Calculation of the width of the strip foundation

The foundation design principles must be understood when organizing any construction project. In order to ensure structural stability and longevity, the width of a strip foundation—which bears the weight of a building and distributes it evenly to the ground below—is essential. This measurement is not arbitrary; rather, it is determined by taking into account both the weight of the structure supported by the soil and its ability to support loads.

The kind of soil and the weight of the building it will support are the main factors determining a strip foundation’s width. The ability of various soils to support weight without settling or shifting over time varies. For instance, compared to sandy soils, clay soils can support a greater weight per unit area. The width needed is determined by engineers and architects to make sure the foundation distributes the weight evenly enough to avoid settlement or structural damage.

The entire weight of the building, the anticipated load on each section of the foundation, and the bearing capacity of the soil are all taken into account when calculating the width. Throughout the building’s lifetime, this process guarantees that the foundation can support the structure without experiencing undue settlement or movement. Through meticulous computation of these measurements, constructors can guarantee the steadiness and security of the complete construction.

What determines the optimal tape width?

Design calculations are used to determine the strip foundation’s cross-sectional dimensions, which are based on variables like:

• soil type at the construction site;
• soil freezing depth;
• groundwater level;
• calculated weight of the building taking into account snow cover;
• wind loads on walls and roofs;
• material from which the foundation will be built.

Hydrogeological surveys on the site and the use of reference literature are required in order to gather preliminary data.

Regulations

The soil’s bearing capacity is the main factor that determines the strip foundation’s width. To achieve the ideal tape width in dense, stable soil, add 70–100 mm on each side of the wall thickness. However, it needs to be raised considerably in cases of loose and loose soil; in certain situations, it can go up to 900 mm.

Composite structures are used from a wide concrete support at the top and bottom of the tape, the width of which depends on the wall thickness, to prevent excessive consumption of concrete or stone materials. Wide strip foundations’ primary drawback is that they cannot be used in areas with high groundwater levels or mobile heaving soils.

Three regulatory documents ought to be consulted when making calculations:

• SNiP 2.01.07-85 – Loads and impacts;
• SNiP 2.02.01-83 – Foundations of buildings and structures;
• SP 131.13330.2012 – Construction climatology.

The first describes the foundation calculation process. Standard specifications for foundation structures are given in the second. The third shows the soil’s freezing depth based on climate zones for the majority of major cities.

Only with the help of hydrogeological surveys conducted on the property is it feasible to ascertain the type and structure of the soil as well as the depth of groundwater. Test drilling is done by specialized organizations to a depth that is lower than freezing. Although this service is pricey, you should use it because they don’t compromise on security and dependability.

Determining the width of a strip foundation during construction is essential to guaranteeing the longevity and stability of a building. The soil’s ability to support a given weight, local building codes, and the weight of the supported structure all affect how wide the foundation strip should be. Builders can choose the ideal width that evenly distributes the weight of the building and guards against settling or structural damage over time by carefully weighing these factors. This article provides useful information for both builders and homeowners by examining the important factors and techniques involved in determining the width of strip foundations.

Minimum width

The process for figuring out a tape’s cross-sectional dimensions yields a value for the width that is less than what it should be rather than a precise numerical value. The minimum width of the strip foundation is required to determine the ideal width and lower construction costs; the actual foundation is typically 10–20% larger.

A compromise option is occasionally used when the soil is dense and stable and the calculations call for a minimum foundation width of 200–250 mm. Construct the lower portion of the narrow; the upper 300–400 mm will depend on the wall thickness. This technique is frequently used in the building of home structures, verandas, and light baths.

Maximum width

The maximum width of the foundation tape is not mentioned in any of the regulatory documents mentioned above. The goal of the strip foundation’s width design calculation should be the opposite: figuring out the ideal size to cut expenses.

Nonetheless, there is a crucial detail regarding the layout of the basement that needs to be considered when building a structure. In these situations, the foundation’s maximum width is restricted. It is related to the weight pressure on the ground and is contingent upon the length of each wall as well as the composition of the wall.

