Foundation is the part of structure below plinth level up to the soil. It is in direct contact of soil and transmits load of super structure to soil. Generally it is below the ground level. If some part of foundation is above ground level, it is also covered with earth filling. This portion of structure is not in contact of air, light etc, or to say that it is the hidden part of the structure.
Fig 1 :- Parts of a foundation
Whenever construction workers begin work on a new building, they must first assess where and how they will build the foundation.
When engineers design the foundation of a building, they must keep in mind how much the soil will settle beneath it, as well as how much weight will go on top. If calculated incorrectly, the foundation may fail and place the entire structure in peril.
The bearing capacity of soil is the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil.
- Ultimate bearing capacity – is the theoretical maximum pressure which can be supported without failure;
- Allowable bearing capacity- is the ultimate bearing capacity divided by a factor of safety.
Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing capacity is based on the maximum allowable settlement.
The processes executed in the foundation works
- Excavation of earth work in trenches for foundation.
- Laying out cement concrete.
- Setting out for footing.
- Reinforcement for footing and column shaft and formwork for footing.
- Laying the footing in case of raft or column construction.
- Lying Anti termite treatment.
- Setting out and formwork for column shaft.
- Laying Column shaft work up to plinth level.
- Laying Damp proof course on the walls.
- Refilling of earth around the walls
- Refilling of earth in the building portion up to the required height according to plinth level
Fig 2:- Procedure for foundation
LOADS ON BUILDINGS-
The occupant load describes the number of square feet allocated to each person within a building.
- Vertical loads due to weight of building and any permanent equipment
- Dead loads of structural elements cannot be readily determined b/c weight depends on size which in turn depends on weight to be supported initially weight must be assumed to make a preliminary calculation, then actual weight can be used for checking the calculation
- Easily calculated from published lists of material weights in reference sources
- IBC requires floors in office buildings and others with live loads of 80psf or less here partition locations are subject to change be designed to support a minimum partition load of 20psf & is considered part of the live load.
Total load = Dead load + Live/Imposed load
SETTLEMENT OF GROUND-
Settlement of a foundation that is caused by seasonal factors is especially noticeable during the hot dry summer months. Below ground level, depending upon climate and environmental conditions. The drying of the soils occur because of both evaporation and transpiration (water being removed because of trees and shrubs). As the soils dry, they tend to consolidate; as they consolidate, many times, a slab-on-grade foundation settles.
?- Bearing capacity (safe)
W- Weight of a unit volume of earth
?- Angle of repose
Fig 3–Rankine’s Formula
Precautions while designing ‘Foundation’
- A foundation should be designed to transmit combined dead load, imposed load and wind load to the ground.
- Net loading intensity of pressure coming on the soil should not exceed the safe bearing capacity.
- 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.
- Whole design of the foundation, super structure and characteristics of the ground should be studied to obtain economy in construction work.
Note: For structures being built in colder climates, engineers must consider frost heaves as well. Frost heaves occur when moisture in the soil freezes, thereby changing the density of the building’s support. A frost heave can cause damage to the foundation, thereby compromising the structural integrity of the entire building.
Drier, warmer climates are not entirely exempt from such worries, however; certain soils will expand and contract when moisture is added or taken away, and engineers must factor in such movement when considering where and how to lay a foundation.
Precautions during Excavation of Foundation Work
The depth and width of foundation should be according to structural design.
- The depth of the foundation should not be less than 1 meter.
- The length, width and depth of excavation should be checked with the help of center line and level marked on the buries.
- The excavated material/ earth should be dumped at a distance of 1 meter from the edges.
- Work should be done on dry soil.
- Arrangement of water pump should be made for pumping out rain water.
- The bottom layer of the foundation should be compacted.
- There should be no soft places in foundation due to roots etc.
- Any soft/ defective spots should be dug out and be filled with concrete/ hard material
Footings that support walls and isolated pad footings that support concentrated loads. The footings themselves are usually made of concrete and its wide bases placed directly beneath the load-bearing beams or walls.
THE FUNCTION OF FOOTINGS AND FOUNDATIONS
The function of a structure is to do nothing. The most successful structures stay still. We can look at footings and foundations as having two functions:
- Transfer Loads – To transfer the live and dead loads of the building to the soil over a large
enough area so that neither the soil nor the building will move.
