the underground soil. It is provided when a single column is to be provided. A foundation is the part of the substructure which makes a direct contact or rested inside the ground.
This type of footing is employed for a single column. This isolated footing is further classified into 3 types:-
- Stepped footing.
- Simple spread footing.
- Sloped footing.
3. Combined Footing.
When individual footing has to accommodate two or in exceptional case more than Two columns is known as the combined footing or When two or three columns are located very near to each other, we provide a single substructure for them and the footing is known as a combined footing.
When two or more columns are available in a row then this kind of footing is made. In this, there are two kinds of footing:-
- Rectangular-shaped combined footing. Columns are uniform or identical.
- Trapezoidal-shaped combined footing. Columns aren’t equal or same & have space limitations.
4. Strap Footing.
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
Strap footing is a combination of two or more individual footings connected strap beam. It’s additionally sometimes called as cantilever footing or pump handle foundation.
5. Continuous Footing.
A continuous footing is the one which supports more than two columns. The footing is analogous to the strip footing for wall. The loads from the individual columns are transferred either directly to the footing slab, or through a longitudinal beam running longitudinally when the loads are heavy.
This sort of foundation is suitable within the earthquake area and also prevents differential settlement.
6. Inverted Arch Footing.
An inverted arch or invert is a civil engineering structure in the form of an inverted arch, inverted in comparison to the usual arch bridge. In the simplest case, the arches simply spread the downwards loads of viaduct piers into a wider ground area, exactly as for an inverted arch bridge.
This sort of foundation employed in places where the bearing capacity of the soil is extremely poor and also the load of the structure is concentrated over the walls & deep excavations aren’t possible.
7. Grillage Footing.
A foundation consisting of one, two or more layers of beams (typically steel) superimposed on a concrete layer to disperse the load over a large area is a foundation for grilling. This type of foundation is generally used for pillars and column scaffolds with a heavy structure.
A grillage footing utilizes to transmit ponderous loads from the steel column to the soils having low bearing power.
8. Raft Foundation.
A raft foundation, also called a mat foundation, is essentially a continuous slab resting on the soil that extends over the entire footprint of the building, thereby supporting the building and transferring its weight to the ground.
Raft foundation is actually a merged footing that covers the whole area just underneath a structure & holds all the walls & columns.
In this foundation, an R.C.C. slab is provided with a beam or without the beam of suitable thickness.
Concrete of Spread Footing:
The footing depth should be deep enough to resisting punching shear and direct shear transferred from column load. The reinforcement within the footing should be designed to resist bending moments.
The dowels at the column-footing interface should be sufficient to transfer column loads. To feature additional support, spread footings are developed with concrete & reinforced with steel. Since spread footing transfers the load of the building over a sizeable area, there is a little risk of failure compare to spot footers.
Where moment framed structure is employed, spot footings will be reinforced to resist moment forces generated by wind load or earthquake load.
Spread Footing Design:
- At first, We need to determine the structural loads and sizes of different members at the foundation level.
- Then we need to collect data and set the proposed footing
- After that, We determine the depth and location of the footing element.
- We also need to determine the bearing capacity of the soil.
- Next, We determine the total and differential settlement and the strength of concrete also.
- Then we select the steel grad, footing dimensions, and thickness.
- After this process, We design the substructure and superstructure connection.
- Finally, We check the uplift and stability against sliding conditions.
Formula for Determining Area of Spread Footing:
Using the subsequent formula, the bottom area of the spread footing is calculated:-
The total load approaching the spread footing is Qt & q is the base area of spread footing.
Advantages of Spread Footing:
For providing the spread footing as a foundation within the building, there are the subsequent advantages such as:-
- For providing greater stability, the spread footing conveys & disseminates the load from the structure over the sizeable area of soil beneath it.
- As compared to other kinds of footing, spread footing has little or no risk of foundation failure.
- By controlling the damage within the structures, this sort of footing increases the lifetime of the buildings.
- These footings are accustomed provide support to the structure continuously.
- These footings are effortless to supply within the building structures & easy construction of basements.
- As compared to a load-bearing foundation a variety footing foundation contains a wider bottom portion and for greater stability over a sizeable area, the broader bottom spreads the burden of the structure.
- Concrete and reinforced steel are the main material to construct this type of footing.
- In residential construction, these footings are ordinarily utilized.
- Reduces cracking caused by settlement.
- Stabilizes soil at the base of the structure.
- Cost savings in construction, design & internal control.
- Differentiates in size and quantity.
Disadvantages of Spread Footing:
- Not suitable for every soil type.
- You have to fill all the lot in one go, but preparing such an amount of concrete on your own is quite problematic.
- There is quite a decent amount of work with pouring the concrete, so you can’t do it without help.
Mat Foundation Vs Spread Footing
- A spread footing (or isolated or pad) footing is provided to support a single column.
- A spread footing is a circular, square or rectangular slab of invariable murkiness.
- Sometimes, it’s stepped to spread the load over an oversized area.
- A spread footing foundation, which is common in residential buildings, features a wider bottom portion than the load-bearing foundation walls it supports.
- This wider part “spreads” the burden of the structure over more area for greater stability.
- A mat or raft foundation may be a sizeable slab supporting variety of columns and walls under the whole structure or a sizeable part of the structure.
- Mat is required where the columns & walls are so close that individual footings would overlap each other.
- Mat foundations are useful in diminishing the differential settlements on non-homogeneous soils or where there’s an oversized variation within the loads on individual columns.
- Mat-slab foundations disseminate ponderous column & wall loads across the whole building area.
- Mat-slab foundations may be developed near the bottom surface, or at the underside of basements.
Shallow Spread Footings
If the depth of foundation is a smaller amount compared to the breadth of foundation then it’s referred to as Shallow or stepped Foundation.
Shallow foundation is basically a form of building a foundation that conveys building loads to the earth very close to the surface, in lieu of a subsurface layer or a range of depths as in a deep foundation.
It may be used where the bearing capacity of soil on which the structure is required to be constructed is maximum. Minimum depth of this Foundation is 800 mm and a maximum depth is no greater than 4 meters.
Spread Footing with Pier:
A pier foundation is an assembly of sizeable diameter cylindrical columns to hold up the superstructure and convey sizeable super-imposed loads to the firm layer below. It stood several feet aloft the bottom. It is also referred to as “post foundation”.
Technology of Spread Footing with Pier
The most ordinary way is digging a square or rectangular borehole. Then formwork is made ready in the pit which settles the shape of the future foundation; then the reinforcement cage is settled, & concrete is slooshed. Then, the formwork is detached and canopied with a pillar.
Pier foundation is employed utilizing the below conditions:-
- When decomposed rocks are present in the top strata, & there are underlying strata of sound rock beneath them
- As stiff clays offer a plenty of resistance when driving a bearing pile
- If structure required to be built on a slope
- The soil must have a little bearing capacity of water unless the pillars will sink under the ponderous load of the house
Spacing of Piers
Pier foundations are generally built 1-1.5 feet aloft the bottom to resist the moisture because the moisture impair the wooden structures.