The subject of foundation engineering deals with design of various types of sub structures under different soil and environmental conditions During the design the designer has to make use of properties of soil , the theories pertaining to the design and his practical experience to adjust the design to suit the field condition
TYPES OF FOUNDATION:
Residential foundation systems can be divided into six general categories:
• No foundation
• Partial foundation
• Post and pier throughout
• Perimeter footing with interior posts
• Continuous perimeter and interior footings
• Continuous footings with a slab floor on grade.
With no foundation, or too small a partial foundation, the horizontal forces in the building cannot be transferred safely into the ground (Fig. 5.1).
There is a similar discontinuity in the load path with a post and pier foundation (Fig. 5.2).
You will learn more about post and pier systems in the next section, Retrofitting Post and Pier Type Houses.
When a building has a continuous perimeter footing, the horizontal force can transfer from the shear wall to footing and then into the ground. Interior supports are usually posts and girders or continuous cripple walls (Fig. 5.3 & 5.4).
Unless there are shear walls above, these systems provide vertical support only and are not part of the horizontal force-resisting system. They are simply part of the gravity force-resisting system.
... – feasibility analysis – finalspecifications – hardware and software study – system design – system implementation –system evaluation – system modification. Role of systems analyst – attributes of asystems analyst – tools used in ...
The foundation would normally consist of one or more the following
· concrete block
If the concrete block is fully grouted and reinforced, it will tend to act as concrete. If it is not grouted or reinforced, it will act as brick or stone Continuous concrete footings and foundation walls are the best material type. This is one reason their presence is required in some prescriptive standards. Expansion anchors require concrete and are not approved for connections to masonry. Some adhesive anchor products can be used for reinforced masonry. Reinforced masonry is not as strong as concrete but when fully grouted is strong enough to resist the seismic loads of light wood frame buildings. Hollow masonry is usually unreinforced.
Concrete is a composite material composed of coarse granular material the aggregate or filler embedded in a hard matrix of material (the cement or binder) that fills the space among the aggregate particles and glues them together.
Sand should be sieved before mixing with the aggregate. For this purpose many sand seivers of various sizes are used. The size is measured in microns and millimicrons.
Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes. For example, the particles of stone used to make concrete typically include both sand and aggregate.
A brick is a block or a single unit of a ceramic material used in masonry construction. Typically bricks are stacked together or laid as brickwork using various kinds of mortar to hold the bricks together and make a permanent structure. Bricks are typically produced in common or standard sizes in bulk quantities. They have been regarded as one of the longest lasting and strongest building materials used throughout history.
But the type of foundation which are current practice are
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A shallow foundation is a type of foundation which transfers building loads to the earth very near 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 typical in residential building, 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 will support as well as 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 building it is not sufficient. A spread footing which changes elevation in several places in a series of vertical “steps” in order to follow the contours of a sloping site or accommodate changes in soil strata, is termed a stepped footing.
Mat slab foundation:
Mat-slab foundations, also called on-grade mat foundations for expansive soils, are used to 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 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.
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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, Styrofoam panels), or heating provisions such as hydronic heating can be built into the slab (an expensive installation, with associated running expenses).
Slab-on-grade foundations are commonly used in areas with expansive clay soil, particularly in California and Texas. 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.
Care must be taken with the provision of services through the slab. Copper piping, commonly used to carry natural gas and water, reacts with concrete over a long period, slowly degrading until the pipe fails. Copper pipes must be lagged or run through a conduit or plumbed into the building above the slab. Electrical conduits through the slab need to be water-tight, as they extend below ground level and can potentially expose the wiring to groundwater.
Rubble trench foundation:
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.
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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 steel-reinforced concrete daro beam is poured at the surface to provide ground clearance for the structure.
If an insulated slab is to be poured inside the grade beam, then the outer surface of the grade beam and the rubble trench should be insulated with rigid XPS foam board, which must be protected above grade from mechanical and UV degradation.
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
When concrete foundation walls are constructed without expansion joints, hairline crackling will normally occur. Cracks that are wider at the top than at the bottom are often caused by expansive soil. When the crack is wider at the bottom than at the top, there is likely a problem with soil settlement. These problems can prevent the seismic loads from safely dissipating through the The effects of expansive soils are best reduced with deepened footings and control of adjacent watering. Keeping roof and surface water away from footings is always a good idea since settlement can also occur with excessive water in the soil. Underpinning, roof gutters with downspouts to yard drains and new concrete paving can help alleviate expansive soil problems. The presence of expansive soils or foundation settlement indicates the need for professional advice. Geotechnical engineers specialize in solving these Depending on the size of the crack, concrete cracking can be repaired with various epoxy or cementitious mortars. These products require special inspection and careful quality control by the approved applicator. These products should be used only under the qualified advice of the engineers.
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A foundation is an integral part of structure and its stability depends upon the stability of the supporting soil. Two important factor to be considered are
The foundation must be stable against shear failure of the supporting
The foundation must not settle beyond a tolerable limit to avoid damage to the structure.