Understanding Strip Foundations

Strip foundations, often employed in low to medium-rise residential buildings, are one of the many types of shallow foundations used in construction...
Strip Foundations

In the world of construction, the selection of the right type of foundation is crucial for the stability and longevity of any building. Foundations act as the intermediary between the building and the ground below, ensuring that the structure is adequately supported and that loads are evenly distributed.

There are various types of foundations used in construction, each with its own unique set of advantages and disadvantages. These types include, but are not limited to, strip foundations, pad foundations, raft foundations, and pile foundations.

Among these, strip foundations, also referred to as strip footings, hold a significant place due to their use in many low to medium-rise residential buildings. They are a type of shallow foundation that provides a continuous line of support for a linear structure, such as a wall.

The principle behind strip foundations is to distribute the weight of the building over a large surface area, reducing the load on any particular point of the ground.

In this article, we delve deep into strip foundations, exploring their characteristics, their application, benefits, limitations, examine the key steps involved in constructing strip foundations, and the vital role of site evaluation and soil testing in the process.

Strip foundations: what are they?

Strip foundations, often employed in low to medium-rise residential buildings, are one of the many types of shallow foundations used in construction. This type of foundation is often selected for its simplicity, cost-effectiveness, and suitability for relatively stable soil conditions with good load-bearing capacity. But what exactly are strip foundations, and how do they work?

As the name suggests, strip foundations form a continuous strip that supports a linear structure, such as a load-bearing wall or closely-spaced rows of columns. Constructed from concrete, strip foundations are placed beneath the wall or column alignments, essentially acting as a continuous base on which these structural elements rest.

The strip foundations’ width and depth are proportioned based on the nature of the load they support and the underlying soil’s properties. The aim is to ensure that the load from the structure is sufficiently spread across a wide enough area, preventing excessive stress on any single point of the soil. This principle helps mitigate risks associated with uneven settlement, contributing to the overall stability of the built structure.

An important feature of strip foundations is their adaptability. In relatively uniform ground conditions, strip foundations can be laid flat, providing an even base for the structures above. However, in sites where the ground level varies, strip foundations can be stepped or even sloped to follow the terrain while still providing an effective distribution of load.

In essence, strip foundations are a versatile, efficient foundation solution in the right circumstances. In the following sections, we will delve deeper into the factors that determine the suitability of strip foundations, explore their construction process, and discuss their advantages and potential limitations.

Suitability of strip foundations

The decision to use strip foundations in a construction project is largely determined by two key factors: the nature of the building itself and the condition of the underlying soil. Understanding these factors is essential to assess the appropriateness of strip foundations.

Nature of the Building: Strip foundations are commonly used in structures that have linear load-bearing elements, such as walls or closely spaced rows of columns. The load distribution of these structural elements aligns well with the continuous supporting capacity of strip foundations. Buildings with light to moderate loads, such as low-rise or medium-rise residential buildings, are typical candidates for strip foundations.

Soil Condition: The type, consistency, and load-bearing capacity of the underlying soil significantly influence the suitability of strip foundations. They are best suited for stable ground conditions with good load-bearing capacity. Subsoils such as gravel, sand, or solid clay can provide suitable conditions for strip foundations, provided they are of good bearing quality and relatively free of organic materials, water seepage, or other unstable elements.

In situations where the soil consistency is not uniform across the building footprint, careful design considerations are required. In areas where the load-bearing capacity of the ground is reduced, wider strip foundations or even a different type of foundation might be necessary.

It’s also important to consider other site-specific factors, such as the water table level, slope of the site, and potential for frost heave or soil shrinkage, all of which can impact the performance of the strip foundations.

Finally, it’s worth noting that while strip foundations are suitable for many situations, they are not universally applicable. Their use should be based on a thorough geotechnical survey and a professional assessment of ground conditions. In the next section, we’ll explore the process of constructing strip foundations, shedding light on the necessary steps and considerations involved in their installation.

Strip foundation construction

The construction of strip foundations involves a series of carefully executed steps. The process begins with a thorough understanding of the building design and the ground conditions, followed by meticulous planning and execution. Here’s a step-by-step overview of the process:

1. Site Survey and Marking: The first step is to identify and mark out the location of all load-bearing walls or columns. This can be done using the building’s plans and a string line or laser level for accuracy.

2. Excavation: Once the site is marked, the next step is to excavate trenches along the marked lines. The width and depth of these trenches depend on the design load, soil type, and local building regulations. The centreline of each wall or column should serve as a guide for trench excavation.

