NewsThe foundation is one of the most critical components of a two-storey building. It provides stability and support, ensuring the structural integrity of the entire structure. The depth of the foundation plays a crucial role in ensuring that the building can withstand the imposed loads and avoid issues related to soil settlement or instability. In this article, we will explore the various aspects of foundation depth for a two-storey building and provide insights into proper construction practices. From determining the width of the footing to understanding the importance of keyways and reinforcement, we will cover all the essential details to help you make informed decisions when it comes to building a strong and reliable foundation. So, let’s dive in and discover the optimal depth required for a solid foundation for your two-storey building.
Footing Construction for Two-Story Residential Structures
When it comes to the footing construction for two-story residential structures, simplicity and efficiency are key. Since the imposed load on a two-story building is relatively small compared to larger structures, addressing soil-related issues becomes more manageable with a good degree of certainty.
The width of the footing is determined by the thickness of the wall it supports. Typically, the footing is expanded by 100 mm on each side of the wall. However, if the underlying soil’s bearing capacity is lower than expected, it is crucial to increase the width of the footing to tackle potential problems. For a residential building with a maximum of two storeys, a bearing capacity of 96 KPa is considered adequate for the footing.
In addition to the width, keyways also play a significant role in footing construction. Keyways are designed to resist lateral loads at the bottom of the wall. They are typically provided when filling work begins before slab construction. The normal dimensions of a keyway are 2.5 cm in depth and 2.5 mm to 3.8 mm in width. Alternatively, dowels can be used as keyways, with a minimum design of bar No. 13 (No. 4) placed at a spacing of 61 cm in the center. Dowels should extend 15 cm into the footing and 30 cm above the footing.
For bridging penetrations or trenches under the footing, it is essential to provide longitudinal reinforcement. A minimum of two bar No. 13 steel (No. 4) should be delivered, with a length that extends 61 cm on both sides of the penetration. If the trench or penetration span exceeds 91 cm, it is necessary to calculate the reinforcing area required.
Excavation depth is another critical consideration for footing construction. The depth of the footing should exceed the frost line of the project site. If it does not, frost protection measures need to be implemented. If the excavation exceeds the required depth, it should be filled with concrete or designed fill material.
To address any short soft areas in the excavated area, it is advisable to use reinforcement in the footing. Soil boring can be conducted to identify different types of soil, such as sands, clays, and silts. It is recommended to drill at least two borings, with a total area of 150m². The depth of the soil boring should be approximately 1.5m from the bottom of the footing. A thorough soil investigation report should include information about soil-bearing capacity, soil type, and soil layer placement.
During the excavation process, it is important to avoid placing fill at the bottom of the excavation as it is difficult to assess the quality of the fill material and ensure proper compaction. However, if fill material cannot be avoided, using sand or gravel and adequately compressing it can help achieve the desired specific gravity. It is also crucial to avoid standing water, mud, wet or loose soil, and any other items washed into the footing near the excavation’s bottom.
Lastly, in frost-prone areas, it is essential to protect the ground from freezing. This can be done by laying straw, mineral wool, polystyrene sheets, blankets, batt insulation, or polyethylene films. Removing the frozen ground or applying heat can also help eliminate frost penetration. However, if the frozen soil thickness is not more than 5 cm, the heat generated by the concrete itself is usually enough to prevent frost penetration.
By following these recommended practices for footing construction, you can ensure a solid foundation for your two-story residential building, providing stability and longevity for your structure.
Keyways and Their Role in Resisting Lateral Loads
Keyways play a significant role in the construction of footings for two-story residential structures. Their purpose is to resist lateral loads at the bottom of the wall, providing stability and preventing excessive movement or displacement.
When the filling work begins before slab construction, it is necessary to include a keyway. The dimensions of a keyway are typically 2.5 cm in depth and 2.5 mm to 3.8 mm in width. The keyway is incorporated into the footing to create a secure connection between the wall and the foundation, ensuring that the wall remains stable and does not shift or tilt under lateral pressure.
Alternatively, dowels can be used as keyways. Dowels are reinforcing bars, typically bar No. 13 (No. 4) in size, placed at a spacing of 61 cm in the center. These dowels should extend 15 cm into the footing and 30 cm above the footing to provide the necessary strength and resistance against lateral forces.
The utilization of keyways or dowels is crucial because they effectively distribute the lateral loads across the entire length of the footing. This helps prevent the concentration of forces at specific points, which could lead to instability or failure of the foundation.
