Constructing footings for residential buildings with a maximum of two stories is a relatively straightforward process due to the relatively small amount of weight that needs to be supported. As a result, issues related to the underlying soil can typically be addressed with a high level of confidence.
The ACI 332.1R-06 building construction code provides comprehensive instructions for designing and building footings that are appropriate for a two-story residential structure. These guidelines cover the necessary dimensions and construction techniques required to ensure that the footings can adequately support the weight of the building. By following these guidelines, builders can have greater confidence in the stability and longevity of the structure.
Footing Width
The width of a building’s footing is determined by the thickness of the wall. Typically, the footing is widened by an additional 100 millimeters on each side. However, if the soil’s ability to bear weight is lower than anticipated, it may be necessary to increase the footing’s width in order to address this issue.
In residential construction projects of up to two storeys, a bearing capacity of 96 KPa is generally deemed satisfactory for the footing. In the event that the soil’s bearing capacity is lower than this, adjustments to the width of the footing may need to be made in order to ensure that the building is adequately supported.
Footing Thickness
When it comes to footing thickness, it’s generally recommended to aim for around 200 mm or 250 mm. These measurements have been found to be adequate in most situations. It’s important to consider the specific context of the project and any unique factors that may affect the necessary thickness, but in general, 200 mm or 250 mm should be sufficient. By following this guideline, builders and contractors can ensure that their structures have a solid foundation that will withstand the test of time.
Keyways
The keyway plays an important role in resisting lateral loads at the bottom of a wall. Therefore, it is necessary to provide a keyway if filling work begins before the construction of the slab. The dimensions of a typical keyway are 2.5 centimeters in depth and 2.5 to 3.8 millimeters in width.
Another option to consider as a keyway is the use of a dowel. For a minimum dowel design, it is recommended to place bar No. 13 (No. 4) at a spacing of 61 centimeters in the center. The dowels should be extended 15 centimeters into the footing and 30 centimeters above the footing. This approach can also serve as an effective alternative to the traditional keyway in reinforcing the wall’s foundation.
Penetrations
When it comes to bridging penetrations or trenches beneath the footing, it is important to ensure that proper reinforcement is in place. This can be achieved by providing longitudinal reinforcement that extends 61cm on either side of the penetration. A minimum of two bar No. 13 (No. 4) steel should be used in this case.
However, if the span of the trench or penetration is more than 91cm, it is necessary to calculate the required reinforcement area. This will help to ensure that the structure is properly reinforced and can withstand the expected load. By calculating the required reinforcement area, the appropriate amount of steel can be determined and installed in order to strengthen the structure and prevent any potential failures or collapses.
Excavation
When constructing a building, it is important to ensure that the depth of the footing is adequate to exceed the frost line of the project site. If the excavation depth does not meet this requirement, then it is necessary to consider frost protection measures. In cases where the excavation depth exceeds the designated depth, the area should be filled with concrete or engineered fill material to prevent any issues.
To ensure the strength and stability of the footing, it is recommended to use reinforcement in the footing to bridge any short soft areas in the excavated area. Soil boring can be conducted to determine the different types of soil present, such as sands, clay, and silts.
It is advisable to drill a minimum of two borings for a maximum area of 150m2, with the depth of boring being around 1.5m from the bottom of the footing. The soil investigation report should contain important information such as soil bearing capacity, soil type, and location of the soil layer.
Fill placement at the bottom of footing excavation should be prevented as it is difficult to check the quality of fill material and achieve proper compaction with available testing and compaction equipment functional at majority of the residential construction sites. In cases where the provision of fill material cannot be prevented, it is recommended to use sand or gravel and compact it properly to obtain the required specific gravity.
The bottom of the excavation should be free from standing water, mud, saturated or loose soil, and materials that are washed into the footing to prevent any damage or issues.
Finally, in cases where frost is expected, it is necessary to take measures to prevent the ground from freezing. This can be achieved by using straw, mineral wool, polystyrene sheets, blanket or batt insulation, or polyethylene films. If the frozen soil thickness does not exceed 5 cm, then the heat from concrete is enough to remove frost penetration. However, if the frozen ground exceeds this thickness, then it will be necessary to remove or heat the frozen soil to remove the frost.
