JavaScript seems to be disabled in your browser. For the best experience on our site, be sure to turn on Javascript in your browser. However, you may not be aware that there are a number of different types of foundations, each of which has both advantages and disadvantages, depending on the project and the ground on which the extension is to be built.

Raft foundations are a common solution for small home extensions and conservatories, and can be tackled by a competent DIYer, providing accurate measurements are used and due care is taken. Looking for reinforcement? We've got all you need. Take a look at our store here. 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. Raft foundations are shallow foundationsand are therefore not suitable for every building project. The advice of a building surveyor or structural engineer should always be sought before committing to a project.

Generally speaking, for domestic projects that have low structural load and a fairly small surface arearaft foundations are ideal. This assumes that the ground is stable enough to provide the load bearing capacity required for the build. If the soil is particularly soft, raft foundations may not be appropriate, and an alternative approach would be required, as advised by your surveyor.

There are four main types of raft foundation: solid slab rafts, slab beam rafts, cellular rafts and piled rafts. Solid slab raft foundations can be further categorised as flat raft mat, blanket raft, wide toe raft and slip plane raft. Solid slab rafts are perhaps the most common type of raft foundation, and they are suitable for projects where the load bearing requirements are relatively light and spread evenly across the structure.

Slab beam rafts are used when the load bearing points are not distributed evenly through the structure. Where heavier loads are anticipated, or where the soil is loose and prone to settlement, a cellular raft may be the most suitable approach, as this type of foundation is much more rigid. Finally, piled raft foundations might be used where a more traditional slab raft would not fit with the overall design of the building or would not provide the required load bearing capacity at all points.

As they are classed as shallow foundations, raft foundations are fairly quick and inexpensive to construct, as they do not require deep and extensive digging. This makes them an ideal solution for smaller builds such as extensions and conservatories. Where site access is restricted, a raft foundation could prove easier to complete than traditional trench foundations.

One of the disadvantages of raft foundations is that erosion can occur at the edges of the raft, unless protective measures are taken. It may also be necessary to install some form of drainage beneath the slab, along with a membrane to protect the base hardcore from becoming clogged by soil and sediment.

Because every site and building project is different, it can be difficult to suggest likely costs for raft extensions. Factors that influence the overall price might include access to the site, soil stability, the proximity of trees and of course, the size and design of the extension or conservatory.

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For those on a budget, it can be worthwhile hiring a mini digger and doing the initial excavations yourself, as well as the hardcore and sand layer. If you are able to dispose of the excavated soil elsewhere in your garden, or even by offering it for free online, this will also save a significant amount, as waste disposal costs are high. The quantity of concrete required, and the need for this to be of a consistent quality, mean that it makes sense to get help at this stage of the project, sourcing the concrete from a professional supplier.

Additional reinforcement structures may be added depending on the type of raft foundation that is being constructed. Reinforcement Mesh Overlap - Find out how far to overlap mesh reinforcement here. We can follow your designs exactly, offer advice as to what would be appropriate, or design a complete solution for you for a very reasonable price.

Call us today to discuss your project on Context Reinforced Concrete RC structures represent a significant percentage of existing infrastructures in the United Kingdom. It is therefore expected that costs related to maintenance and repair or strengthening these constructions represent an increasing percentage of total infrastructure costs allocated by government entities and private owners.

Moreover, the increasing complexity of human societies and the [ Improve On-Site Productivity by Utilizing Prefabricated Steel Reinforcement Anyone who has had experience of working in the UK construction industry recently can see that times are changing and there is no longer a glut of skilled construction workers for construction companies to employ.Welcome to our community forums, full of great discussions about Structural Engineering.

Please register to become a part of our thriving group or login if you are already registered. By Engr. Usman JavedApril 28, in Foundation Design. You need to be a member in order to leave a comment. Sign up for a new account in our community. It's easy! Already have an account? Sign in here. Howard Roark posted a topic in General DiscussionJune Fatima Khalid posted a topic in General DiscussionDecember 3, Palash Engr posted a topic in Concrete DesignJuly 26, Fatima Khalid posted a topic in Foundation DesignJuly 23, By Aye Min Khaing Started 9 hours ago.

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Raft Foundations for Home Extensions Explained

Mat or Raft Footing design raft. Recommended Posts. Usman Javed 1 Posted April 28, Posted April 28, Link to post Share on other sites. EngrUzair Posted April 29, Posted April 29, Dear Usman, Here are a few links, containing examples of raft foundation design :- 1. UmarMakhzumi 1, Posted April 30, Posted April 30, Mat foundations are also called Raft foundations. These are combined foundations supporting several columns organized in one or more Column and raw this raft meaning.

When determining which foundation is the most economical foundationthe engineer should consider the superstructure load, the subsoil conditions, as well as the desireable tolerable settlement.

In general, foundation of building foundations of bridges and buildings might be divided into two major categories as belowv. Common types of shallow foundations include continuous wall footing, spread footing, grillage foundation, strap footing, combined footing, raft or mat foundation, and so on.

