Shallow Foundation and its types

The shallow foundation is categorized as follow :-

1. Spread Footing
2. Combined Footing
3. Raft Foundation
4. Annular Slab or Ring Foundation.

Type 1. Spread Footing: Foundation that transmits the load from a wall or column to a deeper width is called spread foundation or footing. The spread footing intended to the walls of a load bearing structure is called wall footing, continuous footing, or strip footing.

i. Strip Footings: Strip footing is made of stone masonry or concrete and it contains a stepped cross section. Now-a-days, the concrete is gaining popularity in construction industries and the load bearing structures & strip footings become outdated to some extent exclusive of small lightly loaded residential buildings. It is also called continuous footing or wall footing.

ii. Isolated Footing: Spread footing arranged to the columns of a framed structure is known as isolated footing, column footing, or pad foundation.

It is arranged underneath the column to disperse the loads securely to the bed soil. This type of footing is provided to support single-columns and when the columns are placed comparably at long span. It is very cost-effective.

Type 2. Combined Footing: Combined footing is effective when footings of two adjoining columns are provided too narrowly or overlap. It is categorized as follow :-

i. Rectangular Footing: Combined footing is usually rectangular in shape to resist equivalent column loads.

ii. Trapezoidal Footing: For irregular column loads, trapezoidal footing should be utilized to make sure that the centre of gravity (CG) of the column loads overlaps with the CG of the foundation in plan.

iii. Strap Footing: For footings located adjacent to the property lines, a strap footing should be applied to make sure that the edge of the footing adjacent to property line is not expanded into the nearby site. Under the situation, the footing adjacent to the property line (exterior footing) is attached with the footing inside the site (interior footing) via a strap beam.

The strap beam transmits the load of the exterior column footing merely to the interior footing during structural action.

Type 3. Raft Foundation: Raft foundations also known as raft footings or mat foundations, are provided with reinforced concrete slabs having consistent thickness (normally 150 mm to 300 mm) which are extended for a wide area, often the entire footprint of a building. They disperse the load enforced through a number of columns or walls over the area of foundation.

Raft foundation is mainly arranged when the entire area of all footings surpasses 50% of the loaded area. It is also suitable for heavy structures situated over highly compressible and weak soils expanding to large depth.

Type 4. Annular Slab or Ring Foundation: A ring foundation is sometimes arranged for a bigger water tank with its columns attached through a ring beam and supported across an annular slab.

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Detailed information on Shallow Foundation

Shallow foundation

A shallow foundation is defined as the foundation that transmits the building loads to the earth adjacent to the surface. It’s depth is equivalent to or under its width.

Categories of shallow foundation:

1. Spread footingThe role of Spread footing is to extend the super imposed loads of the structure over a extensive zone.

Spread footing is subcategorized as below :

• Single footing for a column.

• Stepped footing for a column.

• Sloped footing for a column.

Concrete is utilized to create the base of these types of footings.

• Wall footing without steps and with steps.

• Grillage foundationgrillage foundation is considered as most cost effective while transmitting the heavy structural loads concerning a column to the soil of low bearing capacity. Depth of such a foundation is restricted to 0.9 to 1.6 meter.

2. Combined footing

The combined footing is developed for two or more columns.

Shape of combined footing is balanced in such a ratio so that the center of gravity of the supporting area remains consistent with the center of gravity of the two column loads. It is formed as either rectangular or trapezoidal.

A combined rectangular footing is useful where both the columns bear identical load or interior column bears superior load. A combined trapezoidal footing is arranged under any situations of loading.

3. Strap footing Strap footing is undertaken when two or more footings are associated with a beam. It is required when the distance among the columns is so excessive that a combined trapezoidal footing turns to be little constricted, containing great bending moments.

4. Mat or Raft foundation Mat or raft foundation is described as a consolidated reinforced concrete slab that covers the entire region of the bottom of the structure.

Mat or raft foundation is useful in the following conditions :-

• If the soil located below contains bearing capacity as well as the building loads are substantial.

• If the combined area of individual footing exceeds half of the total area of the structure, then application of mat or raft foundation will be cost effective.

Article Source : www.civilengineeringterms.com

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Published By

Rajib Dey

www.constructioncost.co

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Different types of foundation and their usefulness

Maximum structures are built with two parts like super structure and sub-structure of the foundation. Super Structure is located over the ground and the other sub-structure of the foundation is located beneath the ground level. Foundation alias footing of a structure associates and transfers the load from the structure to the ground soil. The foundation is developed on the solid ground and it is called as the foundation bed. The foundation transfers the load of the structure and it’s self-weight to the soil with the purpose of keeping the final bearing capacity of the soil under control (the shear failure does not occur) as well as keeping acceptable settlement.

