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Strip foundations are the most popular in private construction. They can be used for different houses on different types of soil, you can do the calculation yourself. This does not require knowledge of higher mathematics or sopromat. There is a method in which everything is simple, however, cumbersome: you have to collect a lot of data. This calculation of the strip foundation is called “according to the bearing capacity of the soil.” But first you will need to collect loads from the house: calculate how much mass will fall on each square meter (centimeter) of the base. Then, choosing the width of the sole of the foundation, choose its optimal width.
Calculation method
The strip foundation can be calculated in two ways: by the bearing capacity of the soil under the sole and by their deformation. The first way is simpler. Let’s consider it.
We know for sure that the foundation is built first. But it is designed last. His task is to transfer the load from home. And we will know it only after we decide on the type of all building materials and their volumes. So before starting the calculation of the foundation, you must:
- draw a plan of the entire building with all the walls;
- decide whether or not a basement is needed, and how deep it should be, if needed;
- know the height of the base and the material from which it will be made;
- determine the type and thickness of the materials used for insulation, wind protection, waterproofing, finishing both inside and outside.
For all materials used during construction, you need to find their specific gravity. It is advisable to make a table: it will be easier to work. Only after that you can start the calculation.
The strip foundation is most often made monolithic or prefabricated concrete. Brick or rubble-concrete tapes are made much less often today: they are less reliable, but at the same time, more material is required for their construction, although its cost may be less.
Conventionally, the calculation of the strip foundation can be divided into several stages:
- Determining the load on the foundation.
- Select ribbon options.
- Adjustment according to conditions.
Now about all the stages in more detail.
Collection of foundation loads
At this stage, the mass of all building materials that are used for construction is summed up:
- walls – external and internal (the total area is taken, not taking into account cutouts on doors and windows);
- floor coverings and materials for it;
- ceiling and ceiling;
- rafter system and roofing materials;
- stairs and other interior elements of the house;
- external heat and wind insulation and finishing;
- basement and foundation (for starters – approximately);
- fasteners (nails, screws, studs, etc.)
Table of average loads from different types of nodes at home. it can be used at a preliminary stage – when you estimate the approximate level of costs
As already mentioned, by this time a building plan with more or less exact dimensions should already be ready. The calculation of the mass of building materials used is simple: find the area on which it will be located, multiply by the specific gravity, you get the mass.
If the calculated element is rectangular, find its area by multiplying the length of the sides. If you count in meters, you get m2. Multiplying by the thickness of the material in the same units (in meters) you get the volume in cubic meters – m3. It will be more convenient to work this way: most of the specific gravity of building materials is given in kilograms per cubic meter (kg / m3). Multiplying the found volume with the specific gravity of the material, you get the mass of material for this plane.
Wall mass calculation example
To make it clearer, let’s take an example. Let’s calculate how much a wall made of profiled pine timber 150 * 150 mm will weigh, with sheathing made of linden lining 14 mm thick, a crate made of pine bar 50 * 20 mm. The wall is 4 m long and 2,8 m high.
Specific weight of purchased pine timber (may vary) 570 kg/m3, lining 530 kg/m3, bar 510 kg/m3.
Wall area: 4m * 2,8m = 11,2m2.
The volume of timber in the wall will be 11,2 m2 * 0,15 m (beam thickness) = 1,68 m3.
Multiplying the volume by the specific gravity of the beam, we get the mass of the wall: 1,68 m3 * 570 kg/m3 = 957,6 kg.
Now we find the volume of lining on the wall: 11,2 m2 * 0,014 m (lining thickness) = 0,16 m3.
We find out how much the lining weighs by multiplying its specific gravity by volume: 0,16 m3 * 530 kg/m3 = 84,6 kg.
The number of crates is considered differently: we determine how many planks are nailed. We will nail the crate along with a step of 60 cm. It will turn out 5 strips 4 m long. There will be a total of 20 linear meters. Now we find the volume: 20 m.p. * 0,05 m * 0,02 m = 0,02 m3.
Now we find the mass of the crate: 0,02 m3 * 510 kg/m3 = 10,2 kg.
Now find the mass of all materials for the wall: 957,6 kg + 84,6 kg + 10,2 kg = 1052,4 kg.
We think the principle is clear. But it takes a long time to count each wall like that. Then you can make it easier: determine how much one square meter of a wall weighs, then find the area of uXNUMXbuXNUMXball the walls that have the same finish and get their total mass.
We calculated that the mass of a wall with an area of 11,2 m2 will be 1052,4 kg. It turns out that one square weighs 1052,4 kg / 11,2 m2 = 93,96 kg/m2. Now, having calculated the area of u42buXNUMXball the walls with such a finish, we can find their total mass. Let their total area be XNUMX m2. Then they will weigh 42 m2 * 93,96 kg/m2 = 3946,32 kg.