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All these are shown in as per below figure. These are used when the natural soil at the site has a reasonable safe bearing capacity, acceptable compressibility, and the column loads are not very high. A typical deep foundation is shown as per the above figure b. Deep foundations are much like shallow foundations except the load coming from columns or superstructure is transferred into the soil vertically.

These are used when column loads are extremely large, the topsoils are weak, and the soils that have a good strength and compressibility characteristics are in a reasonable depth below ground level. Further, earth retaining structures can also be classified under heavy foundations. Usually reinforced concrete RCC can be used for the construction of foundations.

Brick Plain, stone and concrete pieces can are also be used for wall footings once the loads transmitted into the soil are relatively small. Engineers also use other materials like steel beams and sections like in grillage foundations and pile foundationswood as piles Such as temporary structuressteel sheets Such as temporary retaining structures and cofferdams and other composite materials.

Sometimes, these can are also be encased in concrete depending on the load and strength requirements Bowles, ; Tomlinson, All these are also known as spread footings. An isolated footing might be of a square, rectangular or circular shape. It might be axially loaded or eccentrically loaded.

The design of square footings amounts to calculating the size and depth of the footing and amount of main reinforcement and dowels.

The bending moments and shear forces are got in the critical sections, as discussed above. Rectangular footings could be used where space is restricted, and it might be impossible to provide a square footing. The rectangular footing is also provided for rectangular columns or pedestals. The design of rectangular footings is equal to that of a square footing, except it needs to be designed in both directions separately.

Thus, bending moment and one-way shear actions have to considered in both directions. When the total area needed for the footing was determined, the dimensions of the footing has to be chosen such that the maximum bending moment on the equivalent of the two adjacent projections is equivalent, that is, the projections on all sides of this column should be equal.

For the design of a circular footing, that supports a circular column or pedestal, the expressions applicable to your circular slabs have to be used. These are given in 1. The shape of structures is circular in plan. The floor of circular water tanks or towers. Alternatively, the circular round footing is replaced by an equal square footing that could be inscribed in its perimeter. Wall footings carrying direct vertical loads might be designed either in plain concrete or in reinforced concrete.

Since a wall footing deflects essentially in one way, it is analyzed by considering as a strip of unit width and its length. The critical sections for calculating maximum bending moments in different types of wall footing are given in according to the below figure. Also, read: Lab Test on Aggregates at Site. Under normal conditions, square and rectangular footings such as those described in are economical for supporting walls and columns. But, under certain conditions, it can be desired to construct a footing that supports a line of two or more columns.A raft slab is 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. Raft slabs in Australia are designed to comply with Australian standard AS At Cornell Engineers, we call the strengthening beams strip footings because they run in parallel strips.

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The beams around the outside are called edge beams. Sometimes the slab is made thicker, up to mm 8 inches thick, to make it stronger or able to span further.

The strip footings vary in depth depending on the soil conditions on the site. A site classification by a soil tester tells us how reactive the clays are. In residential buildings, the minimum thickness of a slab is 85mm 3.

Smart builders use a surveyor to make sure the house is in the correct location and on the correct site! The footings around the outside of the house are called edge beams. The edge beams are dug into the ground. The width of the edge beams is normally mm unless soil conditions require the edge beams to be wider.

Raft foundation

The amount of steel is specified by the structural engineer depending on the site reactivity and the depth of the edge beams. If the soil where the edge beams are required, the builder sometimes digs the edge beams deeper so that the footing is dug into hard ground. The steel reinforcement is then suspended at the depth specified by the structural engineer.

raft footing

Steel starter bars are placed into the wet concrete at regular centres and are left sticking out of the footing to connect the slab to the edge beams. If you are an engineer, the diameter and spacing of the starter bars are a shear flow problem.

There needs to be enough bars to make the slab and beams work as a single beam. The starter bars are not required if the edge beam and slab are poured at the same time. Once the edge beam concrete has hardened, the concreters will place the sand that goes under the slab to make it the correct height.The foundation distributes the load over a large area. So that pressure on the soil does not exceed its allowable bearing capacity and restricts the settlement of the structure within the permissible limits.

Foundation increases the stability of the structure. The settlement of the structure should be as uniform as possible and it should be within the tolerable limits. Think for a Two storeyed building how much concrete needed? How much quantity of bars needed? Depending on Soil bearing capacity of a particular location.

Different Types of Footings are selected and constructed. Different types of Foundations:- Foundations are mainly classified into two types:. If depth of the footing is equal to or greater than its width, it is called deep footingotherwise it is called shallow footing.

Footing is a part of foundation which is constructed with concrete or brickwork masonry and acts as a base to the floor columns and floor walls. The main function of footing is to transfer the vertical loads directly to the soil. The term footing is used in conjunction with Shallow foundation commonly. Foundations may be shallow or deep foundations depending upon the load and type of foundation soil. If the load to be supported is very high and soil is of low bearing capacity, Deep foundations are provided.