By incorporating keyways or dowels into the footing construction, the overall structural integrity of the two-story building is greatly enhanced. Lateral forces, such as those caused by wind or soil pressure, are effectively resisted, minimizing the potential for any detrimental effects on the building’s stability. Proper implementation of keyways ensures that the foundation remains strong, ensuring the long-term durability and safety of the structure.
Penetrations and Longitudinal Reinforcement
When constructing the footing for a two-storey residential building, it is important to address penetrations and provide proper reinforcement to ensure structural integrity and stability.
Penetrations refer to any openings or trenches that are made under the footing for various purposes, such as utility lines or plumbing. These penetrations can weaken the footing if not properly accounted for. To mitigate any potential issues, it is crucial to provide longitudinal reinforcement around these penetrations.
For bridging penetrations or trenches under the footing, it is recommended to deliver a minimum of two bar No. 13 steel (No. 4) reinforcing bars. These bars should extend 61 cm on both sides of the penetration. By incorporating longitudinal reinforcement, the load-bearing capacity of the footing is enhanced, minimizing the risk of settlement or failure around the penetrations.
Additionally, if the trench or penetration span exceeds 91 cm, it is essential to calculate the required reinforcing area to ensure sufficient strength and stability.
The inclusion of longitudinal reinforcement around penetrations helps distribute the load and minimize any concentration of stress or pressure in a particular area. This reinforcement acts as a safeguard against potential weaknesses and ensures that the footing can withstand the imposed loads and maintain its structural integrity over time.
By properly reinforcing the footing around penetrations, you can ensure the stability and strength of the foundation for your two-storey residential building, minimizing the risk of any structural issues and ensuring the long-term durability of the structure.
Excavation Depth and Frost Protection
When it comes to excavation depth for the foundation of a two-storey building, careful consideration is necessary to ensure stability and protect against potential frost-related issues.
The depth of the footing excavation should exceed the frost line of the project site. If the excavation does not reach below the frost line, it is crucial to implement frost protection measures to prevent any adverse effects caused by freezing temperatures.
In areas where frost is a concern, it is recommended to fill the excavation with concrete or designed fill material if it exceeds the required depth. This additional depth helps prevent the foundation from being affected by frost heave, which can lead to structural damage.
It is also important to use reinforcement in the footing to bridge any short soft areas in the excavated area. By reinforcing these weaker sections, the foundation’s overall strength and stability are enhanced, reducing the risk of settlement or failure.
To identify the various soil types present, such as sands, clays, and silts, conducting soil borings is a recommended practice. Drilling at least two borings, with a total area of 150m², provides valuable information about the soil’s characteristics, including its bearing capacity and layer placement.
When performing the soil borings, it is advisable to drill to a depth of approximately 1.5m from the bottom of the footing. This allows for a thorough evaluation of the soil conditions and helps in designing an appropriate foundation based on the specific properties of the soil.
To ensure the best possible foundation, it is important to avoid placing fill material at the bottom of the footing excavation. This is because it becomes challenging to assess the quality of the fill material and achieve adequate compaction using standard equipment at typical home building sites.
However, in situations where fill material cannot be avoided, it is recommended to use sand or gravel and compress it sufficiently to achieve the desired specific gravity. This helps maintain the stability and integrity of the foundation.
Furthermore, it is crucial to avoid the presence of standing water, mud, wet or loose soil, and other foreign items near the excavation’s bottom. These elements can compromise the strength and stability of the foundation, leading to potential issues down the line.
In areas where frost is anticipated, it becomes essential to take measures to protect the ground. This can be done by using materials such as straw, mineral wool, polystyrene sheets, blankets, batt insulation, or polyethylene films. These materials help insulate the ground and prevent frost penetration.
If frost is encountered, it is necessary to either remove the frozen ground or apply heat to eliminate the frost penetration. In cases where the frozen soil thickness does not exceed 5 cm, the heat generated by the concrete during placement is often sufficient to prevent frost penetration.
By carefully considering excavation depth and implementing appropriate frost protection measures, you can ensure the stability and longevity of the foundation for your two-storey building, even in frost-prone areas. These measures provide safeguards against frost-related issues and help maintain the structural integrity of the entire building.