Using this method, find the mass of all the listed elements. If they have complex geometry, break them down into simple shapes and determine the area in this way. The rest shouldn’t be a problem.
Payload at home
In addition to building materials, the whole situation in the house will put pressure on the foundation: furniture, appliances, people, etc. It is too long to count all this, so when planning it is assumed that the payload per square meter of area is 180 kg / m2. To find out the total payload of the house, multiply its area (of all floors) by this figure.
Snow load
In most regions, it is also necessary to take into account the load on the foundation from snow. Snow loads are determined by regions (see photo), their values are given in the table.
But since the roofs are different, and they accumulate a different amount of snow. Therefore, depending on the angle of the slope, the coefficients are applied:
- the angle of inclination is less than or equal to 25° – the coefficient is 1 (the snow load is taken from the table without changes);
- the angle of inclination is greater than or equal to 60° – the coefficient is 0 – the snow load is not taken into account.
In all other cases (the angle of inclination of the roof is from 25° to 60°), the values are chosen from 0 to 1 (a graph is built and the coefficient is determined from it).
How to calculate the snow load on the roof? You have found your region, you know the average load per square of the roof, and you have determined the coefficient. Now you need to multiply the total area of uXNUMXbuXNUMXbthe roof by all these numbers.
Example: let the snow load in the region be 180 kg/m2, total roof area 65 m2, the factor for taking into account the angle of the roof slope is 0,82 (the angle of inclination is about 30 °). We find the snow load: 65 m2 * 180 kg/m2 * 0,82 = 9594 kg.
This load will need to be added to the mass of the house and its payload.
Calculation of the strip foundation: determine the width of the sole
When calculating the strip foundation, it will be necessary to determine its two parameters:
- laying depth + base height = height;
- tape width;
The third – length – is known. This is the sum of the lengths of all the walls under which the foundation will be laid.
The depth of laying is largely determined depending on the type of soils under the sole. General recommendations can be found in the table, and for a description of determining the depth of laying, see the article “How deep should the foundation be.”
Let us assume that the depth of the foundation for our conditions is below the freezing level of the soil, the height of the basement is 20 cm. The soil freezes in our region by 1,4 m. According to the recommendations, the foundation should be 15 cm below the freezing level. We get the total height: 1,4 m + 0,2 m + 0,15 m = 1,75 m.
Now you need to calculate the width of the strip foundation. It depends on the distance at which the walls are located and the material from which we will build it. Recommended values are shown in the table.
Calculation of the load on the foundation
Now you need to find how much force the house will press on the foundation. To do this, we divide the total mass of the house (the mass of all elements + payload + snow) by the area of uXNUMXbuXNUMXbthe foundation.
The area of the strip foundation is found by multiplying its length by the width selected in the previous paragraph. Then we divide the total load from the house by the area of uXNUMXbuXNUMXbthe foundation in square centimeters. We get the specific load per square centimeter of the strip foundation.
Example. Let the load from the house be 408000 kg, the area of the strip foundation (length 4400 cm, width 30 cm) – 132000 cm2. Dividing these values, we get: 3,09 kg presses on each centimeter.
Now you need to find out if the soils under the base of the foundation can withstand this value. Any soil is able to withstand some pressure. These values are calculated and entered in the table. We find the type of soil under the base of the foundation (determined by geological research) and look at its specific bearing capacity.
If the bearing capacity of the soil is greater than the load from the house, everything is chosen correctly. If not, adjustments need to be made.
Adjustment of parameters
If the load transmitted through the strip foundation is large for these soils, there are two ways out: use lighter materials during construction or increase the width of the strip.
Changing the material is very laborious: often changing one material entails a chain of changes in the parameters of a number of others. As a result, the mass calculation has to be redone. Because more often increase the thickness of the tape in the foundation. This increases the specific load decreases. But too wide a strip foundation (wider than 60 cm), especially a deep one, is economically unprofitable: high material consumption and labor costs. In this case, it is necessary to compare the cost of several types of foundation.
Remember to recalculate the mass of the tape after changing the width of the tape and adjust the mass of the building accordingly.
Read about the calculation of reinforcement for a strip foundation here.
How to calculate the cubature of the foundation
It is better to take into account the mass of the foundation by calculating its volume: this figure will come in handy when pouring the foundation: you will know how much concrete to order or how many materials you need to purchase.
All the initial data is already known: the height, width and length of the tape. You multiply them, you get the cubic capacity of the foundation.
For example, let’s calculate the volume of the foundation for the previously calculated tape: length 44 m, width 30 cm (0,3 m), height 1,75 m. Multiply: 44 m * 0,3 m * 1,75 m = 23,1 m3. In fact, the consumption is likely to be a little more: about 25 cubic meters. Be guided by this figure when ordering concrete.