All the structures have foundation at the base that offers the following functionalities:

• To disperse the load of the structure across an extensive bearing area.
• To load the bearing surface equally to get rid of asymmetrical settlement.
• To resist the lateral movement of the supporting material.
• To enhance the strength of the structure all together.

Foundation is based on the following points :-

Foundation is categorized depending on the dispersion of load to the ground into two sub-categories like shallow foundation and deep foundation.

Shallow Foundation

Shallow foundation belongs to the foundations where depth of the foundation remains below the width of the foundation (D < B). Shallow foundations are usually known as spread footing because they transfer the load of the super structure laterally into the ground.

Categorizarion of Shallow Foundation:

Based on the design, the shallow foundation is classified as:

• Wall Footing
• Isolated column or Column Footing
• Combined Footing
• Cantilever (Strap) Footing
• Mat (Raft) Foundation
• Wall Footing

This type of foundation runs consistently along the direction of the wall and facilitates transferring load of the wall into the ground. Wall footing are mostly applicable where transferable loads are small and cost-effective in compact sands and gravels. For this type of foundation, the width remains 2-3 times the width of the wall at ground level. Wall footing is built up with stone, brick, plain or reinforced cement concrete.

Column Footing

Column footing are useful and inexpensive for the depth surpassing 1.5m. For this type of foundation, the base of the column is distended. Column footing comes in the shape of of flat slab and is developed with plain or reinforced concrete.

Combined Footing

Combined footings belong to the foundations which are built in common for two or more columns in a row. It is generally formed if the footing for a column is spreaded outside the property line. It is applicable if the two columns are placed narrowly and the soil on which the structure is developed contains low bearing capacity. The shape of the combined footing appears as rectangular or trapezoidal.

Strap Footing

If an edge footing fails to spread outside the property line, it is connected with the other interior footing through a strap beam. Such footings are defined as strap footing. or sometimes cantilever footing.

Mat Foundation

A mat foundation belongs to a combined footing that covers the whole area below a structure and supports all the walls and columns. It is also called as raft foundation. Mat foundation is mostly suitable for the following reasons:

Permissible bearing pressure is low.

The structure is weighty.

The site is located with highly compressible layer.

The mat foundation is categorized into following types:

Flat slab type.

Flat Slab thickened under column.

Two way beam and slab type.

Flat slab with pedestals.

Rigid frame mat.

Piled mat.

To read the complete article, visit civileblog.com

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Rajib Dey

www.constructioncost.co

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Brief explanation of wall footing design

Wall footing design is performed on the basis of some theories which must be taken into consideration at the time of designing. The first theory is to examine the soil pressure that is circulated linearly all through design process. This can be obtained, if resultant of soil pressure conforms with the resultant of soil force. Due to this, rotation of footing can be prevented. Usually, the following strength design method is applied while designing the footings.

WALL FOOTING DESIGN EXAMPLE STATEMENT

A 10” thick wall bears a service dead load of 8k/ft and service live load of 9k/ft. At the base of footing the permissible soil pressure is 5000psf and base of footing is 5’ underneath the present ground surface. Now your responsibility is to design the wall footing for;

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Concrete compressive strength= f’c = 3ksi

Yield strength of steel = fy = 60ksi.

Soil density = 120lb/ft3.

SOLUTION:

Note: Typically low strength of concrete in footing is applied in columns. It necessitates the use of dowel to adapt this modification in strength.

STEP 1: ESTIMATE THE SIZE OF FOOTING AND FACTORED NET PRESSURE.

At the time of designing wall footing, generally one feet strip of the wall and footing is undertaken to simplify the calculation process. The permissible soil pressure is 5ksf. The permissible net soil pressure is 5ksf. As, the thickness of footing is unknown at this stage a acceptable thickness of footing equivalent to (1-1.5)x The wall thickness can be guessed.

1.5 x 9’’ = 13.5’’ take 13’’

STEP 2: FIND THE PERMISSIBLE SOIL PRESSURE

qn=5−[(1.083times0.150)+(5−1.083)times0.120]

[q_{n}=4.367 ksf]

[Area-of-footing=frac{8+9}{4.3676}]

Try 47’’ (3.91’) Wide footing

Factored Net Pressure = qnet = 6.138ksf

STEP 3: CHECK THE TOLERABILITY OF FOOTING DEPTH AGAINST SHEAR

Only one way shear or beam shear is substantial in wall footings. The crucial section for this type of shear is at distance ‘d’ from the face of wall. Where ‘d’ denotes the actual depth of footing.

Supposing that one will use #4 bar.

d = 13 – 3 – ¼ = 9.75″

The Tributary area for one way shear

Tributary area = Shaded area for one way shear

D = H – CLEAR COVER – 1/2 BAR DIAMETER

d = 13 – 3 -1/4

d = 9.75″

To get the complete article visit 

www.engineeringintro.com/concrete/reinforced-concrete/wall-footing-design-example/

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Published By

Rajib Dey

www.constructioncost.co

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