Different Types of Forms for Footing Construction
There are various types of forms that can be used for footing construction in two-storey buildings. The choice of form depends on factors such as material availability, construction method, and the specific requirements of the project. Here are some common types of forms used in footing construction:
1. Wood Forms: Wood is a widely used material for forming footings. It is readily available, cost-effective, and can be easily fabricated on-site. Wood forms are typically constructed using plywood sheets, supported by wooden stakes or beams. They offer flexibility in creating different shapes and sizes of footings.
2. Aluminum Forms: Aluminum forms are lightweight and durable, making them an excellent choice for footing construction. They are typically prefabricated and can be easily assembled and disassembled, saving time and effort during construction. Aluminum forms provide a smooth finish to the concrete, ensuring quality and aesthetics.
3. Steel Forms: Steel forms offer strength and stability for footing construction. They are commonly used in projects that require high load-bearing capacity or complex footing shapes. Steel forms are durable and can withstand multiple uses, making them a cost-effective option for larger construction projects.
4. Metal Fabrics: Metal fabrics, such as wire mesh or steel grids, can also be used as forms for footing construction. These materials are flexible and easy to shape, allowing for customization and versatility in creating different footing designs. Metal fabrics provide reinforcement for the concrete, enhancing its strength and stability.
5. Synthetic Fabrics: Synthetic fabrics, like geotextiles or polypropylene sheets, can be utilized as forms for footing construction. These materials are lightweight, easy to handle, and resistant to water and chemicals. Synthetic fabrics offer excellent formwork for footings, ensuring proper shape and support during concrete placement.
6. Plastic Forms: Plastic forms are an economical and lightweight option for small-scale footing construction. They are typically made of high-density polyethylene (HDPE) or polyvinyl chloride (PVC). Plastic forms are easy to set up and remove, reducing labor and material costs. They are ideal for simple footing shapes and projects with limited budgets.
In some cases, the footing trench itself can also serve as a form. This method eliminates the need for additional formwork, saving time and expenses. However, it is crucial to ensure that the excavation is done accurately and that the trench provides sufficient support for the concrete placement.
Choosing the right form for footing construction depends on the specific requirements of the project, budget constraints, and the expertise of the construction team. By selecting the appropriate form type, you can ensure efficient and effective footing construction for your two-storey building, resulting in a strong and durable foundation.
Reinforcement and Concrete Placement
Reinforcement and proper concrete placement are crucial elements in ensuring a strong and durable footing for a two-storey building. Reinforcement helps strengthen the concrete and distribute loads evenly, while correct concrete placement ensures its integrity and longevity. Here are key considerations for reinforcement and concrete placement:
Reinforcement:
Reinforcement in footings helps enhance their load-bearing capacity and structural performance. Longitudinal reinforcing bars are sometimes used to bridge soft spots, minor trenches, or narrow excavations. This reinforcement strengthens the footing, especially in situations where soil-bearing capacity is poor or wall loads are excessive.
Grade 280 or 420 deformed steel bars are commonly used for reinforcement. The recommended bar sizes are typically No. 13 (No. 4) and No. 16 (No. 5). Additionally, it is essential to provide a 7.6 cm concrete cover on the bottom and sides of the reinforcement. Adequate spacing between bars, typically at least 30 diameters apart, ensures effective reinforcement distribution.
Concrete Placement:
Proper concrete placement is vital to ensure the strength and durability of the footing. There are various methods and equipment available for concrete placement based on the project’s requirements and accessibility. Here are some key points to consider:
– Slump Value: The slump value refers to the consistency of the concrete mix. For footing construction, the concrete should have a slump value of no more than 15 cm. However, if a high-range water reducing admixture is used, a slump value of 20 cm is acceptable.
– Compressive Strength: The minimum compressive strength of the concrete at 28 days should be 17 MPa. If the soil conditions or design specifications require additional strength, higher-strength mixtures can be used. This is particularly important when transverse reinforcement is required.
– Concrete Placement Methods: Concrete can be placed using various traditional means, such as chutes, wheelbarrows, cranes, pumps, or conveyors, depending on the accessibility and scale of the project. The chosen method should ensure accurate placement and even distribution of the concrete throughout the footing.
– Quality Control: Throughout the concrete placement process, quality control measures should be in place to ensure consistent and proper concrete placement. These measures may include monitoring the mix proportion, curing conditions, and adherence to specified standards.
Proper reinforcement and careful concrete placement significantly contribute to the overall strength and stability of the footing. By ensuring the correct positioning of reinforcement bars and employing appropriate concrete placement techniques, you can create a robust and reliable foundation for your two-storey building, capable of withstanding the imposed loads and providing long-lasting structural integrity.
