There is no doubt that the construction structures exert considerable weight. A structural element that bears the weight of the structure is called the footing. Even a small mistake in the design of the footing could collapse the entire structure. This makes the proper foundation design crucial for the construction project. Before we move on to the article’s main theme, it is important to understand what footing is. It plays a crucial role by distributing the load to the wider surface of the soil to achieve structural stability. This prevents the heavy load from being entirely concentrated on the building’s columns, increasing longevity. However, different construction projects use different footings based on the requirements. The combined footing is the most suitable one for irregular column placement to distribute loads effectively. In this article, we will discuss the rectangular combined footing, design considerations, and calculations involved in rectangular combined footings.
What is a Rectangular Combined Footing?
The shape of this footing is rectangular, as the name suggests. This footing is used whenever there is a difference in loads between two columns and when the centres are not aligned on a single line. The rectangular lengths create a platform to provide support to both columns. This type of footing, like others, transfers the load from the structure to the underlying soil.
Design Considerations for Rectangular Combined Footings
The design considerations vary with or without a beam in the rectangular combined footing. Measure the column loads and the self-weight of the footing. Then, the soil is tested to determine the bearing capacity after the area of footing for the load is calculated. In the external faces of the external column, the length is always more than the distance. The C.G. of column loads coincides with the C.G. of the area of the footing by fixing the footing beyond the column in a particular way. This ensures that the pressure is uniformly distributed to the soil. For design purposes, critical values and respective locations are mentioned on the S.F.D and B.M.D.
Steps involved in the rectangular combined footing without a beam are,
- The designed depth is fixed after measuring the depth of the slab and considering one-way, and two-way shears.
- The depth of the slab determines the maximum bending moment.
- Slabs are treated considering them as longitudinal beams spanning between design and main reinforcement.
- If needed, shear resistance is prepared to resist one-way shear.
- To avoid the slab bending in the transverse direction and taking the form of a saucer, the load below the column should be distributed across the full width.
- Like isolated footing, the reactive pressure on the above area is calculated for the design of cantilever projections.
- Distribution bars are positioned in the transverse direction.
Steps involved in the design consideration of rectangular footing with a beam are,
- In the case of a rectangular combined footing, the main and shear reinforcement of the beam is designed and maintained.
- The usual method is used to design the projected cantilever slab in the transverse direction.
Step-by-Step Calculation of Rectangular Combined Footing
Careful calculations and safety, stability, and serviceability are important while taking steps to calculate the rectangular combined footing. For specific design requirements and guidelines, it is advisable to refer to relevant codes and standards. When it comes to design calculation, one must consider axial loads and uniaxial moments to which the beam is subjected. Below are the steps involved in the design process of a rectangular combined footing.
- Calculate the axial forces and moments acting on the columns about the axis of interest.
- Based on the column sizes and spacing, measure the initial footing width and length.
- Estimate the depth of the footing as well.
- Using the formula for the sum of axial loads from the columns, calculate the total load.
- Using the formula q=W/A, calculate the bearing pressure.
- The moments exerted by the column moments and any eccentricity are measured.
- Calculate the shear forces acting on the footing.
- The footing is assessed to determine the strength to resist sliding and friction between the footing and soil. Measure the overturning moment as well if the weight of the footing is heavier.
- Maximum moment formula will help us calculate the required area of steel reinforcement.
- The formula of maximum moment is, Mu=0.87fyAsd Where, As is the area of steel, fy is the yield strength of steel, and d is the effective depth of the footing.
- Ensure adequate shear capacity concerning the shear forces.
- To meet the serviceability requirements, ensure the shear capacity of the footing is adequate.
- To avoid cracking, necessary steps should be taken regarding the spacing of reinforcement.
- Provide the number and spacing of bars for shear resistance.
- Draw a construction plan showing dimensions, reinforcement layout, and any other relevant details.
Construction Process of Rectangular Combined Footing
Using stakes and strings, mark the outline of the footing on the ground. To accommodate formwork and reinforcement, dig the soil to a depth slightly larger than the footing dimensions. Wooden planks and steel sheets with strong formwork are used in the construction process to withstand the weight of the concrete. The reinforcement is positioned and tied together using wire ties. Following the preparation of the concrete, the concrete is poured into the formwork and vibrated to consolidate it. This prevents air bubble formations in the concrete. The levelling is done to a desired elevation. Once the concrete is cured, the formwork is removed carefully without causing any damage to the structure.
Rectangular Combined Footing Vs Other Footing Types
This type of footing offers several advantages over other footings. It is highly preferred in closely spaced columns or poor soil conditions. They reduce the load over a larger area to reduce soil pressure. Compared to other footings, they can easily accommodate unequal column loads. The flexibility in its design gives an edge in design and construction. If the footing is designed and constructed properly, the rectangular combined footing will be reliable and cost-effective compared to other types.
Conclusion
Rectangular combined footings are an efficient and economical option for supporting more than two columns, especially when three or more columns are located nearby or when the soil is poor. They minimise soil pressure by increasing the area of loading and improving stability. Units combined have a greater advantage over isolated footings in terms of the relationship between the slants of the columns and the vertical unevenness of the soil. Even though mat foundations have been found to work for very porous soil, it is very complicated and expensive. Even pile foundations which are best placed on the porous ground, are the most specialised and machine intensive. By taking into account various aspects, including soil type, the load of the columns, and structural properties, engineers can produce rectangular combined footings that perform safely and economically for a long time.