Structural Foundations Design in Residential Projects: An Overview and Design Considerations

Structural foundation design is essential to any residential construction project, as it provides the base on which the entire structure rests.

In general, the foundation is responsible for transferring the weight and loads from the building to the ground, making it a critical aspect of the overall stability and safety of the structure (Rao, 2010).

The foundation design process involves several steps, beginning with soil investigation to determine the soil properties and site conditions. This information is critical as it helps the engineer determine the type of foundation suitable for the structure and the depth required to support the loads. The engineer considers factors such as soil-bearing capacity, soil type, groundwater conditions, and seismic activity in the area to design a stable and secure foundation.

Different types of foundations are used in residential construction, including shallow and deep foundations.

• Shallow foundations are suitable for structures with low loads and are typically used for small buildings such as residential homes.
• In contrast, deep foundations are used for structures with higher loads and designed to reach deep into the ground to provide the necessary support.

In addition to the type of foundation, other critical design considerations include the size and shape of the foundation and the type of soil beneath it.

It is also crucial to consider the impact of the surrounding environment, including factors such as temperature changes and moisture content, which can affect the foundation's stability over time.

A poorly designed foundation can lead to a range of problems, including foundation settling, cracking, and even structural failure. Therefore, it is essential to accurately analyze and design foundations to ensure their capabilities in withstanding external loads and maintaining structural stability.

This article gives an overview of foundation design in residential projects and highlights some critical design considerations.

What is Structural Foundation?

A foundation is a structural component responsible for supporting and transferring the loads from the building to the ground. Foundation plays a critical role in ensuring the stability and safety of the building. In general, the foundation must be designed to withstand the loads imposed on it by the structure, such as the weight of the building, the occupants, and any additional loads like furniture or snow.

The Role of Foundations in Residential Structures

In residential structures, the foundations serve as the base on which the entire structure rests. They support the entire weight of the building, ensuring it remains stable and safe for occupants while protecting the building against external forces like wind and seismic activity and insulating against moisture, preventing water from entering the building and causing damage.

Additionally, foundations distribute the weight of the building evenly across the soil or rock, preventing localized stress and ensuring the building remains level.

What Does Foundation Design Entail, and Who is Involved in the Process?

Indeed, foundation design is an essential stage of any construction project, and it involves a thorough analysis of the soil and other site-specific factors to determine the appropriate foundation type and design that will provide adequate support for the structure.

The foundation design entails several critical steps, including soil investigation, foundation type selection, design calculations, and construction specifications.

This process starts with a geotechnical investigation to determine the soil characteristics and conditions of the site, such as soil type, bearing capacity, water table level, and potential for soil settlement or liquefaction (Som & Das, 2003).

Based on the site's geotechnical report, the foundation engineer or geotechnical engineer selects the appropriate foundation type.

Once the foundation type is selected, the foundation engineer performs design calculations to determine the required dimensions, reinforcement, and strength of the foundation elements. These calculations consider various factors, such as the weight of the building, the soil-bearing capacity, and the potential for soil settlement or uplift.

The engineer also prepares construction specifications and drawings that provide detailed instructions for the contractor on how to build the foundation and what measures to take while constructing it.

The foundation design process generally involves various professionals and experts, as outlined in Table 1, including structural engineers, geotechnical engineers, architects, and contractors. These professionals work together to ensure that the foundation meets the project's specific needs, including the required size, shape, and load-bearing capacity.

Structural Engineers

Structural engineers are responsible for designing the structural components of the foundation, including the footings, foundation walls, and other load-bearing elements. They work closely with architects to ensure that the foundation is designed to support the weight of the building and any anticipated loads, such as wind and seismic forces. They also ensure that the foundation is structurally sound and meets local building codes and regulations.

Geotechnical Engineers

Geotechnical engineers study the soil and geology of the building site to determine the appropriate foundation design. They perform soil tests, analyze the results, and provide recommendations for foundation types and depths. They also evaluate the site's slope stability and potential for landslides or other natural hazards.

Architects

Architects design the overall layout and aesthetics of the building, including the size and shape of the foundation. They work with structural and geotechnical engineers to ensure that the foundation design supports the architectural vision of the building while meeting safety and structural requirements.

Contractors

Contractors are responsible for constructing the foundation based on the designs provided by the engineers and architects. They prepare the site, excavate the foundation, pour the concrete, and install any necessary reinforcing materials. They must follow the building plans and ensure that the foundation is constructed to the required standards and specifications.

What are Design Considerations for Foundations?

Several design considerations must be taken into account when designing a foundation. Critical considerations include foundation type, depth, soil bearing capacity, soil type, frost protection, foundation materials, and load transfer.

Foundation Types

Foundation types vary depending on different factors, including the type of soil on the building site, the region's climate, the building's weight, and the local building codes and regulations. Indeed, it is important to choose the right foundation type to ensure the stability and safety of the building. The following section describes some of the common foundation types.

Isolated footing (spread footing)

Figure 1: Isolated footing (spread footing)

It is a type of foundation used to support a single column or post. It is typically a concrete pad poured at the base of the column, which spreads the load over a larger area of soil.

Strip foundation (combined footing)

Figure 2: Strip foundation (continuous footing)

It is a type of foundation used to support a load-bearing wall. It is a long, narrow concrete strip that runs along the length of the wall and spreads the load over a larger area of soil. Strip footings are used for buildings with multiple stories.

