When constructing buildings with multiple columns that are in close proximity to each other and have overlapping foundations, combined footings are often used. The purpose of a footing or foundation is to transfer the load from the structure to the underlying soil. The appropriate type of footing to use depends on various factors, including the depth at which the bearing strata is located, the condition of the soil, and the type of superstructure being constructed.
In the case of multiple columns that require overlapping foundations, combined footings are a suitable solution. The design of these footings must take into account the load-bearing capacity of the soil, as well as the weight and placement of the columns they will support.
To illustrate the design of combined footings, an example can be considered. Suppose there are two columns located 2 meters apart with a total load of 100 kN. The bearing capacity of the soil is 200 kN/m², and the depth of the bearing strata is 3 meters. In this scenario, a combined rectangular footing can be designed to support both columns.
The dimensions of the footing can be determined by considering the load distribution from both columns and ensuring that the soil bearing capacity is not exceeded. Once the dimensions are calculated, the reinforcement required for the footing can be determined, and the design can be finalized.
Overall, the choice of footing type is an essential aspect of building construction and must be carefully considered to ensure the stability and safety of the structure.
Combined Footings
A combined footing is a type of foundation that is used when two or more columns need to be supported on a single spread footing. However, isolated footings for each column are generally more economical and are the preferred option. Combined footings are typically only provided when it is absolutely necessary to do so.
One situation where a combined footing may be necessary is when two columns are situated very close together, causing overlap of adjacent isolated footings. In such cases, it may be more practical to use a combined footing to support both columns instead of trying to fit separate footings in the limited space available.
Another reason for using a combined footing is when soil bearing capacity is low. In such cases, overlapping adjacent isolated footings may cause excessive stress on the soil, which could lead to structural failure. A combined footing can help to distribute the load from multiple columns over a larger area, reducing the stress on the soil and providing a more stable foundation.
Finally, a combined footing may be necessary when there are limitations on the available space for the foundation. For example, if a building column is located close to a building line, an existing building, or a sewer line, it may be difficult or impractical to install separate isolated footings for each column. In such cases, a combined footing can provide a practical solution for supporting the columns while minimizing the impact on surrounding structures or infrastructure.
Types of Combined Footing
Combined footings are used to support two or more columns of a structure. These footings come in different shapes such as rectangular, trapezoidal, and Tee-shaped. The shape and proportions of the footing are carefully chosen so that the centroid of the footing area coincides with the resultant of the column loads. This ensures that the pressure underneath the entire area of the footing remains uniform.
When one column load is much larger than the other, a trapezoidal footing is used. This type of footing restricts both projections of the footing beyond the faces of the columns. On the other hand, rectangular footings are used when either the width of the footing is restricted or one of the projections of the footing is restricted.
Overall, the choice of the shape and proportions of the combined footing depends on the specific requirements of the structure and the loads it will be subjected to. By selecting the appropriate footing shape, engineers can ensure that the footing can effectively support the weight of the columns and distribute the load evenly to the soil beneath it.
Rectangular combined footing
The footing of a structure behaves as an upward loaded beam that spans between columns and extends beyond them in a cantilever fashion. To determine its structural integrity, shear force and bending moment diagrams in the longitudinal direction are drawn using statics. It is crucial to check the moment at the faces of the column, while the shear force is critical either at a distance ‘d’ from the column faces or at the point of contraflexure.
In addition to longitudinal loading, the footing is also subjected to transverse bending. The bending moment resulting from this load is distributed over a transverse strip near the column. To ensure the footing’s stability, two-way shear is checked under the heavier column. It is essential to thoroughly examine and verify these aspects of the footing’s behavior to ensure its ability to support the overall structure safely and efficiently.
Steps for Design of Combined Footing
To design a proper footing, it is essential to determine the point of application of the column loads on the footing. The footing should be proportioned in such a way that the resultant of the loads passes through the center of the footing. Afterward, the area of the footing should be computed, taking into account the allowable soil pressure to ensure that it is not exceeded.
Once the area of the footing has been determined, the next step is to calculate the shear forces and bending moments at the salient points. These calculations will then be used to draw the shear force diagram (SFD) and bending moment diagram (BMD). The depth of the footing can be fixed based on the maximum bending moment.
The transverse bending moment must also be calculated, and the transverse section designed for depth and reinforcement. This design should take into account anchorage and shear. The footing must also be checked for longitudinal shear, and the longitudinal steel designed accordingly.
After designing the reinforcement for the longitudinal moment and placing them in the appropriate positions, the development length for longitudinal steel must be checked. To optimize the design and reduce costs, the longitudinal bars may need to be curtailed.
Finally, the reinforcement must be drawn and detailed, and the bar bending schedule prepared. By following these steps, a properly designed footing can be constructed, which will ensure the safety and stability of the structure.
Detailing of Combined Footing
In designing a combined footing with steel reinforcement, the detailing of both the longitudinal and transverse steel should be similar to that of a conventional beam, as specified in SP-34 guidelines. Additionally, the detailing requirements for beams and slabs should be followed as appropriate.
This means that the specifications for steel reinforcement detailing in SP-34 should be followed for the combined footing design. This includes detailing of the longitudinal and transverse steel in the footing, which is critical for ensuring the structural integrity of the foundation.
The SP-34 guidelines provide detailed requirements for the placement and spacing of steel reinforcement in beams and slabs, which should be applied to the design of the combined footing as appropriate. Following these guidelines will ensure that the steel reinforcement is properly distributed and provides sufficient strength to the foundation.
Overall, the proper detailing of steel reinforcement in a combined footing is essential for ensuring the stability and safety of the structure. By adhering to the guidelines outlined in SP-34, designers can ensure that the foundation is built to withstand the required loads and provide long-lasting support for the structure above.