When to use Raft Foundations
- A raft foundation is used for large loads, which is why they are so common in a commercial building that tends to be much bigger and heavier than residential homes.
- A raft foundation is used when the soil has a low bearing capacity, so the weight of the building needs to be spread out over a large area to create a stable foundation
When to use a raft foundation?
Raft foundation is generally used to support structures like residential or commercial buildings where soil condition is poor, storage tanks, silos, foundations for heavy industrial equipment etc. Working Principle of Raft Foundation To get a better idea about when to use raft foundation, it is important to understand how raft foundation works.
Which type of raft foundation is supported on piles?
This type of raft foundation is supported on piles. A piled raft is used when the soil at a shallow depth is highly compressible and the water table is high. Piles under raft help in reducing settlement and provides resistance against buoyancy.
What are the steps of raft foundation construction?
Key steps of Raft foundation construction are noted below. Identify the desired depth at which foundation is to be provided. Excavate soil up to the required depth. Compact the soil. Provide a waterproofing membrane.
What is strap footing construction in raft foundation?
Raft Foundation: Strap footing Strap footing construction uses a strap beam to connect with an eccentrically loaded column foundation into the foundation of an interior column. (See as per above figure).
Where is raft slab used?
A raft slab is a reinforced concrete slab on ground that is strengthened with integral concrete beams in both directions. Usually, a raft slab is used as the foundation for new houses and extensions.
Do I need a raft foundation?
Typically if the footings would cover over half of the construction area then raft foundation would be used. If the walls of the building are so close that it would cause the individual footings to overlap, then raft foundations should be used.
Why raft foundation was used on weak soil?
A raft foundation is often used when the soil is weak, as it distributes the weight of the building over the entire area of the building, and not over smaller zones (like individual footings) or at individual points (like pile foundations). This reduces the stress on the soil.
Which soil is suitable for raft foundation?
Raft foundation is preferred when the clayey soil is compact and hard. Even strap foundation can also be used. Soft clayey sand is prone to expansion and shrinkage. They have a low bearing capacity and are unsuitable for footing and strap foundation.
When should a raft foundation be used?
Raft foundations are suitable where a building's footprint is reasonably small, and the structural load requirements are not onerous. They are also suitable for basement constructions where the foundation slab can receive direct live loads depending on the use of the building.
Why would you use a raft foundation?
A raft foundation is a reinforced concrete slab under the whole of a building or extension, 'floating' on the ground as a raft floats on water. This type of foundation spreads the load of the building over a larger area than other foundations, lowering the pressure on the ground.
What is the strongest foundation for a house?
Foundations are mostly constructed from strong material so they can hold the house in place even during earthquakes and cyclones. Therefore, they are generally made up of concrete which is the strongest construction material.
How deep does a raft foundation need to be?
The foundations should have a minimum depth of 450mm, measured from finished ground level to their underside, to avoid the action of frost.
Is raft foundation shallow or deep?
Raft foundations are a type of shallow foundation. They are typically formed by reinforced concrete slabs that cover a wide area, often the entire footprint of a building.
What are the disadvantages of raft foundation?
Disadvantages of Raft Foundation In some cases, large amount of reinforcement is required for raft foundation which increases the cost of project. Special attention on raft foundations is required in case of concentrated loads. If they are not treated properly, there is a chance of edge erosion.
Are raft foundations more expensive?
Raft foundations are relatively inexpensive and fast to put down, when compared with other types of foundation. Ground excavations are less onerous than for trench excavations, for example, and the foundation and floor slab can often be combined, giving significant savings on materials and time.
Which type of foundation is best for residential building?
Concrete Slab Foundation. A concrete slab is one of the most common types of house foundations, requiring the least amount of lot prep to begin building. ... Crawl Space Foundation. Homes with a crawl space foundation are elevated a few feet off the ground. ... Basement Foundation.
What are the disadvantages of raft foundation?
Disadvantages of Raft Foundation In some cases, large amount of reinforcement is required for raft foundation which increases the cost of project. Special attention on raft foundations is required in case of concentrated loads. If they are not treated properly, there is a chance of edge erosion.
Are raft foundations good?
Generally speaking, for domestic projects that have low structural load and a fairly small surface area, raft foundations are ideal. This assumes that the ground is stable enough to provide the load bearing capacity required for the build.
What is the minimum depth of raft foundation?
There are certain requirements that need to be considered during the of raft foundation in order to produce adequate design such as minimum depth of 50cm, required excavation depth of approximately 2.5m, and rebar cover of 50mm.
What are the 3 types of slab foundations?
3 Types of Concrete Foundations - Slab on Grade, T-Shaped, Frost Protected - Concrete Network.
What is a Raft Foundation?
A raft foundation, also called a raft 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.
When to use rafts?
Rafts are most often used these days when the strata is unstable or (because of this) a normal strip foundation would cover more than 50% of the ground area beneath the building. There are also situations (usually in areas where mining has occurred) where there may be areas of movement in the strata.
What are the different types of shallow foundations?
Common types of shallow foundations include continuous wall footing, spread footing, grillage foundation, strap footing, combined footing, raft foundation, and so on. All these are shown in as per below figure. Analysis & all design and considerations of shallow foundations are discussed in as per structure design.
What is strap footing?
Strap footing construction uses a strap beam to connect with an eccentrically loaded column foundation into the foundation of an interior column. (See as per above figure).
What is a trapezoidal footing?
Trapezoidal combined footing ( According to below Figure ) is sometimes used as an isolated spread foundation of columns carrying large loads in which space is tight. The size of this foundation that will uniformly distribute pressure on the soil can be obtained in another manner:
What is a flat slab foundation?