Form Types
When constructing a footing, there are several materials that can be used including wood, aluminum, steel, metal fabrics, synthetic fabrics, and plastic forms. In some cases, the trench that is excavated for the footing can also serve as a form.
To begin the construction process, the trench must be excavated in the ground to the appropriate depth and shape to ensure that it can hold the concrete during placement. Once the trench is prepared, the concrete can be placed and compacted to reach the desired level.
Properly constructing the footing is essential to ensure the stability and strength of any structure. Choosing the appropriate materials and taking the time to excavate and shape the trench correctly can help ensure a successful construction project.
Reinforcement
When dealing with weak soil or narrow excavations, longitudinal reinforcing can be utilized to bridge soft spots or minor trenches and improve overall strength. To address low soil-bearing capacity, high wall loads, or when specified by the designer, transverse reinforcement should be employed.
For longitudinal reinforcing, it is recommended to use deformed steel bars of grade 280 or 420, with No. 13 (No. 4) and No. 16 (No. 5) bar sizes being the recommended options. In addition, a minimum concrete cover of 7.6 cm should be provided to both the bottom and sides. To ensure proper reinforcement, the bars should be overlapped by at least 30 times their diameter (30 db).
Concrete Placement
There are various conventional methods for placing concrete for footings, such as using a direct chute, wheelbarrows, crane, pump, or conveyor. However, it is important to note that the slump value of foundation concrete should not exceed 15 cm to ensure the integrity of the structure. In the case of concrete containing high range water reducing admixture, a slightly higher slump value of 20 cm is acceptable.
It is crucial to maintain a minimum compressive strength of 17 MPa for concrete used in footings, which should be achieved by 28 days after pouring. In cases where poor soil conditions are present and transverse reinforcing is required, higher-strength mixtures may be necessary. By using higher-strength concrete, the structure’s stability and longevity can be improved in challenging soil conditions.
Footing Types
Two types of foundation that are commonly used for residential builds with a maximum of two storeys are continuous footing and pad footing. Continuous footing and pad footing are both designed to distribute the weight of a building evenly across the soil. Continuous footing is a long, horizontal strip of concrete that runs along the perimeter of the building’s foundation. It provides support for the load-bearing walls of the building. On the other hand, a pad footing is a single, isolated block of concrete that supports a single point load, such as a column or pier. Both types of footings are essential in ensuring the stability and structural integrity of a building, and the choice between them depends on the specific requirements of the project.
Continuous Footings
Strip footings, also known as continuous footings, are a type of foundation that extends along the length of a wall and beyond its edges by a short distance. These footings have a width that extends beyond both sides of the foundation wall.
However, when continuous footings extend beyond the edge of the wall by a dimension that is greater than the footing thickness, additional measures may be necessary. This is where transverse reinforcement comes into play. Transverse reinforcement refers to the use of reinforcing bars or other materials that are placed perpendicular to the longitudinal axis of the footing.
The purpose of transverse reinforcement is to increase the stability and load-carrying capacity of the footing. This is particularly important when the footing is subjected to lateral loads or other external forces that could cause it to fail. By providing additional reinforcement, the footing is better able to resist these forces and maintain its structural integrity.
Overall, while continuous footings are a common type of foundation used in construction, it is important to consider the need for transverse reinforcement when designing and constructing footings that extend beyond the edge of the wall.
Pad Footing
A pad footing or spread footing is used to transfer concentrated loads, such as those from a column, to the soil. In some cases, a pad footing can be combined with a continuous footing in a monolithic cast. This is commonly done in areas where a beam rests on a wall, creating a higher concentrated load, particularly when the wall’s height is 1.2 meters. The size of the pad footing, including its width and depth, is determined based on the load that it will transmit through it and the soil-bearing capacity.
Thickened Slab
A type of footing known as the thickened slab or shovel footing is created in a single pour with the floor slab. This type of footing features a thickened section at one edge or sometimes in the middle of the floor slab. The primary function of the shovel footing is to provide support for a bearing wall within the building, and it can also serve as an option for supporting columns. By utilizing this type of footing, construction professionals can provide necessary structural support while minimizing the number of columns required in the interior of the building.
