Types of Foundations in Construction Industry

Today, we shall talk about the common sorts of foundations in buildings. Generally, all foundations are divided into 2 categories: shallow and deep. The words ‘shallow’ and ‘deep’ check with the depth of soil during which the inspiration is created.

Shallow foundations may be created in depths of as very little as 3 feet, whereas deep foundations may be created at depths of sixty – 200 feet. Shallow foundations are used for little, lightweight buildings, while deep ones are for giant, serious buildings. The following sorts of Foundations In Building Construction may be elaborated below.

Raft or Mat Foundations: Raft Foundations, conjointly referred to as Mat Foundations, are most frequently used once basements are to be made. In a raft, the complete basement floor block serves as the foundation; the burden of the building is unfold equally over the entire footprint of the building. it's referred to as a raft as a result of the building is sort of a vessel that ‘floats’ in an exceedingly ocean of soil.

Mat Foundations are used wherever the soil is weak, and thus building hundreds have to be compelled to meet an oversized space, or wherever columns are closely spaced, which suggests that if individual footings were used, they might hit one another.

Shallow Foundations: Shallow foundations are referred to as unfold footings or open footings. The ‘open’ refers to the very fact that the foundations are created by 1st excavating all the planet until all-time low of the footing, so constructing the footing. Throughout the first stages of labor, the complete footing is visible to the attention, associated is so referred to as an open foundation.

The concept is that every footing takes the focused load of the column and spreads it out over an oversized space,so that the particular weight on the soil doesn't exceed the safe bearing capability of the soil.

There are many forms of shallow footings: individual footings, strip footings and raft foundations.

In cold climates, shallow foundations should be protected against freezing. This can be as a result of water within the soil round the foundation can freeze and expand, thereby damaging the inspiration. These foundations ought to be engineered below the frost line, that is that the level within the ground higher than which freeze happens.

If they can't be engineered below the frost line, they must be protected by insulation: commonly a touch heat from the building can permeate into the soil and forestall freeze.

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www.constructioncost.co
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Common thumb rules for civil engineering works

Thumb Rules is very important for any civil engineer, Site engineer or civil supervisor to obtain instant decisions on the construction site. By applying thumb, the engineers can get the solution with a simple mathematical formula and take proper decisions wherever required. Before applying these thumb rules, it should be kept in mind that the thumb rule can only provide fairly accurate results never the correct results.

The following types of thumb rules for civil engineers are commonly used in construction work :-

Thumb rule for measuring the Concrete Volume relating to the area:
The volume of concrete necessary = 0.038 m3/square feet area.

As for instance, if Plan Area = 40 x 20 = 800 Sq. m., total necessary volume of concrete will be as follow :-
= 800 x 0.038m3 = 30.4m3

Thumb rule for Steel quantity necessary for Slab, Beams, Footings & Columns:
Essential quantity of steel in residential buildings = 4.5 Kgs – 4.75 Kgs / Sq. Ft.
Essential quantity of steel in commercial buildings = 5.0 Kgs-5.50 Kgs/Sq. Ft.

Thumb Rules For Civil Engineers recommended by B N Datta for the Steel quantity that will be applied for several members of the building :-

Proportions of Steel in Structural Members:

1) Slab – 1% of the total volume of concrete
2) Beam – 2% of the total volume of concrete
3) Column – 2.5% of total volume of concrete
4) Footings – 0.8% of the total volume of concrete

As for instance, suppose the length, width and depth of the slab are 5m, 4m and 0.15m. Now, the quantity of steel for the slab will be computed as follow :-

Initially, it is required to work out the concrete volume.
The total volume of concrete for the slab = 5x4x0.15 = 3m3

Secondly, work out the quantity of steel with formula as follow :-
Based on the guidelines provided in B. N. Dutta reference book, the quantity of steel in slab is 1% of the total volume of concrete used.
Thumb rule to work out the quantity of steel in above slab = Volume of concrete x density of steel x % of steel member.

The weight of steel necessary for above slab = 3x7850x0.01 = 235 kgs

To make perfect calculation, use bar bending schedule.

To learn how thumb rules are applied to calculate the shuttering area and the quantity of cement, sand, course aggregate in several grades of concrete, click on the following link civiconcepts.com

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Types of reinforcement or mesh in several footings (foundations)

Different types of Reinforcement in footings or types of mesh used in foundation:-

Several types of reinforcement exist in footings. The reinforcement should be provided in footings for tension requirements. Normally, the percentage of reinforcement in footings should remain among 0.5% to 0.8%. Based on the load analysis, the structural engineer design the type of Mesh in footings. Given below, the details about he types of mesh (reinforcement) implemented at several types of footings or foundations.

Usually, four different types of reinforcement in footings or foundations are found :–

1. Plain Mesh: This type of Mesh is normally implemented at plain or isolated or combined footings. It is specifically useful for low-rise buildings. Prior to use plain mesh to high rise buildings, the load should be analyzed in accordance with this mesh and determine either the type of mesh is balanced with the load or not.