- Resist Frost – In areas where frost occurs, to prevent frost from moving the building
Types of foundations and their uses
There are different types of foundation designs and each serves a different specific purpose, but generally, every one works to transfer the weight load of a structure to the soil beneath. Types of building, nature of soil and environmental conditions are the major determinant of type of foundation of the building.
Shallow foundation (spread FDN):
Most small and medium homes are built upon a shallow foundation (spread FDN). These are usually comprised of concrete strips that are laid about 3.3 feet (1 meter) beneath the soil, or of a single large concrete slab that is also set about 3.3 feet (1 meter) beneath the soil. The considerations for a shallow foundation, engineers must consider weight and settlement, as well as scour water eroding soil beneath the structure. Can be classified as spread footings, wall and continuous (strip) footings, and mat (raft) foundations.
Larger buildings use a deep rather than a shallow foundation. A deep foundation uses long pylons of steel or concrete to penetrate beyond the weaker surface soils into the deeper and more stable soils or bedrock beneath. The load from the walls above is transferred deep into the earth, thereby providing support for the intense weight above.
- Strip foundation–
- This is the most common type, it is mainly used where you have strong soil base and non-waterlogged areas. Most small buildings of just a floor are constructed with this type of foundation .
- Depends on the structural engineers recommendation , the depth of your foundation could be from 600mm to 1200mm mostly for small scale buildings .
- When the soil is excavated, a level at which the concrete will settle evenly is established, then concrete is poured this may be from 150mm(6”) thick to 450mm(18”) thick depending also on building after that block is set round the trenches at the center of foundation ,the foundation usually follows the block lines. The blocks are then layed to d.p.c level before another concrete is poured on top this is the german or oversite concrete. This type seems to be the cheapest.
Fig 6- Strip foundation
2. Pad foundation–
- This is where isolated columns (pillars) are casted from the foundation to carry a slab at the top of the ground.
- This is mostly used when you want to make use of the under of building as parking space or when the other space is not conducive to have foundation. This columns are thus isolated and their foundations are referred to as pad.
Fig 7- Pad foundation
3. Raft foundation–
Raft foundation is a thick concrete slab reinforced with steel which covers the entire contact area of the structure like a thick floor. This concrete transfers loads from walls and columns to the underlying rock or soil. That is laid on a soft ground consisting of an extended layer (soil are sandy and loose). It is also recommended in waterlogged areas 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.
Fig 8- Raft foundation
Combined footing is foundations supported more than one column and useful when its two columns are so close that single footings cannot be used or is located at or near a property line. Combined footing is usually support two or three columns not in a row. Combined footings are used where the bearing areas of closely spaced columns overlap.
Fig 9- Combined footing
Ramps are an important feature in accessing a home or agricultural building. This applies not only to people who use wheelchairs but also to those who have difficulty climbing stairs, such as people who have arthritis or hemiplegic and those who use walkers, crutches or canes. To be safe and most effective, ramps should be built with a few basic guidelines in mind.
While constructed ramp the slope is extremely important because it affects how difficult it is to travel up and down the ramp. If the slope is too steep, the ramp may be too difficult for someone to use or may even be unsafe. The 1 to 12 slope should be seen as the steepest slope to be built and may be too steep for some people.
The most expensive and the strongest type of foundation, this requires specialist engineering to do. The soil are bored deep down the earth and filled with concrete to be able to support loads of multistory building on top. Most skyscrapers are constructed with this foundation type, a waterlogged area of high building may also require this. It is the costliest hence it is used for high rise building mostly.
Fig 10- Drilled Piles, Drilled Shafts
Pier, in building construction, vertical loadbearing member such as an intermediate support for adjacent ends of two bridge spans. In foundations for large buildings, piers are usually cylindrical concrete shafts, cast in prepared holes, while in bridges they take the form of caissons, which are sunk into position. Piers serve the same purpose as piles but are not installed by hammers and, if based on a stable substrate, will support a greater load than a pile. Especially adapted to large construction jobs, pier shafts having widths of more than 1.8 m (6 feet) have been excavated to depths greater .
On sloping sites the so-called stepped foundation must be used, which is in fact just a special form of the strip foundation.
Cantilever footings are designed to accommodate eccentric loads.