3. Preparation of the Trench: If the ground conditions are poor, it may be necessary to dig the trenches deeper than the required foundation depth and then backfill with compacted hardcore. This provides a stable base for the concrete.

4. Formwork Construction: Formwork, temporary structures that hold the concrete in place while it sets, is constructed to support the poured concrete. The formwork should be sturdy and accurately aligned as per the marked lines.

5. Positioning of Reinforcement: If the design requires, steel reinforcement bars or mesh panels are placed within the formwork. The reinforcement improves the tensile strength of the foundation, preventing cracks and enhancing durability.

6. Pouring of Concrete: Next, concrete is poured into the formwork, ensuring a consistent and level coverage across the entire trench. The concrete should be well-compacted to eliminate air voids and ensure uniform strength throughout the foundation.

7. Curing: The concrete is then left to set for a specific period, typically a few days to a week, depending on the concrete mix and weather conditions. During this time, it’s crucial to protect the concrete from adverse weather conditions that could affect its strength.

8. Backfilling: Once the concrete has adequately set, the formwork can be removed. The excavated soil (spoil) can then be used to backfill the space around the newly formed foundation.

It’s essential to note that this is a generalized overview. The actual process can vary based on the specific project requirements, local regulations, and site conditions. Always consult with a structural engineer or other qualified professional before commencing the construction of strip foundations.

What are the advantages and disadvantages of strip foundations?

Strip foundations, like all building methods, have both advantages and disadvantages. These need to be carefully weighed when deciding whether this type of foundation is suitable for a specific construction project.

Benefits of using strip foundations

When applied in the appropriate scenarios, strip foundations offer numerous benefits, contributing to their popularity in the construction of low to medium-rise buildings. Here are some of the primary advantages of strip foundations:

1. Cost-Effectiveness: Strip foundations require relatively less construction materials compared to other types of foundations, such as raft or pile foundations. They also demand less intensive labor and machinery, which can significantly lower the overall cost of construction.

2. Simplicity and Speed of Construction: The construction process of strip foundations is straightforward and less time-consuming than other types of foundations. This simplicity extends to both the excavation process and the actual laying of the foundation, providing time efficiency.

3. Flexibility: Strip foundations can adapt to a variety of ground conditions and can be laid flat, stepped, or sloped as required. This makes them suitable for a wide range of construction scenarios.

4. Suitability for Light Structures: For low to medium-rise structures, such as residential buildings, strip foundations can provide an efficient and effective load-bearing solution.

5. Reduced Ground Disturbance: Strip foundations only require excavation along the line of the load-bearing elements, which reduces the extent of ground disturbance and maintains more of the site’s original soil structure compared to other foundation types.

While these advantages make strip foundations a preferred choice in many situations, it’s important to recognize that they are not suitable for all conditions. In the following section, we will discuss some of the limitations and potential challenges associated with strip foundations.

Limitations of strip foundations

Despite their numerous benefits, strip foundations also come with certain limitations. Understanding these restrictions is crucial in deciding whether to use strip foundations for a construction project.

1. Not Suitable for Heavy Structures: Strip foundations are best suited for light to medium structures. They are not designed to support the heavy loads of high-rise buildings, industrial facilities, or large commercial structures.

2. Dependency on Soil Condition: The performance of strip foundations is highly dependent on the quality of the underlying soil. They are unsuitable for weak, unstable soils or those with poor load-bearing capacity.

3. Difficulty in Poor Access Conditions: If the construction site is constrained or hard to access, it can be challenging to excavate and pour strip foundations.

4. Limited Resistance to Differential Settlement: While strip foundations distribute the load of the structure evenly to the ground, they may still suffer from differential settlement if the soil beneath them is not uniform. This can lead to cracking and other structural issues.

5. Inadequate for Expansive Soils: Strip foundations may not perform well in expansive soils, which significantly expand and contract with moisture changes. These movements can cause damage to the foundation and the structure it supports.

6. Unsuited for Sites with High Water Table: If the site has a high water table, strip foundations may not be the best choice, as they can be susceptible to buoyancy forces, and the excess water can lead to instability.

As with any construction decision, a professional evaluation is essential when choosing the foundation type. Given their specific advantages and limitations, strip foundations should be used only when they are suitable for the ground conditions and the nature of the structure. Consulting with a structural engineer or a geotechnical expert can provide necessary guidance, ensuring that the foundation choice is sound and safe for the intended construction project.

