Spread Foundation
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Spread Foundation
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"Floating foundation" redirects here. For Floating raft system, see Floating raft system .
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A shallow foundation is a type of building foundation that transfers building loads to the earth very near to the surface, rather than to a subsurface layer or a range of depths as does a deep foundation . Shallow foundations include spread footing foundations, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations and earthbag foundations.
A spread footing foundation, which is common in residential buildings, has a wider bottom portion than the load-bearing foundation walls it supports. This wider part "spreads" the weight of the structure over more area for greater stability.
The design and layout of spread footings is controlled by several factors, foremost of which is the weight (load) of the structure it must support, penetration of soft near-surface layers, and penetration through near-surface layers likely to change volume due to frost heave or shrink-swell .
These foundations are common in residential construction that includes a basement, and in many commercial structures.
But for high rise buildings they are not sufficient. A spread footing that changes elevation in a series of vertical steps so that it follows the contours of a sloping site or accommodates changes in soil strata, is called a stepped footing .
Mat-slab foundations distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Mat-slab foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, mat-slab foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer.
Slab-on-grade or floating slab foundations are a structural engineering practice whereby the concrete slab that is to serve as the foundation for the structure is formed from a mold set into the ground. The concrete is then placed into the mold, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor. That being said, Frost Protected Shallow Foundations (or FPSF) which are used in areas of potential Frost Heave, are a form of Slab on Grade Foundations. [1]
The advantages of the slab technique are that it is cheap and sturdy, and is considered less vulnerable to termite infestation because there are no hollow spaces or wood channels leading from the ground to the structure (assuming wood siding, etc., is not carried all the way to the ground on the outer walls).
The disadvantages are the lack of access from below for utility lines, the potential for large heat losses where ground temperatures fall significantly below the interior temperature, and a very low elevation that exposes the building to flood damage in even moderate rains. Remodeling or extending such a structure may also be more difficult. Over the long term, ground settling (or subsidence ) may be a problem, as a slab foundation cannot be readily jacked up to compensate; proper soil compaction prior to pour can minimize this. The slab can be decoupled from ground temperatures by insulation, with the concrete poured directly over insulation (for example, extruded polystyrene foam panels), or heating provisions (such as hydronic heating ) can be built into the slab.
Slab-on-grade foundations are commonly used in areas with expansive clay soil. While elevated structural slabs actually perform better on expansive clays, it is generally accepted by the engineering community that slab-on-grade foundations offer the greatest cost-to-performance ratio for tract homes. Elevated structural slabs are generally only found on custom homes or homes with basements.
Copper piping, commonly used to carry natural gas and water , reacts with concrete over a long period, slowly degrading until the pipe fails. This can lead to what is commonly referred to as slab leaks. These occur when pipes begin to leak from within the slab. Signs of a slab leak range from unexplained dampened carpet spots, to drops in water pressure and wet discoloration on exterior foundation walls. [2] Copper pipes must be lagged (that is, insulated ) or run through a conduit or plumbed into the building above the slab. Electrical conduits through the slab must be water-tight, as they extend below ground level and can potentially expose wiring to groundwater .
The rubble trench foundation , a construction approach popularized by architect Frank Lloyd Wright , is a type of foundation that uses loose stone or rubble to minimize the use of concrete and improve drainage. It is considered more environmentally friendly than other types of foundation because cement manufacturing requires the use of enormous amounts of energy. However, some soil environments (such as particularly expansive or poor load-bearing (< 1 ton/sf) soils) are not suitable for this kind of foundation.
A foundation must bear the structural loads imposed upon it and allow proper drainage of ground water to prevent expansion or weakening of soils and frost heaving. While the far more common concrete foundation requires separate measures to ensure good soil drainage, the rubble trench foundation serves both foundation functions at once.
To construct a rubble trench foundation a narrow trench is dug down below the frost line . The bottom of the trench would ideally be gently sloped to an outlet. Drainage tile, graded 1:8 to daylight, is then placed at the bottom of the trench in a bed of washed stone protected by filter fabric. The trench is then filled with either screened stone (typically 1-1/2") or recycled rubble. A stem wall is then constructed, or a steel-reinforced concrete daro beam is poured at the surface to provide ground clearance for the structure.
The rubble-trench foundation is a relatively simple, low-cost, and environmentally-friendly alternative to a conventional foundation, but may require an engineer's approval if building officials are not familiar with it. Frank Lloyd Wright used them successfully for more than 50 years in the first half of the 20th century, and there is a revival of this style of foundation with the increased interest in green building.
The basic construction method begins by digging a trench down to undisturbed mineral subsoil. Rows of woven bags (or tubes) are filled with available material, placed into this trench, compacted with a pounder to around 1/3 thickness of pre-pounded thickness, and form a foundation. Each successive layer has one or more strands of barbed wire placed on top. This digs into the bag's weave to prevent subsequent layers from slipping, and also resists any tendency for the outward expansion of walls. The next row of bags is offset by half a bag's width to form a staggered pattern. These are either pre-filled with material and delivered, or filled in place (often the case with Superadobe ). The weight of this earth-filled bag pushes down on the barbed wire strands, locking the bag in place on the row below. The same process continues layer upon layer, forming walls. A roof can be formed by gradually sloping the walls inward to construct a dome. Traditional types of roof can also be made.