Under this type, bars are arranged as a grid. It may contain bars with various diameter and spacing in either direction. The spacing may or may not vary in both directions.

2. Mesh with hooks (Hook Mesh): It is suitable for both low rise and high rise buildings. The footing is reinforced as grid and the bars are arranged with hook at the ends of the mesh. The perfect anchorage of the reinforcement can be obtained by bending the bars ends. Normally, the standard length of hook is 10D where D stands for the diameter of the bar.

3. Footing Mesh up to the depth of Footing: It has similarity with Plain footing. Under this type of footing, the bars are bent at ends up to a height of footing. The concrete cover of 1″ to 4″ should be arranged in all the sides of footing.

4. Raft Mesh: This type of Mesh is ideal for raft footing. Raft footing is suitable when the bearing strength of soil is very low. Under this type, mesh is segregated into two parts like top mesh and bottom Mesh.

Initially, the bottom mesh is arranged on covering blocks, ends of a bottom mesh are bent at an angle of 90 degree up to a height of 50D where D stands for Dia of Bar. After that top mesh is attached with the bottom mesh in opposite direction. Besides, the top mesh equivalent to bottom mesh is bent with 90 degrees but an additional bar of 50D is not arranged since it is already equipped on bottom mesh.

The 50D extra bar is arranged either on bottom or top mesh.

Single ring or double rings are attached with top mesh and bottom mesh to retain the proper framework. The rings allow the steel reinforcement not to distort in any direction. Least diameter of bars used for rings should be 6 mm.

In single ring raft mesh, rings are arranged in only one direction either horizontal or vertical, while in double ring system, the rings are arranged in both the direction.

The following points should be taken into consideration :-

1. Concrete cover differs from 1" to 4" depending on the size of the footing.
2. Hook length in Hook mesh is always 9D, where D stands for Dia of bar.
3. Additional bar is arranged either on top or bottom mesh and additional bar length is 50D.



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Methods of transmitting stresses from RCC columns to footings

To make the design of the footings, it is necessary to have sound understanding about the transmission of stresses (caused by applied loads) from columns to the footings or piles. Factored forces and moments at the base of columns are delivered to the supporting foundations by providing support on concrete as well as by the extension of longitudinal reinforcement into the footing or arrangement of dowels.

Any of the above method can be employed on the basis of the specifications of the relevant design codes like ACI 318-14. The detailing of reinforcement should be done sufficiently for dispersing the stresses to footings perfectly and then further above the basic soil layer.

Methods of transmitting the stressed from Columns to Footings:

1. Expansion of the Longitudinal Steel into the Footing

Expanded reinforcement should be prepared to transmit the compressive forces that surpassed the lesser of the concrete bearing capacities of either the supported member or the foundation, and any computed tensile force across the column-footing interface.

Minimum four bars should be expanded.

The stress-transfer bars should extend into the base with adequate compression-embedment distance to transmit the stress in the column bars to the base concrete.

Whatever the case may be, a minimum steel area of 0.5% of the column-area should be arranged for transmitting the load.

2. Dowel

a. Dowels should be prepared to transmit the compressive forces which go beyond the lesser of the concrete bearing capacities of either the supported member or the foundation, and any computed tensile force across the column-footing interface.

b. There should be minimum four dowels.

c. The diameter of the dowel should not go over the column-bar diameter in excess of 0.15 in.

d. If the desired dowel length remains greater than the footing depth less 3 in, any smaller-diameter bars containing the same area should be applied or a monolithic concrete cap should be included for raising the concrete depth.

e. The dowels should deliver minimum one-quarter of the tension strength of the column bars on each column face.

f. The dowels should be expanded into the column with a distance equivalent to that necessary for compression lapping of column bars.



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Process for creating design of Seismic Tie Beams

Tie beams are constructed to support the differential settlement among the isolated footings other than the vertical loads of the block works.

With reference to ACI 318-08 section 21.12.3.2, grade beams which are built up to function as horizontal ties among pile caps or footings should be proportioned in such a way so that the smallest cross-sectional dimension is identical to or exceeding the clear spacing among associated columns divided by 20 but should not be over 18 inches. Closed ties should be arranged at a gapping not surpassing the lesser of one half the lowest orthogonal cross-sectional dimension and 12 inches.”

But, it becomes difficult to understand that what force these tie beams are required. In this regard, IBC 2009 section plays an important role.

IBC2009 section 1809.13 for shallow foundations and 1810.3.13 for deep foundations.

For SDC C, D, E or F, ties have the capacity to bear, in tension or compression, a force similar to the smaller of the product of the larger pile cap or column design gravity load times the seismic coefficient, Sds, divided by 10, and 25 percent of the smaller pile or column design gravity load.

In brief, Tie beam force; FT = Larger of ( Pu_large x Sds /10 , Pu_small x 25%)

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www.constructioncost.co

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