It took me almost 10 years to understand and observe various footings available across different projects. Initially I used shock seeing different footings at different projects. Then I started my study on understanding direct connection of footing with upcoming structure and soil properties. At the end footings are structures which transfers structural load to soil. bigger the structure larger the footings or deeper the footing, however there come many other factors as well, which structural designer considers during design of footings. We use to come in picture whenever there is any change in design of structure and footings are already casted. Initially we use to understand like broadening the footings. hen working for my previous organisation in the infrastructure, I came across reaction frame all the time. First in Ahmedabad when Afcons was launching TBM in 2017-18, I came across fixing reaction frame. So from that time I got many opportunities with various infrastructure companies, even great minds from RVNL and how serious they are about this topic. I hope this article is useful to you and save your lot of time, which will take you to the same platform discussing at the time of TBM launch or during the planning of launch. So intention of writing this article is to make new structural engineers or anyone shifting from one type of projects to another, should have detailed knowledge about the same. I request to write your experienced comments to make everyone aware of upcoming engineering enhancement.
If you're planning a construction project, whether it's a building, a deck, or any other structure, proper footings are essential for providing stability and structural integrity. In this article, we'll cover everything you need to know about footings, including their definition, types, functions, design considerations, retrofitting required Incase of FSI change and construction process.
What are Footings?
Footings, also known as foundation footings or simply foundations, are the structural components of a building or any other structure that support the weight of the structure and transfer it to the ground. They are typically constructed of concrete or masonry and are designed to distribute the weight of the structure evenly across the soil, ensuring that the building or structure remains stable and secure.
Types of Footings
There are several types of footings that are commonly used in construction, depending on the design requirements and soil conditions of the site. The most common types of footings include:
Strip Footings: Strip footings, also known as continuous footings, are long and narrow footings that are used to support load-bearing walls. They are typically used in residential and light commercial buildings and are placed below the frost line to prevent frost heave. Strip footings are designed to distribute the weight of the walls evenly across the soil and are usually reinforced with steel bars (rebar) to provide additional strength.
Pad Footings: Pad footings, also known as isolated footings, are individual footings that are used to support columns, piers, or other concentrated loads. They are typically square or rectangular in shape and are used when the loads from the structure are not evenly distributed. Pad footings are reinforced with rebar to provide strength and stability.
Raft Footings: Raft footings, also known as mat footings or slab footings, are large, thick footings that cover the entire area under a building or structure. They are used when the soil has poor bearing capacity or when the loads from the structure are unevenly distributed. Raft footings are typically used in large commercial or industrial buildings and are designed to distribute the weight of the structure evenly across the soil.
Functions of Footings
Footings serve several important functions in a construction project, including:
Load Transfer: Footings transfer the weight of the structure to the ground, ensuring that the building or structure remains stable and secure.
Stability: Footings provide stability to the structure by distributing the weight of the structure evenly across the soil and preventing settlement or subsidence.
Frost Protection: Footings that are placed below the frost line help prevent frost heave, which can cause damage to the structure.
Resistance to Soil Movement: Footings are designed to resist soil movement, such as settlement, expansion, or contraction, which can affect the stability of the structure.
Structural Support: Footings provide support to load-bearing walls, columns, piers, and other concentrated loads, ensuring that the loads are transferred to the ground safely.
Design Considerations for Footings
The design of footings is a critical step in the construction process and should be carried out by a qualified engineer or design professional. Several important factors need to be considered during the design of footings, including:
Soil Conditions: The type and bearing capacity of the soil at the construction site should be thoroughly evaluated to determine the appropriate type and size of footings required.
Load Requirements: The loads from the structure, including the weight of the building, live loads (such as occupants, furniture, and equipment), and environmental loads (such as wind, snow, and seismic loads), should be carefully calculated to determine the appropriate size and reinforcement of the footings.
Building Codes and Regulations: Footings must comply with local building codes and regulations, which may specify requirements for minimum footing dimensions, reinforcement, and construction practices.
