Definition of building substructure and its constituents

The primary constituents of a building substructure belong to the foundation and plinth beam. These components securely transmits the load from the superstructure to the ground.

Foundation: The foundation means the structure situated under the ground level that gets in touch with the superstructure directly. The foundation transmits the dead loads, live loads and all other loads operating on it to the foundational soil.

The construction process of foundation is done in such a manner that the soil over which it stands is stressed inside its safe bearing strength. Any failure of foundation leads to the failure of the building structure.

Therefore, different building structure requires various types of foundations like shallow or deep foundations.

Preliminary, the soil profile is checked with a geotechnical engineer prior to complete a proper foundation for the building structure. The following types of foundation are normally utilized for building structure:

1. Strip Foundation (Shallow Foundation)
2. Raft Foundation (Shallow Foundation)
3. Pile Foundation (Deep Foundation)

1. Strip Foundation: Strip foundation or strip footing offers support for linear structures like a wall or tightly spanned column, in the shape of strips. It is suitable for the soils having strong bearing strength as well as ability to support light structural loading. The size and location of the strip foundation are based on the width of the wall.

2. Raft Foundation: Raft foundation is also known as a mat foundation. It expands over the total building area and combats severe structural loads.

A raft foundation transmits the total load from the building area to the total floor area. As a result, the stress operating on the soil is significantly decreased that results in lessening the scope for shear failure of soil.

3. Pile Foundation: Pile Foundation stands for a type of deep foundation that transmits heavy loads from the superstructure to hard strata underneath the ground. Pile in a pile foundation refers to a deep reinforced concrete column that meets the hard rock strata far below the ground.

Plinth Beam: Plinth beam belongs to a beam that is developed in the plinth level among the wall and the foundation.

The purpose of this type of reinforced concrete beam is to resist the circulation of cracks from the foundation to the walls.

A plinth beam can allocate the load uniformly from the walls to the foundation. These are essential for construction projects planned in the areas susceptible to earthquake.

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Published By
Rajib Dey
www.constructioncost.co
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Some vital guidelines to build up a raft foundation

A raft foundation alias mat foundation refers to a continuous slab that stands on the soil that expands over the whole footprint of the building, thus provides strong support to the building as well as transmits its weight to the ground.

A raft foundation is mostly suitable when the soil becomes weak, since it allows to allocate the weight of the building over the whole area of the building, and not over shorter zones (like individual footings) or at individual points (like pile foundations). It minimizes the stress on the soil.

Stress is merely the weight divided with area. As for instance, when a building measures 5 x 5 weighs 50 tons, and contains a raft foundation, then the stress on the soil is weight / area = 50/25 = 2 tons per square meter.

When the similar building is supported by 4 individual footings with dimension 1 x 1m each, then the entire area of the foundation should be 4 m2, and the stress on the soil should be 50/16, that is around 12.5 tons per square meter. Therefore, if the entire area of the foundation is raised, the stress on the soil is significantly reduced, that means the weight per square meter.

A raft foundation is also effective for basements. Foundations are built up with excavation of soil with the purpose of retaining strong, compact, undisturbed natural soil that remains a few feet underneath ground level.

This soil contains more strength as compared to the loose soil at the surface. When a raft foundation is built up 10 feet under ground, and the concrete walls remain around the boundary to form a sound basement.

GUIDELINES TO BUILD UP A RAFT OR MAT FOUNDATION
Initially, excavate the ground to consistent, flat level to build up a raft foundation.

After that, place a waterproof plastic sheet over the earth, and pour a thin 3" layer of plain cement concrete (PCC) to form a rightly flat and level base for the foundation.

Then, a waterproofing layer is set up, and then reinforcement steel for the raft slab is secured in place. Once all the steel are arranged in exact location, concrete is poured to the required thickness that normally remains in the range of 200mm (8") to 300mm (12") thick for small buildings. The thickness will be increased when it is required to combat heavy loads.

To get more details, click on the link.

