Calculation of reinforcement for the foundation and correct reinforcement

Recommendation points

• Construction fittings – we disassemble the assortment
• Visually about the work of reinforcement
• Selecting the section and density of the bookmark
• Why and how to distribute reinforcement lines
• Protective and separating layers
• Laying, knitting, spacer plugs and other subtleties

The strength of the foundation depends on the correct reinforcement, as well as the integrity of the house standing on it. The foundation is the foundation of a building and should be given very close attention. Let’s talk about how the foundation reinforcement works, how to correctly calculate the required amount of reinforcement and about the correct knitting..

Construction fittings – we disassemble the assortment

In the CIS, the most popular products for reinforcement are made of hot-rolled steel according to GOST 5781. These are metal rods with a diameter of 6–80 mm with profiled notches on the surface. Such rolled metal is distinguished by a high modulus of elasticity – about 200 kPa.

A distinctive feature of metal reinforcement is the presence of the so-called yield area – the temporary state of a substance beyond the limit of elastic deformation before physical destruction. The technical qualities of the reinforcement are determined by the class of steel used in production: from the least durable A-I to the strongest A-VI.

Smooth reinforcement can be used for structural reinforcement. Its main drawback is the reduced adhesion of the metal to the concrete mass; therefore, it is reasonable to design elements made of smooth reinforcement without high axial tensile loads..

Visually about the work of reinforcement

First, consider a model of a reinforced concrete column. Under normal conditions, it is subjected to an axial load leading to a linear expansion of the array from the center outward due to compression. Concrete is not plastic and is subject to fatigue failure in such an environment. The reinforcement of the column takes part of the load on itself and forces the entire massif not to expand, but to bend within acceptable limits. Transverse reinforcement also strengthens the edges and prevents oblique cracks.

The second model is a horizontal beam supported on the edges with a load applied in the center. Concrete without reinforcement in such conditions can break even under its own weight. Steel in concrete gives it elasticity, while the concrete itself prevents point deformation of the reinforcement, so that the applied load is distributed along the entire length of the beam.

The beam model almost completely corresponds to the MZLF, but in deep complex foundations, the column principle works on stiffeners. The load on the foundation falls unevenly due to the presence of openings in the walls and different weights of individual sections, or due to other design features. In turn, the density of the soil under the foundation is also uneven. You can agree on the opinion that the main work of the foundation is to harmlessly take on the load from the structure, and then correctly distribute it along the points of support.

Selecting the section and density of the bookmark

The main distinguishing feature of concrete goods is the cross-section of longitudinal reinforcing elements at the cross section. The ratio of this value to the cross-sectional area of ​​the concrete mass is called the filling density. Depending on the mass, load, type and even the section of the structure, the density can be from 0.1 to 2.5%, for the foundation, values ​​of 0.1-0.3% should be adhered to.

The minimum thickness of longitudinal reinforcement bars and corner D-clamps is determined by the actual span length:

• in areas up to 3 m, reinforcement is not thinner than 10 mm;
• on spans of more than 3 m – not less than 12 mm;
• on point loaded beams (column-skeletal structure) – not less than 14 mm at a filling density of 0.2%.

Reinforcement of corners and abutments of the strip foundation using L-shaped clamps: 1 – longitudinal reinforcement; 2 – transverse reinforcement; 3 – vertical reinforcement; 4 – L-shaped clamps

To summarize: a 400×900 mm strip foundation has a cross-sectional area of ​​36×10 ^ 4 mm², that is, the optimal cross-section of longitudinal reinforcement is 360 mm². According to SP 52-101-2003, for non-stressed concrete, the calculated value is selected upward: either 5 rods of 10 mm (if the span length allows), or 4 rods of 12 mm each (with a significant margin of safety).

Please note that the equivalent density can be achieved, conditionally, with three rods of 14 mm each or even two of 16 mm, so where to stop? On this score, even experienced designers sometimes do not give clear recommendations, however, guided by common sense, one should lay as many rods of the minimum permissible diameter as possible. However, remember that a too dense reinforcement cage can make it difficult for the concrete to spill and compact..

Why and how to distribute reinforcement lines

The above calculation technique is valid for thin beams in which the reinforcement is performed in one row with the same protective layers above and below. In practice, it is never known for certain how a concrete beam will behave, in which direction it will bend, where there will be zones of tension and compression. Since the foundation has a width-to-height ratio of 1: 2 or more, the design line of reinforcement is performed both under the upper and lower edges..

But that’s not all. To stabilize the mass and impart solidity, so-called structural reinforcement is used. It includes primarily vertical and horizontal transverse elements – rods or clamps. They are also calculated according to the density of the bookmark, it is at least 0.025% of the section, but not transverse, but longitudinal along the vertical and horizontal secant plane. Usually clamps are made of reinforcement 1-2 numbers below the main reinforcement with an installation step of 0.8-1.4 meters.

Protective and separating layers

Due to the non-zero water absorption of reinforced concrete, reinforcement is highly corroded. This effect can be minimized by providing cladding covers for each reinforcement line. For the underground part of the foundation, the layer thickness is at least 40 mm, for outdoor structures – 30–35 mm, for insulated ones – 25 mm, and in the presence of waterproofing – 15–20 mm. In any case, the protective layer cannot be thinner than the reinforcement used..

The free space between the lines of the main reinforcement is called the dividing mass. Since deformation phenomena manifest themselves more strongly at the concrete surface, the width of the unreinforced section should not exceed a certain value. Which one? Behind the scenes, a value of 1/4 of the width of a particular face is used, that is, on the sides of the reinforcing frame, you need to add 3 or 4 longitudinal bars 1-2 numbers less than the main reinforcement. The resulting strips wider than 450 mm must be reinforced with a wire mesh.

Laying, knitting, spacer plugs and other subtleties

The reinforcing frame in most cases is assembled as follows:

1. Longitudinal rods of the lower line of reinforcement are laid at the bottom of the pit.
2. They are tied together with an overlap of 20 nominal diameters, and at turns they are fastened with L-shaped elements of the same thickness and with the same overlap.
3. The bottom line is installed on the spacer plugs forming the bottom protective layer.
4. The transverse structural reinforcement is knitted with the set pitch. These can be multidirectional U-clamps or rectangular rings. An important nuance: all longitudinal reinforcement bars, including auxiliary ones, are installed inside the clamps, and not outside.

It remains only to pass the upper strip of the main reinforcement into the clamps, tie it up and separate the edges with constructive longitudinal reinforcement. All elements are recommended to be fastened with wire bundle, preferring it to arc welding. After adjusting the protective layers, you can load insulation plates and pour concrete.