The task of designing or constructing building foundations can seem daunting. The most common mistake people make is assuming that all building foundation are the same, when in fact there are many different types of foundations with different uses and benefits. This blog post will go over some of the more popular types to help you decide which one is best for your project.
- Shallow Building Foundation
- Deep Building Foundation
Shallow Foundation
Shallow foundations are a type of shallow foundation which supports shallow bearing-wall structures, normally used in residential construction. The shallow footing can be cast-in-place or precast concrete. Typical thicknesses range from 200 mm to 1000 mm (8 inches to 40 inches). Shallow footings are typically 300 mm to 500 mm (12 inches to 20 inches) deep. They are not usually used in commercial construction due to shallow bearing pressure, which is mostly attributed to shallow-piled retaining walls.
A shallow foundation is also referred to as shallow ground floor, shallow platform, shallow-piled foundations, and shallow site-prepared foundations. Typically found on dwellings with flat roofs, shallow foundations typically support a timber frame or precast concrete slabs which in turn support the superstructure of the building.
There are several different types of shallow foundations:
- Isolated Spread Footing
- Wall Footing or Strip footing
- Combined Footing
- Cantilever or Strap Footing
- Raft or Mat Foundation
Isolated Footing
The isolated spread footing is the most ordinary type of footing which you can use on shallow and lower foundations. The primary aspect of spread footings is that the footing’s base spreads out beneath the concrete floor, to help push-out against the ground. The foundation addition is expanded and the foundation extends out from the footing to hold against the force caused by the weight of the building.
Isolated spread footings are commonly employed in the segmental construction. It is an advantage as the foundation is built separately. In case of a weak ground, the isolated footing carries extra loads to ensure that there will not be any thrust or overturning forces on the supports.
As we know, the greatest force on any footing is the dead weight of the building. Another major force is the wind. In that case, isolated spread footings are used in unconfined terrain to support a frequently occupied assembly of building above, such as dwelling, school, etc. They are also typically utilized in the seismic countries to face high seismic activity. These types of foundation are chiefly used in moderate-rise buildings like residential buildings, educational institutions, hospitals and so on.
Types of Isolated Footing Foundation
Isolated foundation is further categorized into the following types:
- Single pad footing.
- Stepped footing for a column.
- Sloped footing for a column.
- Wall footing without step.
- Stepped footing for walls.
- Grillage foundation.
Important Note: When the load of the structure is low, it is economical to have shallow spread footings. Also, the load of each column should not be very high. But when the building requires columns to be closely placed, it is somewhat economical to have deep footing. When the bearing capacity of soil is high at a shallow depth, i.e., the soil’s bearing capacity increases with the decrease in the depth of soil, it is economical to have isolated spread footings.
Wall Footing or Strip footing
Wall footing is also known as continuous footing. This type of foundation is used to distribute loads on the walls in such a way that the soil does not fail due to excessive pressure. The width of the wall footing is usually 2-3 times the width of the wall it supports.
In a wall footing, every vertical element of the wall has three points of contact to the ground. Wall footing installation is relatively simple. The challenge is that they are installed in the same excavated trench as the walls so there’s a limit to how much material can be removed from the trench. This narrow removal width and added width of the wall foundation makes it really difficult to determine if the footing is installed at the right depth.
Wall footing is used extensively in buildings, especially in the construction of load-bearing masonry walls. It is often used for walls with heights that exceed 3-4 meters. The wall footing also provides support to foundation wall, sill, lintel or bearing wall. Wall footing is arranged in line with the direction of the masonry wall. A typical wall footing consists of two rows of beams or feet which are embedded in and continuous with the footings for adjacent walls.
Wall footing is economical when the weight of vertical wall, and of the superimposed stories is low compared with the weight of soil per unit length. Wall footing is economical when there are small lateral pressure from the soil against the wall or when the wall is on a dense sand or gravel.
Combined Footing
The combined footing can be provided when the columns of the structure are closely aligned and their footings overlap or are interconnected. The combined footing helps in increasing the actual load bearing area for the entire structure. It is different than that of isolated or indivisible footings because the footing is not separate but rather a blend of various bases in a single footing.
Combined foundations are an alternative form of construction where the footings for each individual column of the building are united into one continuous footing. Combined Foundations have existed for a long time with some of the oldest examples from Imperial Rome, but it’s only in recent engineering history that they have become economically viable. This is because advances in concrete technology (specifically low cost and high strength insitu-prestressed foundations), computational methods to analyze the structure and lower modern building costs.