A slab with constant thickness usually referred as a flat slab type raft foundation.
What is a combined footing?
All these footings are known as combined footings. When more than one (1) line of columns is supported by a concrete slab, The it’s referred to as a raft foundation. also calculation mat calculation/ Raft calculation
What are raft foundations?
Also known as mat foundations or raft footings, raft foundations are basically slabs of concrete that sit on a firm, level bed of soil and extend across the largest part or even the entire footprint of the build. Whilst these are shallow foundations, they can provide much better stress distribution than conventional trench footings. Because the weight of the building is spread across a much larger surface area than with traditional trench footings or pile footings, the stress acting on the soil below is greatly reduced.
How many types of raft foundations are there?
There are five main types of raft foundation:
Why do construction firms need to know about raft foundations?
When looking at a new building project, construction firms need to find the most appropriate design and construction methods for that particular build, in order to come up with a cost-effective solution that can deliver on the project requirements and handle site-specific conditions. When it comes to foundations, traditional trench footings aren’t always the most suitable route, which is why every construction professional should have a firm understanding of raft foundations.
Why are shallow foundations better than trench footings?
Whilst these are shallow foundations, they can provide much better stress distribution than conventional trench footings. Because the weight of the building is spread across a much larger surface area than with traditional trench footings or pile footings, the stress acting on the soil below is greatly reduced.
How do balance rafts work?
They are created through excavation of the soil in a way that the weight of the removed soil (including the water weight) balances (entirely or partially) the weight of the superstructure plus the foundation. This solution might not be always cost-effective as it involves significant amounts of earthworks as well as a lot of work related to the construction of retaining structures (permanent or temporary) prior and during the excavation, to reach the depth of construction of the foundation. This is especially so when the water table is close to the ground surface.
Why are raft foundations so difficult?
Where access to the site is restricted, traditional foundations can be difficult, because the plant needed to dig to the required depth is often too large. When raft foundations are to be constructed close to the ground surface much smaller equipment is required which can be proved beneficial in cases where limited access to the site is provided (e.g. urban environment).
What is a flat raft mat?
Flat raft mats are typically used for small buildings where it’s possible to position columns at regular distances across the footprint of the building, to provide equal stress distribution on the foundation. Steel mesh is typically used at the bottom (in the column-foundation connection proximity) and the top (mid-span between columns) of the concrete mat, to provide resistance in both directions.
What is a raft foundation?
Blog Post. A raft foundation is a reinforced concrete slab under the whole of a building or extension, 'floating' on the ground as a raft floats on water. This type of foundation spreads the load of the building over a larger area than other foundations, lowering the pressure on the ground. This is an alternative if you can't use ...
Where is insulation placed on a raft?
Insulation is usually placed over the top of the raft - take care to avoid cold bridging at the junctions with external walls. Important! A structural engineer designing a raft foundation may not be fully aware of damp coursing, cold bridging or land contamination issues.
Do raft foundations need to be designed?
But it's important to note that raft foundations aren't suitable in all cases and usually need to be designed by a structural engineer.
What is a raft foundation?
Raft foundation purpose is the load distribution on a volume area, which prevents deformation of constructed facilities at ground movements.
What are the advantages and disadvantages of a raft foundation?
Advantages and disadvantages. The major advantage of a raft foundation is its large bearing area, which significantly reduces the pressure on the ground. As a consequence, it is excluded differential settlement and walls deformation. Among other advantages are the following: Use of rafts as base of the first floor.
What type of slabs do builders use?
For small constructions builders usually use monolithic slabs, characterized by ease of molding technology and little laying depth; For industrial and high-rise buildings they use baseplates with stiffeners, assembled from individual details fastened together, or use a fully monolithic construction;
What are the disadvantages of using rafts as base of the first floor?
However, they are justified by a guaranteed reliability to adverse natural conditions.
How deep is a floating floor?
Floating type is laid at a depth of 25-35 inches, which eliminates the possibility of building a ground floor or basement.
What are the two types of foundations?
There are two types of this foundation kind: classic and floating.
Where to place formwork in a future building?
Place formwork on the entire perimeter of a future building.
When Raft Foundations Are Used?
A raft foundation is usually preferred under a number of circumstances:
Why are raft foundations preferable to footings?
They are ideal for poor ground condition where normal footings would not cope well as they cannot spread the load as effectively.
Why use a rafter foundation?
Raft foundations tend to be cheaper and quicker to use than traditional footings. There are a number of reasons why this is the case: 1 The foundation and floor slab is combined, which saves time and materials 2 Less excavation is required
What are the advantages and disadvantages of a raft foundation?
Advantages and Disadvantages of Raft Foundations. Raft foundations tend to be cheaper and quicker to use than traditional footings. There are a number of reasons why this is the case: The foundation and floor slab is combined, which saves time and materials. Less excavation is required.
What is the ratio of individual footings to total floor space?
The ratio of individual footings to total floor space is high. Typically if the footings would cover over half of the construction area then raft foundation would be used. If the walls of the building are so close that it would cause the individual footings to overlap, then raft foundations should be used.
Why did the raft foundation become popular?
Because of the speed and volume of houses required after the second world war, the raft foundation was widely used. The raft foundation was cheaper, easier to install and most importantly, did not require as much excavation as the usual strip foundations.
When to use rafts?
Rafts are most often used these days when the strata is unstable or (because of this) a normal strip foundation would cover more than 50% of the ground area beneath the building. There are also situations (usually in areas where mining has occurred) where there may be areas of movement in the strata.
