Published on March 6, 2014
FOOTINGS • • • Foundation of a structure is like the roots of a tree without which the tree cannot stand. The construction of any structure, be it a residence or a skyscraper; starts with the laying of foundations. Foundations are broadly classified into shallow foundations and deep foundations. The depth of the foundation means the difference of level between the ground surface and the base of the foundation. If the depth of the foundation is greater than its width the foundation is classified as a deep foundation. Shallow foundations are commonly used in smaller structures such as residences and small buildings whose floor height is limited to 10m whereas Deep Foundations are used in Skyscrapers…. Piles are the most commonly used Deep Foundations used in skyscrapers…
SETTLEMENTS OF FOUNDATIONS NO SETTLEMENT * TOTAL SETTLEMENT * DIFFERENTIAL SETTLEMENT Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage
Bearing Capacity of Shallow Foundations
SHALLOW FOUNDATIONS • Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. • Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorlycompacted fill, peat, recent lacustrine and alluvial deposits, etc. Advantages a) Cost (affordable) b) Construction Procedure (simple)] c) Material (mostly concrete) d) Labour (doesn’t need expertise)
TYPES OF SHALLOW FOUNDATIONS PAD FOUNDATION SPREAD FOOTING COMBINED FOOTINGS CONTINOUS FOOTING MAT FOUNDATION
PAD FOUNDATION Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or reactangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.
SPREAD FOOTING Footings are designed to resist the full dead load delivered by the column. A footing carrying a single column is called a spread footing, since its function is to “spread” the column load laterally to the soil. This action will reduce the stress intensity to a value that the soil can safely carry. Spread footings are sometimes called single or isolated footings. They are square or rectangular pads which spread a column load over an area of soil that is large enough to support the column load. The soil pressure causes footings to deflect upward causing tension in two directions at the bottom. As a result, reinforcement is placed in both horizontal directions at the bottom.
Footings can be classified according to their function or shape. The function is how a footing serves: a spread or isolated footing (distributes the column load to an area of soil around the column), a combined footing (combines the loads from two or more columns to the soil), a continuous footing (one dimensional action, cantilevering out on each side of the wall), a pile cap (transmits the column load to a series of piles which in turn, transmit the load to a stronger soil layer at some depth below the surface), and a strap footing (transmits the loads from all the columns to a grid of footings, thereby bridging weak spots on the surface of the soil).
Rectangular Spread Footings Rectangular footings may be used when there is not enough clearance for a square footing. In this type of footing, the reinforcement bars in the short direction are placed in the three bands, with closer bar spacing in the band under the column than in the two ends. The band under the column has a width equal to the length of the short side of the footing, but not less than the width of the column if that is greater, and is centered on the column. Under long narrow columns it should not be less than the width of the column. The reinforcement in the band shall be 2/(b+1) times the total reinforcement in the short direction, where b is the ratio of the long side of the footing to the short side. The reinforcement within each band is distributed evenly, as is the reinforcement in the long direction.
CONTINOUS FOOTING In this type of foundation the base of the column is sufficiently enlarged to act as the individual support. The widened base not only provides stability but is useful in distributing the load on sufficient area of the soil. Column footings are usually used in the foundations of residences and buildings where the soil is hard enough has has sufficient bearing capacity.
COMBINED FOOTINGS 1. Provides a larger footing to support two or more columns, and 2. Transfers an eccentric outer column’s load to a inner footing and column.
Types of Mat Foundation: • 1- Flat plate ---- The mat is of uniform thickness 2- Flat plate thickened under columns 3- Beams and slabs 4- Slab with basement walls
Slab fabric • • • • Slab fabric -e.g. previously called F62 (now SL 62)= fabric 6mm bars welded together in a 200mm square grid. Sheet size 2.4m X 6.0m. Available in 6, 7, 8, 9, 10, mm diameter bar sizes (strength of 500 Mpa). Slab fabric is also available in rectangular grids. Common sizes are RL918 (9mm bars at 100mm centres and 8mm bars at 200 centres) and RL1018 (10mm bars at 100mm centres and 8mm bars at 200 centres).
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