Brief overview of foundation construction

Brief overview of foundation construction

Definition of foundation: The foundation can be defined as follow: Any structure is segregated into 2 parts:
  • 1. Superstructure: It is the portion of structure located over Ground level (GL)

  • 2. Substructure/ Foundation: It is the portion of structure under GL
Constituents of foundation: Generally there are 3 constituents of foundation: (i) Footing, (ii) Column, (iii) Tie beam.

Footing belongs to that portion of substructure that fundamentally delivers the load (dead/live/wind/seismic) to the soil that provides support to the structure, in a manner so as to maintain the safe load bearing capacity of soil under control.

Footing refers to a structure that is built up in brickwork masonry or RCC under the base of a wall or RCC column for allotting the load over an extended area. The footings are very useful for the plan dimension, i.e., the bearing area that is ultimately supported by the soil and distributing the building load to the soil at below.

The plan dimension of the footing is defined with the nature of the soil. The footing that is supported with the rocks will contain less plan dimension with regards to that situated on soft clay or fine sandy soil. RCC Footing should contain the least depth of 150mm for footing on soil, and lowest 300mm over the top of piles for footing on piles. Lowest clear cover (concrete cover from the outline of concrete to the outline of steel) for footings is usually 50mm.

Structural characteristics of footing:
  • Total settlement of the structure is restricted to values as indicated in following table 2 [extracted from IS1904].

  • Differential settlement (variation in total settlement) among various footings has to be circumvented as much as practicable and should be restricted to values as stated in following table 2 [extracted from IS1904].

  • Footing has to be designed and placed at the level where a safe bearing capacity of a soil is recognized from soil testing report.

  • Centre of gravity (CG) of the footing has to be matching with the CG of loading.

  • Stress distribution under the footing should be in a way that maximum permissible stress should not surpass anywhere under the footing.

  • Nowhere tension should be acceptable to develop under the footing. Eccentricity check for footing should be accomplished.
Types of Footings Residential and Commercial Construction:

Slab on Grade: The most typical type of foundation, consist on a footing that surrounds the perimeter of the structure and is covered by a slab set on grade.

Piers: Used to support posts, these are footings for above grade that hold posts for above grade structures like modular buildings, mobile homes, trellises, patio covers and etc.

Wall foundation: Used on retaining walls, restraining walls and perimeter walls for modular and above grade buildings. these types of footings are usually built in combination with masonry or brick walls.

Columns: These footings are used to support heavy loads that will sit on a single bearing point. for example, support beams, front porches, etc...

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