Good afternoon, dear guests!
The article should begin with the fact that SNIP does not regulate the norms for the construction of foundations separately for fences. The foundation is one aggregate (single) structure, the concept of which is uniform the distribution of the load on the ground from the superior structure, whether it be a cottage, pool, porch, fence or outdoor toilet.
A light fence is a fencing structure, during the construction of which light materials were used in relation to the bearing capacity of the soil.
The main idea of the foundation is to create conditions for the structure under which it will not collapse and serve to the homeowner faithfully for many years, which is why many conditions are imposed on the design foundations.
The category of light fences includes:
- Rabitz;
- welded mesh;
- professional sheet;
- slate;
- wooden shield (solid or knocked down).
Due to the use of various materials, for such fences when constructing foundations, the windage of the structure is taken into account. If using a mesh, wind resistance is practically absent, then when using slate or profiled sheet with gusts of wind - stress is created in the foundation, and the bending moment at the base of the support pillar becomes very great.
To build a foundation for a light fence, it is completely inappropriate to arrange a monolithic tape and it is enough to make a good foundation for the support posts of the fence at reasonable and rational costs, since a light fence is one of the few that successfully tolerates a small displacement of the supporting elements relative to each other friend. It is precisely this effect that we need to minimize or completely eliminate.
The displacement of the pillars occurs in three cases:
- subsidence of the soil;
- frosty heaving;
- gusts of wind.
The illustrations show diagrams of deformation of light fences, depending on the reason for the displacement of the posts:
With a wind load, the bending moment of the foundation is very large, since the bending load from sudden gusts of wind can be significant. The deflection of the support can also be caused by frost heaving forces, since the soil around the foundation freezes unevenly due to heterogeneity.
Foundations
1. Foundations on a good foundation (loam, dense sand)
In order to avoid deviation of the support pillars from the vertical axis, on such soils, it is enough to dig or drill a hole for a column with a diameter of 25-30 cm. below the freezing mark. After that, install the support and fill it with sand, followed by tamping, without bringing the upper mark by 0.3 - 0.5 m.
Next, concreting the remaining section with the mandatory installation of a welded mesh or steel reinforcement bars (as shown in the illustration). It is not necessary to lay thick rods, 3-4 mm will be enough.
What is reinforcement for? At the moment of wind load, is the bending moment acting on the supporting elements of the fence, due to the rigidity of the metal pillar - is transmitted to the foundation - and the crushing force falls precisely on the upper zone of the foundation, where the pillar and borders soil.
2. Foundations on poor foundations (loose soil with low bearing capacity)
If the soils are weak, rather loose or thick black soil, then in order to reduce the subsidence of the support, the area of support of the foundation on the base should be increased.
And since the wind load is dynamic and occurs from different sides, then over time on weak soils it can "shake" the rack, therefore the bearing area must be increased not only in the lower zone, but also from the sides, respectively, concreting is carried out to the entire depth of the established supports. There is no need to change the diameter of the pit - you can do the same 25-30 cm.
There are options when, in order to save money, concreting is done in layers: the support is concreted at the base by 15-20 cm, then sand is poured, and then the upper part ~ 30 cm is concreted.
3. Foundations on heaving soils
In case of freezing, soil heaving acts on the support in all directions. For such soils, before the production of the foundation, a damper layer is usually made and it is made from a mixture of fine gravel and river sand.
Thus, a pit of a larger diameter is prepared in advance and the heaving soil is partially replaced with a sand-crushed stone mixture (illustration below).
How to understand that the soil is very heaving? Walk around and assess the condition of the neighboring fences, and if the supports stand in different planes and disperse - then heaving soil is evident (even brick columns on separate foundations deviate from the vertical axis).
Support selection advice
When purchasing metal for a fence device, pay attention to round pipes, they will cost less than a square with the same wall thickness. The round tube has less weight, but due to its streamlining - it works great under dynamic loads such as wind.
If, due to the aesthetic appearance, you do not want to purchase a round product, then replace the square with a rectangular section and install with the narrow side along the plane of the fence.
Due to this arrangement, the windage of the racks decreases and the stiffness of the fence increases for the perception of external loads.
I would be glad if the article became useful to you. Thanks for attention!
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