In a nutshell, the main purpose of combined footings is to distribute the load bearing pressure on a similar basis along the entire foundations. The main types are slab type combined footings, rectangular combined footing, raft, and strap beam type.
Cantilever or Strap Footing
Cantilever or strap footing work on the same principle as combined footings. In cantilever footing, the foundation under the columns is built individually and connected by a strap beam. General rule of thumb is that in cantilever footing, the front edge of the foundation under the column neither can be extended beyond property line, nor beyond end distance from column face. This means cantilever spans are limited to certain range.
In most areas of the country, strap footings are used when there is an obstacle located in the edge of the foundation. The hook from the beam reaches over the obstacle and supports both sides of foundation. Strap footings are to be used when your footing cannot go beyond property line such as buildings and utility lines.
Raft or Mat Foundation
Raft foundations are generally selected in the cases of structures where dispersion phenomena are predominant. In raft foundations, the foundation is of large width as compared to the depth. Mat foundations are usually more economical in cases where a relatively small load has to be transmitted over a large area. However, these foundations are more suitable for buildings subjected to low earthquake excitation.
Raft foundations consist of individual units of concrete, steel or any other metal interconnected by portal frame, decking etc. The units are arranged in groups to support a common load. It is widely employed in large structures like bridges, tanks, railway stations, and hangars.
This foundations are economical to use when the soil is of poor quality and where the structure concerned has a surface area which is very much over the minimum. Thus the site area required for a firm foundation of equal strength is very much reduced by using this type of construction.
Deep Foundation
A deep foundation is the type of foundation that goes deep into the ground for extra support. This type of foundation can be made out of concrete or steel rebar. It is typically used in areas with high water tables because it prevents groundwater from seeping up through cracks in the building’s structure. Deep foundations can be made of: wood; steel and reinforced concrete (RC); precast RC; or cast-in-situ RC. It depends on the depth or bottom of the structure’s root as well as the strength and ground properties.
There are different types of deep foundation are as follows:
- Pile Foundation
- Pier Foundation
- Caisson Foundation
Pile Foundation
Pile foundations are the way to go when in need of ground support in soft and deep soil conditions. They are preferred due to their ability to transmit loads to soil strata which are beyond the reach of shallow foundations. These foundations can be driven, cast-in-situ, or pre-cast concrete piles. Piled foundations are used for pretty much anything including residential homes, commercial buildings, parking structures, marine terminals including offshore platforms. Piles are classified as either driven or drilled.
Pile foundation can be of the greatest importance when it is required to lay the loads at comparatively lower levels. It is generally known that in such cases the soil may fail to carry the load because of its slender strength. This makes it appropriate even if the depth of excavation is required to be lowered or because of some other obstacle encountered at a certain level. In such situations, a pile foundation can act as a solution to the problem.
Types of Pile Foundation
The followings are the types of pile foundations.
- Sheet Piles
- Load Bearing Piles
- End Bearing Piles
- Friction Piles
- Soil Compactor Piles
- Timber Piles
- Concrete Piles
- Steel Piles
- Composite Piles
Pier Foundation
Pier foundations can be utilized for practically any development when the soil is unfit to give adequate support or abatement to underpinning. The pier foundation is best for multi-story buildings because of the tallness of the development. Since base region is determined by the establishment plan strategy for regular footing, single pier load test is wiped out.
Pier foundations provide many advantages over a regular slab. They are faster to construct, use less cement, and cost less to install. It also allows you to expand your structures scope without having to dig it out.
Caisson Foundation
All construction carried out on dams and bridges requires the use of caissons. The caisson has a cylindrical shape and is an open steel structure with sheet and angle iron pans. The caisson foundation is made of cylindrical shape structural components that are watertight with sealed joints that makes them capable of serving as floating foundations. It works on the principle that it can float or sink to required location depending upon the depth of water at any given location and then be sunk by discharging ballast water or sand through specially provided area near the topmost portion of this foundation.
Caisson foundations have been used in numerous applications from highway road slabs to high rises. These are popular is because it can sustain high load and easy to construct. Also this type of foundation are very economical in comparison with column foundations and pile foundations for such structures.
Types of Caisson Foundations
There are several types of caisson foundations.
- Box Caissons.
- Floating Caissons.
- Pneumatic Caissons.
- Open Caissons.
- Sheeted Caissons.
- Excavated Caissons.
Contact Tameerkaro for your Strong Building Construction
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