Raft foundation (mat foundation)

Figure 3: Raft foundation (mat foundation)

It is a type of foundation used to support an entire building. It is a large, flat concrete slab that extends over the entire footprint of the building, distributing the load over a larger area of soil. Raft footings are used for buildings constructed on weak or unstable soil. The main difference between strip and raft foundations is that the strip foundation efficiently utilizes limited space for individual footings, providing a balanced load distribution for closely spaced columns.

It is also important to distinguish the difference between raft foundation and slab on grade (conventional) foundation. Slab on grade foundations are shallow and do not have deep footings or piers. They are essentially a single, thick concrete slab.

Compared to raft foundations, slab on grade foundations generally require less reinforcement since they are not designed to resist differential settlement to the same extent. They are often used in regions with stable soil conditions. They are cost-effective and commonly seen in residential construction.

Pile foundation

Figure 4: Pile foundation

It is a deep foundation used when the soil is too weak to support the weight of the building. It involves driving piles into the soil to transfer the load of the building to deeper, stronger layers of soil.

Foundation Load Transfer

Load transfer is the process of transferring the loads from the building to the foundation. Generally, the foundation must be designed to transfer the loads to the soil safely. The loads imposed on the foundation include dead loads, such as the weight of the building, and live loads, such as the occupants' weight and furniture.

The mechanism of distributing the building load on soil depends on the type of foundation used.

Nevertheless, in all cases, the building load is distributed to the soil through a mechanism known as bearing capacity (it is the ability of the soil to support the weight of the building without excessive settlement or movement). The load from the building is transferred to the footing slab, Figure 1, which is a wide and thick part of the foundation that spreads the load over a large soil area (Das & Sivakugan, 2018). The foundation then distributes the load to the soil beneath it, which supports the weight of the building.

The distribution of load to the soil is based on the principles of soil mechanics. The soil's load-bearing capacity depends on several factors, such as the type and strength of the soil, the depth of the footing, and the width of the footing.

To ensure that the load is distributed evenly, the footing is designed to be wider than the building's walls or columns. The width and depth of the footing are determined by the soil's load-bearing capacity and the building's weight. During construction, the footing is placed on a compacted layer of soil and reinforced with steel bars or mesh to increase its strength.

Figure 5: Load path in buildings (Reference)

Foundation Depth

The depth of the foundation is one of the most critical design considerations. It depends on the type of soil and the loads that will be imposed on the foundation.

For instance, a building located in an area with poor soil conditions will require a deeper foundation to provide adequate support. The foundation's depth also depends on the frost depth in the area, which varies depending on the location. For example, a shallow foundation may be sufficient in shallow frost depths. In contrast, a deeper foundation may be required to prevent frost heave in areas with a deep frost depth.

Soil Bearing Capacity

Soil bearing capacity refers to the maximum load the soil can withstand without failure.

It is essential to determine the soil's bearing capacity to design a foundation that can withstand the loads imposed on it (Das, 2021). Several factors influence the soil's bearing capacity, such as soil type, moisture content, and compaction. A geotechnical engineer can conduct a soil test to determine the soil's bearing capacity and recommend an appropriate foundation design.

Soil Type

The soil type on the construction site plays a crucial role in foundation design. Different types of soil have varying bearing capacities and compaction properties.

Besides, some soil types are more susceptible to swelling or shrinking, which can cause significant damage to the foundation and the structure. For example, expansive soils are known to swell, causing foundation movement and structural damage. Accordingly, understanding the soil's characteristics is essential to design a foundation that can withstand the soil's properties.

Frost Protection

Frost heave can cause significant damage to the foundation and the structure. Hence, designing the foundation with adequate frost protection is essential to prevent the soil from freezing and heaving.

Generally, insulation helps to prevent the soil from freezing and heaving, while footing depth ensures that the foundation is located below the frost line to prevent frost heave. Therefore, selecting adequate insulation and footing depth are critical factors in providing frost protection for the foundation.

Foundation Materials

Several types of foundation materials can be used in residential construction. Among them, reinforced concrete is the most widely used option in residential structures thanks to its high durability and low cost. The choice of material depends on several factors, such as the soil type, load, and structure's design.

Summary

In conclusion, the foundation of a residential structure is a critical aspect of its construction and structural integrity.

The foundation supports and transfers loads from the building to the ground, ensuring stability and safety for its occupants. The design process involves a qualified structural engineer who considers several factors, including soil investigation, load analysis, and design calculations, to determine the foundation's type, size, and shape.

Critical design considerations include foundation depth, soil bearing capacity, soil type, frost protection, foundation materials, and load transfer. A poorly designed foundation can lead to problems like foundation settling, cracking, and even structural failure.

Therefore, it is crucial to work with a qualified engineer to ensure that the foundation is designed to withstand the loads and maintain the structure's stability and safety.

References

• Das, B. M., 2021. Principles of geotechnical engineering. s.l.:Cengage learning.
• Das, B. M. & Sivakugan, N., 2018. Principles of foundation engineering. s.l.:Cengage learning.
• EngineersDaily, 2014. How Loads Flow Through a Building?. [Online]
• Rao, N. K., 2010. Foundation design: theory and practice. [Online].
• Som, M. N. & Das, S. C., 2003. Theory and practice of foundation design. s.l.:PHI Learning Pvt. Ltd.