Groundwater and Drainage: The presence of groundwater and the site's drainage conditions should be taken into consideration during footing design. Proper drainage measures, such as installing drainage pipes or gravel layers, may be needed to prevent water accumulation around the footings, which could compromise their stability. Incase we observe higher water table then passive rock anchoring is the solution especially for hard rock conditions to avoid any up thirst of water on footing cause serious effect on building life. Please contact your geotechnical consultant and contact us for detailed solution.
Construction Sequence: The sequence of construction activities, including excavation, formwork, reinforcement placement, and concrete pouring, should be carefully planned to ensure proper installation of the footings.
Construction Process of Footings
The construction process of footings typically involves the following steps:
Excavation: The construction site is excavated to the required depth and width for the footings, following the design specifications. The excavation should be done carefully, taking into consideration the soil conditions, groundwater level, and required dimensions of the footings.
Formwork: Formwork, also known as shuttering, is installed to create the shape and dimensions of the footings. Formwork can be made of wood, steel, or other materials and is designed to hold the concrete in place until it cures and gains sufficient strength.
Reinforcement: Steel reinforcement, such as rebar, is placed within the formwork to provide additional strength and stability to the footings. The reinforcement is positioned according to the design requirements, and proper clearances from the bottom and sides of the formwork are maintained.
Concrete Pouring: Concrete is poured into the formwork, filling the space between the reinforcement bars. The concrete should be of the specified strength and consistency, and it should be placed evenly to prevent voids or honeycombs. Proper consolidation techniques, such as vibration, may be used to ensure the concrete is properly compacted.
Curing: After the concrete is poured, it needs to cure properly to gain sufficient strength. Curing involves keeping the concrete moist and at the appropriate temperature for a specified period of time to allow it to harden and develop its full strength.
Stripping of Formwork: Once the concrete has cured to the required strength, the formwork is removed carefully, without damaging the newly constructed footings.
Backfilling: After the formwork is removed, the space around the footings is backfilled with soil or other approved materials. The backfill should be compacted properly to prevent settling, and proper drainage measures should be taken to ensure water does not accumulate around the footings.
Conclusion
In conclusion, footings are a crucial component of any construction project, providing stability, load transfer, and resistance to soil movement. Proper footing design, construction, and adherence to building codes and regulations are essential to ensure the safety and integrity of the structure. If you're planning a construction project, it's important to work with qualified engineers or design professionals to ensure that your footings are properly designed and constructed for optimal performance.
Shallow and deep footings
Shallow and Deep Footings: Understanding the Differences and Applications
Footings are an essential part of the foundation system of a building or structure. They are designed to distribute the weight of the structure evenly to the underlying soil, providing stability and preventing settlement. Depending on the site conditions and the design requirements, footings can be categorized into two main types: shallow footings and deep footings.
Shallow Footings:
Shallow footings, also known as spread footings or pad footings, are typically used when the soil near the surface is strong enough to support the loads imposed by the structure. They are shallow in depth and have a wider width, spreading the load over a larger area of soil. Shallow footings are commonly used for smaller structures, such as residential buildings, low-rise commercial buildings, and light industrial structures.
The construction process of shallow footings typically involves excavating the soil to the required depth, placing the formwork, reinforcing the footing with steel reinforcement, and pouring concrete to create a solid foundation. Shallow footings are suitable for soils with good bearing capacity and minimal risk of settlement.
Deep Footings:
Deep footings, also known as pile foundations or drilled shafts, are used when the soil near the surface is weak or unstable and unable to support the loads imposed by the structure. Deep footings are installed deeper into the soil to reach a more stable layer that can support the loads. They are commonly used for larger structures, high-rise buildings, heavy industrial structures, bridges, and structures built on soft or expansive soils.
Deep footings can be categorized into two main types: driven piles and bored piles. Driven piles are pushed or hammered into the ground until they reach the required depth, while bored piles are created by drilling a hole into the ground and then placing reinforcement and pouring concrete into the hole. Deep footings can be made of various materials, such as steel, concrete, or timber, depending on the design requirements and site conditions.
