Calculation of the load on the foundation

Calculation of the load on the foundation is necessary for the correct choice of its geometric dimensions and footprint of the basement. Ultimately, the correct calculation of the foundation depends on the strength and durability of the entire building. The calculation is reduced to determining the load per square meter of soil and comparing it with the permissible values.

For calculation it is necessary to know:

Region in which the building is built;
Soil type and depth of groundwater;
The material from which the structural elements of the building will be made;
Layout of the building, number of storeys, type of roof.

Based on the required data, the calculation of the foundation or its final verification is made after the design of the structure.

Let's try to calculate the load on the foundation for a single-storey house made of full-bodied brick of continuous masonry, with a wall thickness of 40 cm. The dimensions of the house are 10x8 meters. Overlapping of the basement room - reinforced concrete slabs, overlapping of 1 floor - wooden on steel beams. The roof is gable, covered with metal, with a slope of 25 degrees. Region - Moscow region, type of soil - wet loam with a coefficient of porosity of 0.5.The foundation is made of fine-grained concrete, the thickness of the foundation wall for calculation is equal to the thickness of the wall.

Determination of the depth of the foundation

The depth of the deposit depends on the depth of freezing and the type of soil. The table gives reference values of the depth of soil freezing in different regions.

Table 1 - Reference data on the depth of soil freezing

Reference table for determining the depth of foundation foundation by region

The depth of the foundation should generally be greater than the depth of freezing, but there are exceptions due to the type of soil, they are listed in Table 2.

Table 2- Dependence of foundation depth on soil type

Dependence of foundation depth on soil type

Depth of foundation is necessary for the subsequent calculation of soil load anddetermination of its dimensions.

Determine the depth of freezing of soil according to Table 1. For Moscow, it is 140 cm. According to Table 2, we find the type of soil - loam. The depth of laying should be no less than the calculated depth of freezing. Based on this, the depth of foundation for the house is 1.4 meters.

Roof load calculation

Roof load is distributed between those sides of the foundation on which the rafter system rests through the walls. For a conventional gable roof, these are usually the two opposite sides of the basement, for the four-pitched roof, all four sides. The distributed load of the roof is determined by the area of the roof projection, referred to the area of the loaded sides of the foundation, and multiplied by the specific weight of the material.

Table 3 - Specific weight of different types of roof

Reference table - Specific weight of different types of roofing

Determine the area of the roof projection. Dimensions of the house - 10x8 meters, the area of the projection of the gable roof is equal to the area of the house: 10 · 8 = 80 m2.
The length of the foundation is equal to the sum of its two long sides, as the gable roof rests on two long opposite sides. Therefore, the length of the loaded foundation is defined as 10 · 2 = 20 m.
The area of the foundation-loaded foundation is 0,4 m thick: 20 · 0,4 = 8 m2.
Type of covering - metal, angle of slope - 25 degrees, then the design load of Table 3 is 30 kg / m2.
The roof load on the foundation is 80/8 · 30 = 300 kg / m2.

Snow load calculation

Snow load is transferred to the foundation through the roof and walls, so the same foundation sides are loaded as in the calculation of the roof. Calculates the area of the snow cover, equal to the area of the roof. The obtained value is divided by the area of the loaded sides of the foundation and multiplied by the specific snow load determined on the map.

Table - calculation of snow load on the foundation

The length of the ramp for a roof with a slope of 25 degrees is( 8/2) / cos25 ° = 4.4 m.
The roof area is equal to the length of the ridge multiplied by the length of the ramp( 4.4 × 10) · 2 = 88 m2.
Snow load for the suburbs on the map is 126 kg / m2.Multiply it by the roof area and divide by the area of the loaded part of the foundation 88 · 126/8 = 1386 kg / m2.

Calculation of load of overlaps

Overlappings, like the roof, are usually supported on two opposite sides of the foundation, so the calculation is based on the area of these sides. The floor area is equal to the area of the building. To calculate the load of floors, you need to take into account the number of floors and the ceiling of the basement, that is, the floor of the first floor.

The area of each overlap is multiplied by the specific gravity of the material from Table 4 and divided by the area of the loaded part of the foundation.

Table 4 - Specific weight of overlaps

Table calculation of the weight of overlaps and their load on the foundation

The floor area is equal to the area of the house - 80 m2.The house has two floors: one of reinforced concrete and one - wooden on steel beams.
We multiply the area of reinforced concrete overlap by the specific weight from Table 4: 80 · 500 = 40,000 kg.
Multiply the area of the wooden floor by the specific weight from Table 4: 80 · 200 = 16000 kg.
Summarize them and find the load on 1 m2 of the loadable part of the foundation:( 40000 + 16000) / 8 = 7000 kg / m2.

Wall load calculation

Wall load is defined as the wall volume multiplied by the specific gravity from table 5, the result is divided by the length of all sides of the foundation multiplied by its thickness.

Table 5 - Specific gravity of wall materials

Table - Specific weight of walls

The area of walls is equal to the height of the building, multiplied by the perimeter of the house: 3 ·( 10 · 2 + 8 · 2) = 108 m2.
The volume of walls is the area multiplied by the thickness, it is equal to 108 · 0.4 = 43.2 m3.
Find the weight of the walls by multiplying the volume by the specific weight of the material from Table 5: 43.2 · 1800 = 77760 kg.
The area of all sides of the foundation is equal to the perimeter multiplied by the thickness:( 10 · 2 + 8 · 2) · 0.4 = 14.4 m2.
The specific load of the walls on the foundation is 77760 / 14,4 = 5400 kg.

Preliminary calculation of the foundation load on the ground

The foundation load on the ground is calculated as the product of the foundation volume by the specific density of the material from which it is made, divided by 1 m2 of its base area. The volume can be found as the product of the depth of the foundation for the thickness of the foundation. The thickness of the foundation is taken with a preliminary calculation equal to the thickness of the walls.

Table 6 - Specific density of foundation materials

Table - Specific gravity of the material for the ground

The area of the foundation is 14,4 m2, the depth of the deposit is 1,4 m. The volume of the foundation is 14,4 · 1,4 = 20,2 m3.
The weight of the foundation of fine-grained concrete is: 20,2 · 1800 = 36,360 kg.
Ground load: 36360 / 14.4 = 2525 kg / m2.

Calculation of total load per 1 m2 of

The results of the previous calculations are summarized, while the maximum load on the foundation is calculated, which will be greater for those sides on which the roof rests.

The nominal design resistance of the soil R0 is determined by the tables of SNiP 2.02.01-83 "Bases of buildings and structures".

We summarize the weight of the roof, the snow load, the weight of the floors and walls, as well as the foundation on the ground: 300 + 1386 + 7000 + 5400 + 2525 = 16 611 kg / m2 = 17 t / m2.
Determine the conventional design resistance of the soil in accordance with SNiP 2.02.01-83.For wet loams with a coefficient of porosity of 0.5 R0 is 2.5 kg / cm2, or 25 tons / m2.

Calculation shows that the load on the ground is within the permissible range.