Footing is an essential component of the foundation of construction. Some of the essential purposes of footing include providing support to the foundation and prevent settling. In simpler terms, the final point of contact for the entire weight of any building is the footing, which provides the necessary strength and support. The importance of footing is also evident from the fact that if the building’s weight is unequally distributed or remains unsupported, the foundation can topple down.
One of the signs of a weak foundation of the building is the sagging walls and cracked floors. When done right, footing provides advantages as follows:
- Projection from ground and dwelling vibrations.
- Braces adjacent grinders and post to ground connection simultaneously.
- Provides stabilization and anchorage to the crawl spaces of the building.
- Securing wood and metal post to the ground by sheer weight.
- Sliding-resistant ( protects the foundation during earthquakes).
As we have gone through the advantages of footing, now the question arises: How to do it right? In this article, Zarea.pk will answer your question by providing a comprehensive guide of constructing footing for residential buildings with maximum two storeys.
The footing of a two-storey building
The footing of a two-story building is relatively easy due to the smaller imposition of load. Therefore, the problems of the underlying soil are feasible to resolve.
The footing determines the thickness of the wall. Typically, the footing extends at 100mm on each side. In case of low soil-bearing capacity, the footing is increased to tackle this problem. On average, the even footing of a residential two-storey building is 96 KPa.
Footing thickness around 200 mm to 250 mm is considered sufficient.
Functionally, keyways provide resistance to lateral loads, which are present at the bottom of the walls. The provision of a keyway becomes necessary if the filling work is initiated before slab construction. On average, the dimensions of keyways 2.5 cm depth and 2.5mm to 3.8 mm width.
The dowel can also be used as a keyway. In a minimum dowel design, bar no. 13 (No.4) is placed at the center with 61cm spacing. Extend dowels are 30 cm above the footing and 15 cm into it.
Longitudinal reinforcements extending 61 cm at each side of the penetration is provided for bridging penetrations or trenches under the footing. Two minimum steel bars no. 13 (No.14) are also used. The required reinforced area requires commuting if the span of trench or penetration is more than 91cm.
Frost protection is required if the footing depth doesn’t extend beyond the frost line of the project site. If this isn’t the case and the excavation depth exceeds the designated path, it is usually filled with concrete or other fill material. Reinforcement is used in the footing to bridge short soft areas in the respective excavated area. Different types of soils such as sand, silt, or clay are determined through boil-boring.
A minimum of two borings with an area of 150m2 is drilled. From the bottom of the footing, the depth of the boring is kept at 1.5mm. At the bottom of the footing excavation, fill placement should be prevented; otherwise, it becomes difficult to check the quality of fill material. If it cannot be prevented, then sand or gravel is used for compaction to obtain specific gravity as required.
The materials that are washed into the footing, including water, saturated/loose soil, and mud, should be discharged to keep the bottom of the footing clear.
In case of frost protection, batt insulation, polyethylene sheets, straw, or mineral wool is used to prevent the ground from freezing. If the construction is taking place in a cold environment, then the frost must be removed or heated. The heat from the concrete can also remove the frost if its thickness is less than 5mm.
The following types of forms are used for the construction of footing:
- Metal fabric
- Synthetic fabric
- Plastic forms
To maintain the desired shape during concrete placement, the trench in the ground is excavated. The concrete is placed and compacted to reach the desired level.
To increase the strength over narrow excavations and bridge the soft spots and minor trenches, longitudinal reinforcement is often used. When the soil bearing capacity is low, and wall load is high, transverse reinforcement is used. Deformed steel bars of Grade 280 and 420 are used with the recommended bar sizes i.e., No. 13 (no.4) and No.16 (no.5). A minimum concrete cover of 7.6 cm is also provided to the bottom and sides. It is important to note that the bars should overlap each other by at least 30db.
The concrete placement usually takes place by using conventional methods. These methods include using a wheelbarrow, pump, conveyor, crane, or direct chute. The slump value should not exceed 15 cm. However, there is an exception, and the slump value can reach up to 20 cm if the concrete consists of high range water reducing admixture. The compressive strength of the concrete should be at least 17 MPa at 28 days. In the case of poor soils where transverse reinforcing is required, usually, high-strength mixtures are used.
Now, as we have gone through the detailed account of how footing is constructed for a double-storey residential building, it’s time to discuss footing types. Following are the various types of footings that are commonly constructed:
This type of footing is also known as strip footing. It extends along the length of the wall and a short distance beyond the wall edges. The width usually extends beyond both sides of the foundation wall. If the edge of the wall is more significant in dimensions than the wall thickness, then transverse reinforcement may be required in the case of continuous footing.
Pad footing is also known as spread footing. In this type of footing, the concentrated load is transferred through the column in the soil. In some cases, the pad footing is monolithically cast. This is done by developing continuous footing at points where the beam sits on the wall, and a higher concentration load is created. This is also usually done when the height of the wall is 1.2 m. to determine the width and length of pad footing, the transmitted load and soil-bearing capacity is kept into consideration.
Thickened slab footing
This type of footing is also known as shovel footing. It is constructed monolithically along with the floor slab. Usually, it is a thickened part along the edge or center of the floor slab. This type of footing supports the bearing wall in the interior of the building. It also provides an alternative for supporting the columns.
Curing and Protection
The next important step after the construction of the footing is its curing and protection. To protect the concrete from high or low temperatures, the concrete should obtain a compressive strength of 3.5 MPa. Concrete is coated with a protective layer of polyethylene or any other type of moisture retarder. This protects the concrete from excessive drying from sun and wind.
The last and final step is to ensure the footing drainage. This step is essential as it releases the pressure which is generated due to the accumulation of water and soil. Footing drainage also prevents water penetration from cracks and intersections of walls. For ensuring the drainage, a slotted PVC drain pipe or clay pipe is usually used. The pipe is placed at all footings which are adjacent to the storage space.
In some cases, the pipe is installed integrally along the footing. The top of the drainage system coincides with the top of the interior slab. At last, the drainage system is then covered with filter paper or gravel. The drain water is then disposed of into a storm sewer.
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