The foundation sole for walls up to three meters in length should not exceed:

• concrete monolith – 400 mm;
• concrete foundation blocks – 500 mm;
• Button – 600 mm;
• Full brick – 750 mm;
• Ward stone – 800 mm.

The maximum width that is permitted if the walls are longer than three meters is:

• reinforced concrete monolith – 500 mm;
• concrete foundation blocks – 600 mm;
• rubble concrete – 800 mm;
• solid brick – 900 mm;
• rubble stone – 1000 mm.

This information comes from the actual observations of builders and is not mandated by any regulations. They should therefore be considered in computations but not treated as absolutes.

What data will be required for the calculation?

To develop the project, it is necessary to ascertain the building’s total weight, the soil’s bearing capacity, and the wall’s length in addition to the region’s climatological indicators, the soil’s hydrogeological structure, and the composition of the foundation walls.

Determination of building load

A straightforward formula is used to determine the weight load on the strip foundation:

M+P+S+V, Location:

• M – dead weight of the building, including the weight of all building structures and elements, including the foundation;
• P – a payload or weight of everything that will be inside the building and create pressure on the ceilings;
• C – maximum possible mass of snow cover in winter and at the beginning of melting;
• B – wind pressure on walls and roof.

The calculated result should be multiplied by 1.2-1.25 to give the strip foundation structure a 20–25% safety margin.

Bearing capacity or soil resistance

The regulatory literature provides this indicator, which is based on the "Classification of soils" GOST 25100-95. For the most prevalent soil types, the value is (in kg/cm2):

• loam – 1.5-2.8;
• dry dense clay – 1.6-3.0;
• fine-grained sand – 2.2-3.4;
• medium-grained – 2.5-3.6;
• sandy loam – 2.6-3.6;
• coarse sand – 3.6-4.6;
• gravel, crushed stone, pebbles – 5.1-6.5.

When preparing calculation data, it is important to consider how soil moisture, fluidity, and porosity affect the indicator of resistance to weight loads.

An example of calculating the width of the sole for the strip foundation

The following formula is used to calculate the size of the supporting foundation sole:

Breadth equals building mass, wall length, and soil resistance.

Assuming that the preliminary computations performed during the gathering of data revealed:

• a building made of aerated concrete blocks, taking into account the useful, snow and wind loads, creates a weight pressure of 165,800 kgf;
• the total length of the foundation strip in a house of 10 x 8 meters with one transverse lintel is 44 meters or 4400 cm;
• soil – dry dense clay with a bearing capacity of 1.9 kg/cm2 .

We determine the tape’s width for an aerated concrete house using these indicators:

165800: 4400: 2.1 = 19.83 cm, 20 cm is the round number.

It turns out that the tape can have a minimum width of 20 cm. But the aerated concrete blocks have to be 300 mm thick, and the foundation needs to extend at least 5 cm past the wall’s edges. As a result, 400 mm will be the ideal sole width, giving the structure twice as much structural strength. Additionally, a thorough computation of the strip base is provided here, and the topic of the ideal tape-laying depth is covered here.

Any building project’s stability and longevity depend heavily on knowing the strip foundation’s width. The weight of the structure the foundation supports and the soil’s ability to support loads are the main factors that determine the foundation’s width.

The kind of soil that the foundation will be built upon must be taken into account when calculating this width. For instance, narrower foundations may be possible in sandy soils provided they can support the weight, but wider foundations are usually needed in clay soils in order to distribute the load more fairly.

Furthermore, the depth at which the soil can support the weight without experiencing undue settlement determines the foundation’s width. The bearing capacity, which is this depth, affects the determination of the minimum width of the foundation required to guarantee stability over time.

In summary, the strip foundation’s width is determined by taking into account a number of technical factors, but its main goal is to give the entire structure a stable foundation. Builders can choose the ideal width for their construction projects that strikes a balance between durability, safety, and cost-effectiveness by knowing the soil conditions and load requirements.

Video on the topic

Calculation of the foundation of a house based on bearing capacity

Independent calculation/checking of the supporting area of the foundation