If the soil has adequate bearing capacity at reasonable depth then shallow footings are provided. Below we have discussed about the different types of house foundations which we usually use for construction. Shallow Foundations or Spread Foundations:- Spread foundations are used when the soil has sufficient strength within a short depth below the ground level. Shallow foundations need enough area to transfer the heavy loads to the base soil.

As mentioned above, we chose shallow foundations when the soil has adequate Soil bearing capacity. Footings in Shallow foundations may be of Masonry, Plain concrete or reinforced concrete.

The depth of shallow foundations are generally less than its width. Different types of Shallow foundations are further divided into two types, depending on the soil bearing capacity Footings which are provided under each column independently are called as Isolated footings.

They are usually square, rectangular or circular in section. Footing is laid on PCC. Before laying PCC, termite control liquid is sprayed on top face of PCC to restrict the termites to damage the footing.

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Isolated footings are provided where the soil bearing capacity is generally high and it comprises of a thick slab which may be flat or stepped or sloped. This type of footings are most economical when compared with the other kind of footings. It is circular, square or rectangular slab of uniform thickness. These types of footings are constructed in olden days now they are outdated. As from the name its resembling that, footings are stacked upon one another as steps.

Three concrete cross sections are stacked upon each other and forms as a steps. This type of footings are also called as a Step foundation. Stepped footing is used generally in residential buildings.A raft or mat foundation consists of a large concrete slab or a slab and beam system resting on soil or rock and supporting all the loads through a number of columns or walls.

In structures like chimneys, silos, cooling towers, buildings with basements where continuous water proofing is needed. As observed by Terzaghi and Peckthe differential settlement in rafts is only one half that of footings of the same intensity of loading due to the random distribution of compressible zones.

Mat foundation is specified to bridge over pockets of weak spots in moderately weak soil. In situations, where individual footings may touch or overlap each other, and it is advisable to excavate the full site instead of the ground under individual footings. In such situations, careful analysis should be made as to whether individual footings or rafts are more economical. In general, where settlements expected are small, individual strips and footings tend to be cheaper than rafts, which will require much more steel to carry increased shear and bending moments due to continuity.

It is very important that when we adopt a raft foundation, we should carefully check whether there are any weak spots below the foundation. In situations, where there are soil deposits below hard layers, individual footings should be preferred over rafts. It is also a usual practice these days to use mat foundation over piles to reduce settlements.

raft footing

Due to difficulties and hazards in construction, the up-stand beams with compacted granular materials between beams covered with mass concrete is more popular, than down-stand beams. A stiffened edge raft with the peripheral down-stand or stepped down beam can also be used as a cut off against ingress of water to the ground floor and is very popular in practice. Cellular rafts rigid buoyancy rafts for compensated foundations are necessary to avoid differential settlements.

Strip rafts and grid foundations are also generally designed on the same principles as mat foundation. Mat foundation can be subjected to vertical loads, horizontal loads and also to moments from column members. The total loading from a block of flats of R.

C frame and brick infill and plastered finishes will be about In general, the column loads of a four-story residential flat will vary from 20 to 80 tons, with the mean column load of 40 to 50 tons.

raft footing

Horizontal forces like wind and earthquake increase the vertical loads in one part and reduce them in other parts. They may also produce moments at the base of columns, which in turn, will influence the raft pressures. For a successful design, the layout of mat foundation should be carried out with due consideration of both settlements and bearing capacity.

For example, the flat slab analogy which has been very successfully used in many cases has also led to structural failures in many other cases due to lack of understanding of the basics.

It is very evident that the flat slab analogy based on small deflection is valid only where the differential settlement between columns is small.Shallow foundation is commonly accepted as foundation with founding level less than 3m from ground surface.

In case surface loads or surface conditions could still affect the bearing capacity, the foundation which sits on it is called shallow foundation.

Pad foundation refers to the foundation which is intended for sustaining concentrated loads from a single point load such as structural columns. Strip foundation is used to support a line of loads such as load-bearing walls. For instance, closely-spaced columns render the use of pad foundation inappropriate and strip foundation may be a better alternative. Raft foundation consists of a concrete slab which extends over the entire loaded area so that loads from entire structure are spread over a large area leading to a reduction of the stress of foundation soils is reduced.

Moreover, raft foundation serves to avoid differential settlement which otherwise would occur if pad or strip foundation is adopted. Geotechnical-Material Engineer. To know more about me just visit AboutMe. Your email address will not be published. Notify me of follow-up comments via e-mail address. Save my name, email, and website in this browser for the next time I comment.

Notify me of follow-up comments by email. Notify me of new posts by email. Pad Foundation Pad foundation refers to the foundation which is intended for sustaining concentrated loads from a single point load such as structural columns.

Strip Footing Raft Foundation Raft foundation consists of a concrete slab which extends over the entire loaded area so that loads from entire structure are spread over a large area leading to a reduction of the stress of foundation soils is reduced. Raft foundations Share this:. Tags: Foundation. No Comments Feb 17, Amar singh April 15, Reply.

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