FOUNDATION

General rules of Foundation Design

While designing a foundation the following points must be borne in mind.

When a soil is yielding soil, a certain amount of settlement must be reduced as much as possible by bringing down the pressure intensities.

It is necessary that a foundation shall be designed so that if at all a settlement should occur, it will be uniform. In other words, the settlement of all the footings must be more or less the same.

This is a very important point in reinforced concrete structures due to the rigid connection between the different components of the structure.

Types of Shallow foundations

Footings

Footings are structural members used to support columns and walls and to transmit their load to the underlying soils.

Mats or rafts

Combined footings, strap and strip footings

Column Footing

In this type of foundation the base of the column is sufficiently enlarged to act as the individual support. The widened base not only provides stability but is useful in distributing the load on sufficient area of the soil.

Column footings are usually used in the foundations of residences and buildings where the soil is hard enough has has sufficient bearing capacity.

Pressure distribution Under a Foundation

The law of distribution of pressure under a foundation depends on the homogeneity of the soil and flexibility of the base. If really the soil is homogeneous and the base of the foundation is flexible, the pressure distribution under the foundation will be uniform. On the contrary if the foundation base is absolutely rigid, the pressure distribution will not  be uniform but may follow such pattern.

In our designs it is usual to assume a flexible base and hence to regard the pressure distribution to be uniform. This can be achieved by gradually decreasing the thickness of the base towards the edges so that the base is only as much thick as it is regarded to resist the induced moments and shears.

Reinforced Concrete Footings

Footing comprises of the lower end of a column, pillar or wall which i enlarged with projecting courses so as to distribute load.

Footings shall be designed to sustain the applied loads, moments and forces and the induced reactions and to ensure that any settlement which may occur shall be as uniform as possible and the safe bearing capacity of soil is not exceeded.

In sloped or stepped footings, the effective cross-section in compression shall be limited by the area above the neutral plane, and the angle of slope or depth and location of steps should be such that the design requirements are satisfied at every section.

Design Procedure of Column Footings

Here is a step-by-step guide to Column Footing Design:

Step 1

Area required for footing

Step 2

Bending Moment

Step 3

Depth from shear consideration

Step 4

Check for two way shear

Reference: http://www.civilprojectsonline.com/building-construction/guide-to-foundation-design-column-footings/

Depth of Foundation

Depth of foundation depends on following factors. 1. Availability of adequate bearing capacity 2. Depth of shrinkage and swelling in case of clayey soils, due to seasonal changes which may cause appreciable movements. 3. Depth of frost penetration in case of fine sand and silt. 4. Possibility of  excavation close by 5. Depth of ground water table 6. Practical minimum depth of foundation should not be less than 50 cm. to allow removal of top soil and variations in ground level. 7. Hence the best recommended depth of foundation is from 1.00 meter to 1.5 meter from original ground level.  

Footing

Footing is a structure constructed in brick work, masonry or concrete under the base of a wall or column for distributing the load over a large area.

Width of Foundation/ Footings

The width of footings should be laid according to structural design. For light loaded buildings such as houses, flats, school buildings etc having not more than two storeys, the width of foundation is given below. 1. The width of footing should not be less than 75 cm for one brick thick wall. 2. The width of footing should not be less than 1 meter for one and half brick wall.

Different Processes in Foundation Work

The processes executed in the foundation works are given below.  1. Excavation of earth work in trenches for foundation. 2. Laying out cement concrete. 3. Laying the footing in case of raft or column construction. 4. Laying Anti termite treatment. 5. Laying Brick work up to plinth level. 6. Laying Damp proof course on the walls. 7. Refilling of earth around the walls 8.  Refilling of earth in the building portion up to the required height according to plinth level.

Precautions while designing Foundation

1. A foundation should be designed to transmit combined dead load, imposed load and wind load to the ground. 

2. Net loading intensity of pressure coming on the soil should not exceed the safe bearing capacity.

3. Foundation should be designed in such a way that settlement to the ground is limited and uniform under whole of the building to avoid damage to the structure. 

4. Whole design of the foundation, super structure and characteristics of the ground should be studied to obtain economy in construction work.

Reference: http://theconstructor.org/geotechnical/designing-a-foundation/1392/

Types of FoundationShallow Foundations(sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts. Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. These are not strict rules, but merely guidelines: basically, if surface loading or other surface conditions will affect the bearing capacity of a foundation it is 'shallow'. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts.Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc. Kinds of shallows Foundations :

Pad Foundations

Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or reactangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.

Strip Foundations

Strip foundations are used to support a line of loads, either due to a load-bearing wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would be inappropriate.

Raft Foundations

Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact.

A raft foundation normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation.

Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.

Deep Foundations

Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.

Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.

Piers are foundations for carrying a heavy structural load which is constructed insitu in a deep excavation.

Caissons are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.

Compensated foundations are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement.

Reference: http://www.civilengineeringzone.co.cc/2009/10/types-of-foundation.html