Alternatives to strip foundations

While strip foundations serve as an excellent solution for many construction scenarios, there may be instances where they are not the optimal choice due to soil conditions, structural load, or other site-specific factors. In such cases, other types of foundations may be more suitable. Let’s take a look at some of these alternatives.

1. Pad Foundations: Also known as isolated or individual footings, pad foundations are ideal for supporting concentrated loads such as columns. They are generally square or rectangular in shape and spread the load over the ground underneath, making them suitable for buildings where load-bearing walls are not present.

2. Raft Foundations: Raft foundations, also known as mat foundations, are a type of shallow foundation that distributes the building load over the entire building area. They are used when the soil bearing capacity is low, or the building loads are heavy.

3. Pile Foundations: When the surface soil’s bearing capacity is insufficient to support the structure’s load, pile foundations are used. They transfer the load to deeper, more stable soil strata or bedrock. They are typically used for large structures or in areas with poor or variable soil conditions.

4. Pier Foundations: Similar to pile foundations, pier foundations are used to transfer loads to a deeper, more stable layer of soil or bedrock. However, unlike piles, piers are not driven but are drilled into the ground. They are often used in conjunction with grade beams.

Each of these foundation types has its advantages and appropriate applications, based on factors such as the type of structure, soil conditions, and local building codes. It is essential to consult with a structural engineer or geotechnical expert to determine the most appropriate type of foundation for your specific construction project.

Conclusion: The integral role of strip foundations in construction

Strip foundations play a crucial role in the construction of low to medium-rise buildings. They provide a cost-effective, simple, and time-efficient foundation solution in appropriate soil conditions. Nevertheless, it’s essential to recognize their limitations and consider other foundation types where necessary.

The foundation is a vital component of any structure, bearing the load of the entire building and transferring it to the ground. It must be carefully designed and constructed to ensure the stability and longevity of the structure it supports. Strip foundations, when used correctly, are a key part of this process.

While strip foundations are an excellent choice in many situations, professional guidance is always necessary to make the right decision about the foundation type. The choice of foundation should be based on a careful evaluation of the building design, load requirements, soil condition, and other relevant factors.

Whether you are building a small residential building or a larger structure, understanding strip foundations’ basics can help make more informed decisions about your construction project. However, always remember that each construction project is unique, and what works best will depend on the specific circumstances and requirements of the project. With the right advice and planning, you can ensure a safe and successful construction project.

FAQs about strip foundations

Why would you use a strip foundation?

Strip foundations are used in various construction scenarios for several reasons:

1. Load Distribution: Strip foundations are effective at evenly distributing the load of a structure along the line of the wall or structure. This makes them suitable for buildings with load-bearing walls or line structures that need consistent support.

2. Suitable for Light to Medium Structures: Strip foundations are ideal for light to medium structures like residential buildings. They are generally not used for heavy or high-rise structures.

3. Ground Conditions: Strip foundations work well in conditions where the soil has a good bearing capacity. They are typically used where the load of the building is low relative to the strength of the soil.

4. Cost-Effective: Compared to other foundation types, strip foundations are usually less expensive to construct. They require less material and labour, which contributes to their cost-effectiveness.

5. Simplicity: The construction process for strip foundations is relatively simple and straightforward. This can be an advantage in projects where speed and ease of construction are priorities.

6. Reduced Ground Disturbance: Strip foundations only require excavation along the line of the load-bearing elements, reducing the extent of ground disturbance.

However, while strip foundations have many benefits, they may not be suitable in all situations. It is always important to conduct a comprehensive site survey and soil investigation before choosing the appropriate type of foundation for a specific project.

How deep can strip foundations go?

The depth of strip foundations can vary significantly based on the building they’re supporting, local building regulations, and the specific characteristics of the soil. In general, the depth must be sufficient to distribute the building load and to provide stability against various factors, such as frost action and soil movement.

The minimum depth of a strip foundation is often dictated by local building codes. For instance, in the UK, the Building Regulations Approved Document A suggests a minimum depth of 450mm below ground level for most soils, and at least 1 meter on shrinkage or expansive clay soils. However, in areas with significant frost penetration, the depth may need to be greater to prevent frost heave.

In terms of maximum depth, strip foundations would typically not exceed 3 meters, as beyond this depth, other types of foundations, such as piled or raft foundations, may be more appropriate due to the increased bearing capacity and the potential for more significant soil pressure at greater depths.

It’s worth noting that a qualified structural engineer should always be involved in determining the appropriate depth of the foundations based on the specifics of the project. This will typically involve carrying out a detailed geotechnical survey to understand the characteristics and bearing capacity of the soil.

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