The construction process of deep footings typically involves drilling or driving the piles into the ground, placing the reinforcement, and then pouring concrete to create a stable foundation. Deep footings require specialized equipment and expertise in pile installation techniques, and they are generally more expensive than shallow footings due to the additional materials and labor involved.
Key Differences between Shallow and Deep Footings:
Depth: Shallow footings are typically shallow in depth, usually extending only a few feet below the ground surface, while deep footings are installed much deeper into the ground, often reaching tens or even hundreds of feet, depending on the site conditions and design requirements.
Width: Shallow footings are wider in width, spreading the load over a larger area of soil, while deep footings are typically narrower, concentrating the load on a smaller area.
Load Capacity: Deep footings are generally designed to support heavier loads compared to shallow footings, as they are installed in more stable soil layers that can bear higher loads.
Construction Technique: Shallow footings are typically constructed by excavating the soil, placing formwork, reinforcing, and pouring concrete, while deep footings require specialized pile installation techniques, such as driving or drilling, along with additional reinforcement and concrete pouring.
Cost: Deep footings are generally more expensive than shallow footings due to the specialized equipment, labor, and materials required for pile installation.
Conclusion:
In summary, the choice between shallow footings and deep footings depends on the soil conditions, design requirements, and size of the structure. Shallow footings are suitable for stable soils near the surface and smaller structures, while deep footings are necessary for weak or unstable soils and larger structures. Proper design and construction of footings, whether shallow or deep, are crucial to ensure the stability, durability and proper load transfer.
Rectangular and Combined Eccentric Footings: Understanding the Types and Applications
In addition to the distinction between shallow and deep footings, footings can also be further categorized based on their shape and configuration. Rectangular footings and combined eccentric footings are two common types of footings that are used in different scenarios to accommodate specific design requirements and site conditions.
Rectangular Footings:
Rectangular footings, as the name suggests, have a rectangular shape and are typically used when the load from the structure is distributed unevenly, or when the soil conditions require a wider footing to spread the load over a larger area. Rectangular footings are commonly used for column supports in buildings and other structures, where the load is not centered and needs to be distributed over a larger footing area.
The construction process of rectangular footings involves excavating the soil to the required depth, placing the formwork in the shape of a rectangle, reinforcing the footing with steel reinforcement, and pouring concrete to create a solid foundation. The dimensions of the rectangular footing, such as length, width, and depth, are determined based on the design requirements, load calculations, and soil properties.
Combined Eccentric Footings:
Combined eccentric footings, also known as combined footings or strap footings, are used when two or more columns are located close to each other and the loads from these columns need to be combined and transferred to the underlying soil through a single footing. Combined eccentric footings are typically used in situations where the individual footings for each column would overlap, or where the soil conditions require a combined footing to distribute the loads more effectively.
Combined eccentric footings can have various shapes, such as rectangular, trapezoidal, or T-shaped, depending on the specific requirements of the structure and the soil conditions. They are designed to accommodate the eccentricity of the loads from the columns, where the center of the column loads does not coincide with the center of the footing.
The construction process of combined eccentric footings involves excavating the soil to the required depth, placing the formwork in the shape of the combined footing, reinforcing the footing with steel reinforcement, and pouring concrete to create a stable foundation. The dimensions and reinforcement detailing of combined eccentric footings are determined based on the design calculations, load eccentricities, and soil properties.
Key Differences between Rectangular and Combined Eccentric Footings:
Shape: Rectangular footings have a rectangular shape, while combined eccentric footings can have various shapes, such as rectangular, trapezoidal, or T-shaped, depending on the specific design requirements.
Load Distribution: Rectangular footings are used to distribute the loads from a single column, while combined eccentric footings are used to combine and distribute the loads from multiple columns located close to each other.
Load Eccentricity: Rectangular footings are used when the loads from the column are centered on the footing, while combined eccentric footings are used when the loads from the columns are eccentric, i.e., not centered on the footing.