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Published By
Rajib Dey
www.constructioncost.co
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Raft foundation construction details

This construction article provides the detail construction methods for raft foundation of a building. The raft foundation is conducted for the multistory buildings or building having heavy loads.

Step 1: Initially, the surveyor should begin exaction and as soon as the survey is completed then exaction is carried out to the desired level on the basis of the validated shop drawings.

Throughout excavation, precaution for safety should be undertaken, the entire area should be guarded and everybody should utilize PPE at site.

Step 2: As soon as the final phase of the exaction is completed then the surface should be consolidated with plate compacter as the area is small and it is not difficult to apply the roller compacter. But 95% compaction should be performed with compaction test.

Step 3: After getting the passable results of compaction test, 75mm or 100 mm blinding concrete is arranged. Blinding concrete should be garde 20/20 SRC (Sulphate Resisting concrete).

The blinding concrete facilitates to arrange a solid and flat surface for the reinforcement of raft foundation.

The curing polythene sheets should be arranged instantly after finishing and when the concrete is set, it should be substituted with hessian cloth and should be retained wet unless the surface becomes fully dry.

Once the blinding surface becomes dry, water proofing of surface should be started according to sanctioned specification.

Step 4: Once water proofing is done, perfect planning should be undertaken for the shuttering work, all material should be retained on a plate form and steel bars should be installed with adherence to sanctioned shop drawings of the surrounding. All working space should maintain neat & clean to get rid of accidents at site.

Upon the completion of the reinforcement work, the contractor should sanction it from the supervision consultants for pouring the concrete.



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Arka Roy
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Variation among pad foundation, strip foundation and raft foundation

Shallow foundation is generally described as foundation that contains founding level below 3m from ground surface. Alternatively, when the breadth of the lower most part of the foundation is more than the depth of the lower most part of the foundation from the uppermost of the soil, i.e. Ground Level then it is defined as a Shallow Foundation. It transmits building loads to the earth adjacent to the surface. Shallow foundations comprise of spread footing foundations, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations and earthbag foundations.

Pad foundation stands for the foundation that is specifically developed for retaining concentrated loads out of a single point load like structural columns. Pad foundations are specifically designed to provide support to individual or multiple columns, scattering the load to the ground underneath. They are usually square or rectangular in plan, the plan area is defined by the allowable bearing pressure of the soil. The shape in plan will be determined by the arrangement of the columns and the load will be transmitted into the soil.

The thickness of the slab should be adequate to make sure that distribution of the load is perfect. The top of the pad should have been sloping (i.e. the pad is denser in the centre as compared to it is at the edge). It provides a cost-effective solution, though there are construction issues which are associated with casting the slope.

Strip foundation is utilized to provide support to a line of loads like load-bearing walls. As for example, closely-placed columns deliver the imperfect application of pad foundation and strip foundation may be a good choice.

Raft foundation comprises of a concrete slab which expands over the entire loaded area in order that loads from whole structure are expanded over a broader area that results in minimizing of the stress of foundation soils. Besides, raft foundation is useful for resisting differential settlement.

Ref.: www.engineeringcivil.com

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Rajib Dey

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Detailed information on Shallow Foundation

Shallow foundation

A shallow foundation is defined as the foundation that transmits the building loads to the earth adjacent to the surface. It’s depth is equivalent to or under its width.

Categories of shallow foundation:

1. Spread footingThe role of Spread footing is to extend the super imposed loads of the structure over a extensive zone.

Spread footing is subcategorized as below :

• Single footing for a column.

• Stepped footing for a column.

• Sloped footing for a column.

Concrete is utilized to create the base of these types of footings.

• Wall footing without steps and with steps.

• Grillage foundationgrillage foundation is considered as most cost effective while transmitting the heavy structural loads concerning a column to the soil of low bearing capacity. Depth of such a foundation is restricted to 0.9 to 1.6 meter.

2. Combined footing

The combined footing is developed for two or more columns.