Curing and Protection
To ensure that concrete reaches a compressive strength of 3.5 MPa, it is crucial to protect it from freezing during the initial stages of its formation. Adequate protection can be achieved by covering the concrete with a layer of polyethylene or another type of moisture retarder that can prevent the formation of ice crystals. This protective layer can also shield the concrete from harsh environmental conditions such as wind and sun, which can lead to excessive drying and cracking of the concrete surface. By using a moisture-retardant cover, the concrete can retain its moisture content, which is essential for the proper hydration process to take place. Proper hydration is crucial for the concrete to reach its desired strength and durability. Therefore, it is imperative to provide the necessary protection to ensure the concrete’s longevity and structural integrity.
Footing Drainage
The function of a drainage system is to alleviate any lateral pressure on the footing caused by the buildup of water in the soil. It also serves to minimize the likelihood of water seepage through the intersection of cracks or between the footing and wall. As a result, it is recommended to install drains at all footings bordering interior living or storage areas, unless the soil is naturally well-drained. The drain itself is typically made of slotted PVC or clay pipe, and may even be integrated with the footing. When placing the drainage system, it should be positioned beneath the top of the interior slab, and covered with a layer of gravel and filter paper. Finally, the drained water can be directed to a sump pump or storm sewer for proper disposal.
FAQs
How deep is the foundation of a two-storey building?
The given context suggests that a footing thickness of either 200 mm or 250 mm is typically enough. This thickness is likely referring to the concrete foundation that is laid beneath a structure to support its weight and distribute it evenly across the ground. The footing must be able to withstand the load of the structure without sinking or shifting, so it is essential to get the thickness right. A thickness of 200 mm or 250 mm is considered adequate in most cases, but the specific requirements may vary depending on the size and weight of the structure being supported, as well as the properties of the soil or rock beneath it. Nonetheless, it is generally recommended to consult with a qualified engineer or building professional to determine the appropriate footing thickness for a particular project.
What should be the width of strip footing?
The width of a footing is determined by the thickness of the wall it supports. Typically, a footing is extended by 100 mm on either side. However, if the soil’s capacity to bear weight is lower than anticipated, increasing the width of the footing can help address this issue.
For residential buildings with a maximum of two storeys, a bearing capacity of 96 KPa is considered sufficient for the footing. If the soil’s capacity to bear weight is lower than this, steps should be taken to increase the width of the footing and ensure it can support the structure adequately. By doing so, the building’s stability and safety can be ensured.
What is the purpose of keyway in footing of residential buildings?
A keyway is an important component that serves to resist lateral loads at the bottom of a wall. It is crucial to provide a keyway if filling work commences prior to the construction of the slab. The keyway’s typical dimensions are 2.5 cm in depth and 2.5 mm to 3.8 mm in width. These dimensions ensure that the keyway is able to perform its function effectively. Therefore, it is essential to pay close attention to the keyway’s design and installation to ensure that it can withstand the forces it will encounter during construction.
What are the common types of footings considered for residential buildings with a maximum height of two storeys ?
Strip footing, also known as continuous footing, is a type of foundation commonly used in building construction. It is a long, continuous strip of reinforced concrete that runs along the length of the wall, distributing the weight of the building across a wider area of soil. This helps to prevent settling or shifting of the structure over time.
Pad footing, on the other hand, is a type of isolated footing that is used to support individual columns or pillars. It consists of a single block of reinforced concrete that is placed under each column, transferring the load of the column to the soil below.
Thickened slab is a type of foundation that involves a slab of concrete that is thicker at the edges than in the middle. It is commonly used to support walls and other vertical loads, providing a wider base to distribute the weight of the structure across a larger area of soil. This type of foundation is typically used in situations where the soil is relatively stable and can support the weight of the building without the need for additional reinforcement.
What is the purpose of footing drainage?
A drainage system serves two main purposes for a building’s footing. First, it alleviates any lateral pressure that may arise from water buildup in the surrounding soil. Second, it reduces the likelihood of water infiltrating the footing through cracks or where the footing intersects with the walls. By effectively managing the flow of water around the foundation, a drainage system can help ensure the stability and longevity of the building.