Continuous Footings and Pad Footings
When it comes to two-storey residential buildings, continuous footings and pad footings are two common types of foundation structures used for load distribution and support. Let’s delve into each type:
1. Continuous Footings:
Continuous footings, also known as strip footings, extend along the length of the wall, including a short distance beyond the wall edge. These footings distribute the load of the wall evenly along its length, providing stability and preventing differential settlement. Typically, the breadth of continuous footings extends beyond the width of the foundation wall on both sides.
To ensure proper strength and stability, transverse reinforcement may be required when continuous footings extend beyond the border of the wall, particularly if they exceed the footing thickness.
2. Pad Footings:
Pad footings, also referred to as spread footings, are used to support concentrated loads, such as those exerted by columns or points of heavy structural elements. They have a wider base that spreads the load over a larger area and transmits it to the ground below. Pad footings are often used in cases where a beam stands on a wall and provides increased concentrated loads. In such situations, a pad footing may be monolithically cast with a continuous footing, especially when the wall height is approximately 1.2 meters.
The width and depth of pad footings are determined by the load they need to bear and the soil-bearing capacity. They are designed to effectively transfer the load and prevent excessive settlement or displacement.
Both continuous footings and pad footings play essential roles in ensuring the stability and structural integrity of a two-storey building. They distribute the loads efficiently and help prevent uneven settlement, which can lead to structural issues. The appropriate choice between continuous footings and pad footings depends on the building design, load requirements, and soil conditions.
By employing the proper foundation type, whether continuous footings or pad footings, you can ensure the reliable support and stability necessary for your two-storey residential building, ultimately contributing to its overall structural strength.
Thickened Slab and Shovel Footing
In addition to continuous footings and pad footings, two other types of footing structures commonly used in two-storey residential buildings are thickened slabs and shovel footings. Let’s explore each of them:
1. Thickened Slab:
A thickened slab, also known as a shovel footing, is a variation of the conventional slab-on-grade foundation. It involves creating a thicker section along one border or in the center of the floor slab to provide additional support and load-bearing capacity. Thickened slabs are typically used to support a bearing wall within the interior of a structure or as an alternative to supporting columns.
The thickness of the thickened slab is designed based on the load requirements and soil-bearing capability. It provides increased rigidity and strength to accommodate the concentrated loads from walls or structural elements.
2. Shovel Footings:
Shovel footings are another option for supporting walls in two-storey residential buildings. They are shallow footings that extend below the frost line, providing the necessary support while minimizing excavation depth and construction costs. Shovel footings are commonly used in areas with mild climates or when soil conditions allow for shallow foundations.
The width and depth of shovel footings are determined by the structural loads and the bearing capability of the soil. They are designed to distribute the loads evenly and prevent excessive settlement.
Both thickened slabs and shovel footings provide efficient and cost-effective solutions for supporting walls in two-storey residential buildings. They offer alternatives to traditional continuous footings or pad footings, depending on specific design requirements and site conditions.
By implementing the appropriate footing type, whether a thickened slab or a shovel footing, you can ensure adequate support and stability for the walls of your two-storey building. These footing structures contribute to the overall strength and durability of the foundation system, ultimately ensuring the structural integrity of the entire structure.
Curing and Protection of the Foundation
Curing and protection of the foundation are essential steps in ensuring the durability and longevity of the two-storey building’s structure. Proper curing allows the concrete to reach its optimal strength, while protection safeguards the foundation from potential damage. Let’s explore these aspects in more detail:
Curing:
Curing is the process of maintaining favorable conditions for the concrete to hydrate and gain strength. Adequate curing is crucial for the long-term performance of the foundation. Here are some key considerations:
1. Avoiding Rapid Drying: It is essential to prevent rapid drying of the concrete, as it can lead to shrinkage cracks and reduced strength. Methods such as covering the concrete with moisture-retaining materials (e.g., polyethylene sheets) or applying a curing compound can help maintain an optimum moisture environment.
2. Length of Curing Duration: The curing duration depends on various factors, including the type of concrete mix, weather conditions, and project specifications. In general, curing should continue for a minimum period of seven days to ensure sufficient strength gain. However, specific project requirements may necessitate longer curing durations, as per the recommendations of the structural engineer.