Shape of combined footing is balanced in such a ratio so that the center of gravity of the supporting area remains consistent with the center of gravity of the two column loads. It is formed as either rectangular or trapezoidal.

A combined rectangular footing is useful where both the columns bear identical load or interior column bears superior load. A combined trapezoidal footing is arranged under any situations of loading.

3. Strap footing Strap footing is undertaken when two or more footings are associated with a beam. It is required when the distance among the columns is so excessive that a combined trapezoidal footing turns to be little constricted, containing great bending moments.

4. Mat or Raft foundation Mat or raft foundation is described as a consolidated reinforced concrete slab that covers the entire region of the bottom of the structure.

Mat or raft foundation is useful in the following conditions :-

• If the soil located below contains bearing capacity as well as the building loads are substantial.

• If the combined area of individual footing exceeds half of the total area of the structure, then application of mat or raft foundation will be cost effective.

Article Source : www.civilengineeringterms.com

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Rajib Dey

www.constructioncost.co

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Different types of foundation and their usefulness

Maximum structures are built with two parts like super structure and sub-structure of the foundation. Super Structure is located over the ground and the other sub-structure of the foundation is located beneath the ground level. Foundation alias footing of a structure associates and transfers the load from the structure to the ground soil. The foundation is developed on the solid ground and it is called as the foundation bed. The foundation transfers the load of the structure and it’s self-weight to the soil with the purpose of keeping the final bearing capacity of the soil under control (the shear failure does not occur) as well as keeping acceptable settlement.

All the structures have foundation at the base that offers the following functionalities:

• To disperse the load of the structure across an extensive bearing area.
• To load the bearing surface equally to get rid of asymmetrical settlement.
• To resist the lateral movement of the supporting material.
• To enhance the strength of the structure all together.

Foundation is based on the following points :-

Foundation is categorized depending on the dispersion of load to the ground into two sub-categories like shallow foundation and deep foundation.

Shallow Foundation

Shallow foundation belongs to the foundations where depth of the foundation remains below the width of the foundation (D < B). Shallow foundations are usually known as spread footing because they transfer the load of the super structure laterally into the ground.

Categorizarion of Shallow Foundation:

Based on the design, the shallow foundation is classified as:

• Wall Footing
• Isolated column or Column Footing
• Combined Footing
• Cantilever (Strap) Footing
• Mat (Raft) Foundation
• Wall Footing

This type of foundation runs consistently along the direction of the wall and facilitates transferring load of the wall into the ground. Wall footing are mostly applicable where transferable loads are small and cost-effective in compact sands and gravels. For this type of foundation, the width remains 2-3 times the width of the wall at ground level. Wall footing is built up with stone, brick, plain or reinforced cement concrete.

Column Footing

Column footing are useful and inexpensive for the depth surpassing 1.5m. For this type of foundation, the base of the column is distended. Column footing comes in the shape of of flat slab and is developed with plain or reinforced concrete.

Combined Footing

Combined footings belong to the foundations which are built in common for two or more columns in a row. It is generally formed if the footing for a column is spreaded outside the property line. It is applicable if the two columns are placed narrowly and the soil on which the structure is developed contains low bearing capacity. The shape of the combined footing appears as rectangular or trapezoidal.

Strap Footing

If an edge footing fails to spread outside the property line, it is connected with the other interior footing through a strap beam. Such footings are defined as strap footing. or sometimes cantilever footing.

Mat Foundation

A mat foundation belongs to a combined footing that covers the whole area below a structure and supports all the walls and columns. It is also called as raft foundation. Mat foundation is mostly suitable for the following reasons:

Permissible bearing pressure is low.

The structure is weighty.

The site is located with highly compressible layer.

The mat foundation is categorized into following types:

Flat slab type.

Flat Slab thickened under column.

Two way beam and slab type.

Flat slab with pedestals.

Rigid frame mat.

Piled mat.

To read the complete article, visit civileblog.com

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Published By

Rajib Dey

www.constructioncost.co

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