There are a few key challenges when building structures, particularly where columns are spaced irregularly or are somewhat close to property boundaries, ensuring the load is evenly distributed. In these cases, not one, but rather a Combined Footing can be an appropriate, effective, and stable solution.
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Combined Footing is a commonly used foundation type of a stabilization system of multiple columns or pillars to provide balance to the structure and avoid unequal settlement.
For this post, we will describe what a Combined Footing is, the types of Combined Footing, where they are used, and the advantages of Combined Footing for modern construction projects.
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With our construction chemicals, we assist engineers and builders with the durability, strength, and workability of concrete, so that every footing performs at its optimum.
Combined Footing
A Combined Footing is a type of foundation provided for two or more columns that rest on a unified formwork of concrete. In essence, a Combined Footing acts as a platform to evenly distribute the load generated by two or more columns to the underlying strata and/or soil.
Typically, Combined Footings would be used if:
- Two columns are sufficiently close together, and their respective footings overlap.
- A column is located near a property boundary where the isolated footing cannot extend beyond the limit.
- Columns have varying loads, and a larger base area is required to ensure uniform soil pressure.
In simple terms, a Combined Footing provides stability by ensuring the center of mass of the column loads is located directly over the center of the footing area to minimize any differential settlements and stable structure.
Types of Combined footing
Combined footings are shaped and designed based on the loads from each column, their spacing, and soil conditions. Depending on the required designs, the following are the most common types of Combined Footings in construction.
1. Rectangular Combined Footing
The rectangular Combined Footing is the simplest type and is the type most used in construction. The rectangular Combined Footing is rectangular and supports two columns with either equal or unequal loads.
The design distributes the footing area in such a manner that the center of mass of the combined loads will coincide with the center of the footing.
The rectangular Combined Footing is most useful when the columns are spaced evenly and carry similar loads, or when isolated footings are not suitable due to site boundaries.
2. Strap or Cantilever Combined Footing
This type connects two isolated footings using a strap beam that balances the load between them. The strap beam does not bear directly on the soil but transfers load between the footings, helping them act as a single unit.
It’s particularly useful when columns are spaced far apart or when one footing cannot be centrally placed under a column due to boundary restrictions.
3. Trapezoidal Combined Footing
Unequal loads on two columns necessitate the use of a trapezoidal footing. The trapezoidal shape widens the base area, directly beneath the larger column, thereby providing equal pressures to the soil below. Thus, the trapezoidal footing helps stabilize the columns even if their load-bearing requirements differ from each other.
4. Slab Type Combined Footing
In the case of one or more columns being in a straight vertical row, a slab-type combined footing, which is simply a continuous slab supporting all columns at once, is used. This method may often be favored when the soil is low-bearing because it will spread the loads over a larger area.
All types of Combined Footing serve different functions, depending on the particular project’s specifications, load conditions, and site constraints. Choosing an appropriate type of Combined Footing ensures the safety, cost efficiency, and durability of the structure.
Uses of Combined Footing
Combined Foating is a flexible foundation solution and is easy to use in many types of circumstances on-site. Below are the most common uses:
1. Limited Space situations
In urban areas or congested areas where construction takes place, site space is often limited. In situations when two columns are located near each other, there is then time for individual footings, and the individual footings might overlap.
A Combined Footing solves this issue by providing a shared base for both columns, optimizing available space without compromising structural stability.
2. Load Distribution on Weak Soil
In instances of low soil bearing capacity, distributing the load over a larger area is essential to delay or mitigate settlement or failure of the footing.
Combined Footings are designed to spread the load of multiple columns over an enlarged area, resulting in lower pressure on the soil and improved overall stability of the footing.
3. Construction Near Property Lines
When one of the column footings is located near the property line, creating an isolated footing separate from the other column footings would be a very difficult undertaking.
A Combined Footing enables loads to be efficiently transferred within the permissible boundary of the building when it is close to a neighboring property line.
4. Uneven Load Conditions
In multi-column buildings, column loads can be uneven. Combined Footings provide uniform pressure on the soil by modifying the shape or thickness of the footing. This serves to prevent tilt or differential settlement due to uneven load conditions.
5. Cost-benefit for Multi-Column Footings
Utilizing a combined footing instead of an independent footing for each column can reduce excavation, concrete, and reinforcing material.
Material savings mean labor savings to recruit the labor force while shortening the construction sequence, which provides a cost-benefit solution without sacrificing performance.
Benefits of combined footing:
1. Space Efficient
The use of combined footing provides the best use of space when supporting multiple columns with tighter bounds, especially in urban or high construction density applications.
2. Uniform Load Distribution
The force from each column is shared by the combined footing, so there is a more uniform distribution of load over a larger surface area, minimizing maximum localized shears that may cause uneven settlement or cracking due to differential loading.
3. Cost-effective construction
In using combined footing, there are fewer independent footings; therefore, they will use less material and reduce excavation and forms, making them more economical and efficient.
4. Building Design Flexibility
Combined footings can be easily adjusted to satisfy construction conditions, soil bearing conditions, and load distribution. This opportunity allows for adjustment for residential and commercial buildings.
5. Structural Stability
Combined footings can provide increased bonding and strength when paired with construction chemical additives, such as those from Sakshi Chem Sciences Pvt. Ltd., as well as increased durability in resisting wear.
Conclusion
Combined Footings are a vital component in achieving structural stability and load distribution when space is at a premium or when the soil conditions provide added complexity.
The versatility, economy, and load-carrying capacities of Combined Footings are what make it one of the most reliable types of foundations used in construction today.
At Sakshi Chem Sciences Pvt. Ltd., we take great pride in being able to assist engineers and builders with our innovative chemical construction solutions, which improve the overall strength, durability, and performance of every foundation.
Innovation is in our DNA, and quality is what we promise as we continue to build the chemistry of stronger, safer, and more sustainable structures around the world.
Frequently Asked Questions (FAQs)
1. What is the formula to calculate Combined Footing?
The equation is based on equalizing the load from the different columns within the bearing capacity of the soil. For a rectangular Combined Footing, the basic formula is:
A = P/q,
Where A is the footing area needed, P is the total load on the columns, and q is the safe bearing capacity of the soil.
2. What are the dimensions and size of the Combined Footing?
The size of the Combined Footing is dependent on the load effect, spacing of columns, and soil bearing capacity. The basic premise is to provide uniformly distributed loads, ensuring that the center of gravity of the loads is in line with the center of gravity of the footing area.
3. What is the minimum thickness of the Combined Footing?
The minimum thickness is typically within 200 mm to 300 mm due to soil strength, load from columns, and safety factors against bending and shear. Engineering design standards may outline varying requirements due to sites.
4. What are the types of Combined Footing?
Types of Combined Footings are generally categorized into:
Rectangular Combined Footing: For the same load condition.
Trapezoidal Combined Footing: For different loading conditions.
Strap or Cantilever Footing: For columns spaced apart or near borders.
Slab and Beam Footing: A Combination of slab and beam to support multiple columns.
5. What is the main reinforcement used in Combined Footing?
Combined Footings generally include longitudinal and transverse steel bars to resist bending and shear. Having appropriate reinforcement placement helps improve strength, minimize cracking, and ensure long-term durability of the foundation.
Sagar Telrandhe is a Construction Engineer with a B.Tech in Construction Engineering & Management. Passionate about infrastructure development, project planning, and sustainable construction, he specializes in modern construction techniques, project execution, and quality management, contributing to efficient and innovative building.