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Shallow foundation - Wikipedia
1) Spread Footing Foundations
Types of shallow foundations and their characteristics I Geotech d.o.o. I
Spread Foundation - Types and When To Use Spread Footings
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Definition
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Construction
Following are some of the major types of foundations and foundation construction methods:
Foundations provided immediately beneath the lowest part of the structure, near to the ground level are known as shallow foundations. Such foundations are mostly placed on the first hard and firm strata available below the ground level. Shallow foundations are further classified into the following types:
Foundations constructed by increasing the are at the base of the structure by means of offsets, are called spread footing foundations. In such foundations, spread is given under the base of a wall or a column by providing offsets. This spread is known as footing and the foundation itself is called spread footing.
Broadly speaking, all types of shallow foundations can be referred to as spread footing foundations. However ,from design and construction point of view, they have been designated separately. The various types of spread footing foundations are ;
This is a common and the simplest type of spread footing foundation; It consists of a number of courses of bricks, the lowest being usually twice the thickness of wall above. In this type of foundation, the base width of the wall is increased by providing 5cm (one-fourth of brick length) offsets on either side of the wall. The depth of each course is usually 10 cm
In some cases, however, the bottom course is made 20cm deep. In the case of footing for store walls, the size of offset is slightly more than that of the brick wall footing.
For constructing wall footing, a bed of lean cement concrete (1:8:16) is first laid over the entire length of the wall. The thickness of this lean concrete bed is usually kept 15cm and its width is kept 20 to 30 cm more than that of the bottom course.
In no case, the depth of the concrete bed should be less than its projection beyond the wall base. For foundations over firm soil or compacted ground, the concrete bed below the brick wall footing may be dispensed with. After laying the concrete for constructing the concrete bed, it should be properly compacted and cured before laying the base course of the wall.
This is the cheapest type of spread footing foundations and is largely used for walls of ordinary buildings.
Isolated footings are used to support the individual pillars and columns constructed in brick or stone masonry. They are stepped down in the wall footing after providing 15cm offset of foundation concrete on all the sides.
These are either stepped type, slate type or slope type, having projections in the vae concrete. To support heavy loads, reinforcement is also provided at the base. The reinforcement provided is in the form of steel bars and is placed in both directions. Concrete column footings may be either isolated footings or combined footings. Isolated footings are used to support the individual columns, whereas combined footings are used where projections of different columns are not possible on all the sides, due to limited space.
The combined footings are generally rectangular in shape when both columns are of same sections and trapezoidal in shape when the columns are of different sections.
The foundation which consists of one or two tiers of wooden or rolled steel section with space filled up with concrete is known as Grillage foundation. This is so called because the bed constructed in this type of foundation is called grillage. This type of construction avoids deep excavation and provides the necessary area at the base of the structure to reduce the intensity of pressure within the safe bearing capacity of the soil.
The object of this type of foundation is to spread the load over a large horizontal area at the base of a structure.
Depending upon the material used in construction, grillage foundations are further classified into two types.
For timber grillage foundation construction
Steel grillage foundations are useful for structures like columns, piers, stanchions subjected to heavy concentrated loads and hence are employed for foundations of the buildings such as theaters, factories, town, halls etc. Timber grillage foundations re usually provided for timber columns subjected to heavy concentrated loads. This type of grillage foundation can also be safely used for light buildings where the soil encountered is soft and is permanently water-logged.
The foundation consisting of a thick R.C.C slab covering the whole area of a mat is known as raft foundation.
This type of foundation is useful for public buildings, office buildings, school buildings, residential quarters etc, where the ground conditions are very poor and bearing power of the soil is so low that individual spread footing cannot be provided.
Another types of foundation is stepped foundation, For Stepped Foundation Construction, excavation is done into steps having short length and uniform thickness and the masonry work is done on the horizontal bed of concrete thus prepared.
If there is any possibility of slipping of the structure bodily, R.C.C piles can be driven along its base concrete on the sloping side.
The foundation consisting of inverted arches between the piers is known as inverted arch foundation. In this type of foundation, the load from the piers is transferred to the soil by constructing arches in inverted position at their at their base. The rise of the inverted arches is about one - fifth to one - tenth of the span and they are usually build in half-brick rings. The position of arches may be either along the row of piers or across the row of piers in both the directions depending upon the nature of soil and the type of land to be taken by them.
For constructing this type this type of foundation;
This of foundation is not commonly used for buildings, but it is quite suitable for other structures like bridges, reservoirs, tanks, supports for drainage lines etc.
The foundation constructed sufficiently below ground level with some artificial arrangements such as piles, wells etc, at their base are called deep foundations. Deep foundation are further classified into the following types ;
A foundation (spread footing or grillage) supported on piles is called a pile foundation. A pile foundation usually consists of a base of spread footing or grillage supported by piles at their bottom. Piles distribute the load of structure to the soil in contact either by friction alone or by friction combined with bearing at their ends.
This type of foundation is suitable under the following situations ;
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