3. Moisture Management: During the curing process, it is crucial to manage moisture levels by periodically dampening the concrete surface or using moisture-retaining methods. This helps prevent premature drying and ensures the proper development of concrete strength.
Protection:
Protecting the foundation after curing is vital to safeguard it against potential damage or deterioration. Here are some important considerations for protecting the foundation:
1. Moisture Barrier: To mitigate moisture penetration from the ground or exterior sources, a moisture barrier or waterproofing membrane should be applied to the exterior surface of the foundation. This helps prevent water infiltration and protects against moisture-related issues, such as mold growth or corrosion.
2. Drainage System: A properly designed and installed drainage system is crucial for diverting water away from the foundation. It relieves hydrostatic pressure, reduces the potential of water accumulation, and helps prevent damage due to water infiltration. The drainage system typically consists of a slotted PVC or clay drain pipe placed alongside the foundation, covered with gravel and filter paper to facilitate water flow.
3. Insulation: In colder climates, insulation can help protect the foundation from frost penetration and potential damage. Insulating the foundation with materials such as rigid foam insulation or mineral wool can reduce heat loss and minimize the risk of freezing and subsequent frost-related issues.
By effectively curing the foundation and implementing appropriate protection measures, you can ensure the long-term durability and stability of the two-storey building. Proper curing allows the concrete to achieve its maximum strength, while protection measures safeguard the foundation from potential damage caused by moisture, frost, or other external factors.
Importance of Drainage System in Relieving Lateral Pressure
The drainage system plays a critical role in relieving lateral pressure exerted on the foundation of a two-storey building. It is crucial in minimizing the potential risks associated with water accumulation and reducing the likelihood of damage caused by hydrostatic pressure.
By effectively managing the water around the foundation, the drainage system helps to:
1. Prevent Soil Saturation: Excessive water accumulation around the foundation can lead to saturated soil conditions. Saturated soil exerts increased lateral pressure on the foundation walls, which can result in structural instability or even failure. A well-designed drainage system helps divert water away from the foundation, reducing the likelihood of soil saturation.
2. Minimize Hydrostatic Pressure: Hydrostatic pressure occurs when water accumulates in the soil surrounding the foundation and exerts pressure against the walls. This can lead to cracks, bowing, or other structural problems. The drainage system helps alleviate hydrostatic pressure by redirecting water away from the foundation, thus reducing the potential for damage.
3. Prevent Water Penetration: Properly installed drainage systems assist in preventing water from penetrating through cracks or joints in the foundation walls. By effectively channeling water away, the drainage system minimizes the risk of moisture infiltration into the interior of the building, preventing dampness, mold growth, and potential structural deterioration.
4. Maintain Soil Stability: Excessive water accumulation near the foundation can destabilize the soil, leading to soil erosion or shifting of soil layers. A well-functioning drainage system safeguards the soil stability by effectively managing water runoff and reducing the potential for soil movement or erosion.
To achieve effective drainage and mitigate lateral pressure, the drainage system typically includes a slotted PVC or clay drain pipe placed alongside the foundation. This pipe collects water and directs it away from the building. Proper grading of the soil, efficient gutter and downspout systems, and strategic placement of French drains or surface drains are also components of an effective drainage system.
It is important to note that the necessity of a drainage system may vary depending on the site conditions. Areas with naturally well-drained soil may require less extensive drainage systems than areas with heavy clay or poor drainage characteristics.
By implementing a well-designed and properly functioning drainage system, you can significantly reduce the lateral pressure on the foundation of a two-storey building. This promotes structural integrity, prevents water-related issues, and ensures the long-term stability and durability of the entire structure.
How deep should a foundation be for a two-storey house?
When building a two-storey house with strip foundations, the depth of the concrete foundation should be at least 200mm. This is necessary because the load-bearing capacity of a two-storey structure is significantly greater than that of a single-storey extension due to the additional storey. Therefore, a deeper footing depth is required to ensure proper support and stability for the two-storey house.
How deep should a trench fill Foundation be?
For foundation depth in the UK, the rule of thumb states that pad foundation or strip, trench fill foundations should have a minimum depth. This depth varies based on soil type, with a minimum depth of 750 mm (0.75 m) in low plasticity clays, 900 mm (0.9 m) in medium clay, and a British Standard8004 recommendation of 1 m for foundations in high-risk areas. So, How deep should a trench fill foundation be? It depends on the soil type and the specific requirements of the project.