Soil Conditions: Rectangular footings are typically used in situations where the soil conditions allow for a wider footing, while combined eccentric footings are used when the soil conditions require a combined footing to distribute the loads more effectively.
Construction Technique: The construction process for rectangular footings is relatively straightforward, involving a simple rectangular formwork, while combined eccentric footings require more complex formwork to accommodate the combined shape of the footing.
Conclusion:
In conclusion, rectangular footings and combined eccentric footings are two common types of footings used in different scenarios to accommodate specific design requirements and site conditions. Rectangular footings are used for column supports where the load is distributed unevenly, while combined eccentric footings are used to combine and distribute the loads from multiple columns with eccentric loads. The shape, load distribution, load eccentricity, soil conditions, and construction technique are some key differences between rectangular footings and combined eccentric footings.
It is important to consider the design requirements, load calculations, soil properties, and site conditions when selecting the appropriate type of footing for a specific project. Consulting with a qualified structural engineer or geotechnical engineer is recommended to ensure proper design and construction of footings for a safe and stable foundation for any structure.
Footings Retrofitting: Considerations for FSI Changes
Footings retrofitting refers to the process of strengthening or modifying existing footings to accommodate changes in the Floor Space Index (FSI) or other design requirements. FSI is a zoning parameter that determines the permissible built-up area on a given plot of land, and changes in FSI may occur due to changes in local building codes, zoning regulations, or development plans.
When FSI changes, it may result in increased loads on existing footings, as additional floors or structural elements are added to the building. This can potentially exceed the capacity of the existing footings, leading to structural instability, settlement, or other structural issues. Retrofitting footings becomes necessary to ensure the safety and stability of the structure.
Here are some key considerations for footings retrofitting when FSI changes:
Load Calculation: The first step in footings retrofitting is to recalculate the loads on the existing footings based on the updated FSI and any additional structural elements that are added. This involves analyzing the loads from the new floors or structural elements, and comparing them with the capacity of the existing footings to determine if retrofitting is required.
Structural Analysis: A comprehensive structural analysis is essential to assess the capacity and stability of the existing footings in light of the updated loads. This may involve analyzing the soil properties, footing dimensions, reinforcement detailing, and structural design to determine if the footings need to be strengthened or modified.
Retrofitting Techniques: There are several techniques that can be used for footings retrofitting, depending on the specific requirements of the project. These may include underpinning, adding additional reinforcement, increasing the footing size, or modifying the footing shape. The retrofitting technique chosen should be based on the structural analysis and engineering recommendations to ensure the integrity and stability of the footings.
Building Codes and Regulations: Compliance with local building codes and regulations is crucial in footings retrofitting. The updated FSI may have specific requirements for footings design and construction, and it is essential to ensure that the retrofitting work meets these requirements to obtain the necessary permits and approvals.
Construction Process: The retrofitting process should be carefully planned and executed to minimize disruption to the existing structure and ensure the safety of the workers and occupants. Proper construction techniques, materials, and quality control measures should be followed to ensure that the retrofitting work is carried out to the highest standards.
Monitoring and Quality Control: Monitoring and quality control measures should be implemented during and after the retrofitting process to ensure that the footings are strengthened or modified as per the design requirements. This may involve periodic inspections, testing of materials, and verification of construction practices to ensure that the retrofitting work is completed successfully.
Conclusion:
Footings retrofitting is a critical process that needs to be carefully planned and executed when there are changes in the FSI or other design requirements. Proper load calculations, structural analysis, retrofitting techniques, compliance with building codes, and quality control measures are essential to ensure the safety and stability of the structure. Consulting with a qualified structural engineer or geotechnical engineer is recommended to assess the need for footings retrofitting and to ensure proper design and construction practices are followed.
We recommend to get detailed loading details from consultant contact us for solution and get approval from the consultant. This is little time consuming however worth spending time, as we will not fall into using any specific one expensive product. Be proactive being project lead to save overall cost. You can refer our retrofitting blog as well--> click here
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