Do-it-yourself foundation reinforcement - basic rules and diagrams
During the operation of a building, a number of factors influence its foundation. The foundation is under stress from soil shifts, frost heaving and, of course, from the weight of the building itself. In this case, the upper part of the foundation, like a reinforced concrete floor slab, is subject to compression, and the lower part is subject to tension. The diagram of the influence of forces inside the foundation is shown in the figure below. It is very important to remember that the correct manufacture of the structure must necessarily take into account the forces of frost heaving, which sometimes exceed the weight of the building itself and lift it, causing stretching of the foundation in the upper part. In this case, the decisive role is played by correct and competent construction, thanks to which it is possible to protect not only the foundation itself, but also the walls of the house from destruction.
In this lesson, you will be provided with step-by-step instructions and a diagram of the correct installation of reinforcement, as well as methods for tying it and other important points, which are also further discussed in the video. In addition, this construction lesson will provide approximate drawings of laying reinforcement when pouring a strip foundation, as well as a diagram of the correct connection of reinforcement in the corners of the structure.
The maximum load during compression of a foundation or reinforced concrete slab falls on the concrete, and during tension the reinforcement must withstand the load. In this regard, the reinforcement must be located at the top and bottom of the structure. In this case, the middle part of the foundation does not need reinforcement, since practically no forces act on it. How to make structural reinforcement is explained in detail in the video and drawings located in this lesson.
Correct placement of fittings
The main load of any reinforced concrete structure or slab must be withstood by longitudinal reinforcement, which is located in the lower and upper parts. Hot-rolled steel rods of class A3 are usually used as longitudinal reinforcement. When the height of the strip foundation exceeds 1.5 m, transverse and vertical rods of hot-rolled smooth reinforcement with a diameter of 6 to 8 mm of class A1 are also laid. Concrete floor slabs are also manufactured in the factory using the same principle. In this case, it is much better to make vertical and transverse reinforcement with a single clamp to create a connected solid frame. This process is explained very clearly in the video.
The longitudinal rods must be located inside the slab frame or foundation. Thanks to this bond, the likelihood of cracks appearing in concrete and their propagation is reduced, and the reinforcement bars are fixed in the required position. What should be the pitch between the longitudinal bars and transverse reinforcement bars is indicated in paragraph 7.3.4 of SNiP 52-01-2003.
Laying longitudinal reinforcement
In particular, paragraph 7.3.6 of SNiP 52-01-2003 indicates that the spacing between the longitudinal reinforcement bars of a concrete structure must be calculated based on its type (it can be walls, floor slabs, beams or columns), height and width of the cross section. In this case, it is necessary to ensure that the concrete itself is effectively involved in the work, taking into account the uniform distribution of deformation and stress over the entire cross-sectional area of the structure. In particular, the distance between adjacent bars of working longitudinal reinforcement should not exceed the cross-sectional height of the concrete element multiplied by two. However, this distance should not be more than 400 mm. In the case of linear elements eccentrically compressed relative to the bending plane, the step between the longitudinal reinforcement should not exceed 500 mm. And although it is quite difficult to comprehend this principle at first glance, the step-by-step instructions for reinforcing concrete structures with your own hands, given in this lesson, will help you avoid such mistakes.
Principle of transverse reinforcement
How to properly make transverse reinforcement is indicated in paragraph 7.3.7 of SNiP 52-01-2003. In reinforced concrete slabs and foundations, taking into account the fact that the transverse force should not be absorbed only by concrete, it is necessary to lay transverse reinforcement bars. The step between them should not be greater than the value that ensures the inclusion of transverse reinforcement in the work when inclined cracks arise and propagate. It must be remembered that the distance between the transverse rods should be no more than half the working height of the section of the foundation or slab, but this step should not exceed 300 mm.
Proper tying of reinforcement
When tying reinforcement inside the foundation, it is undesirable to use welding, since an increase in temperature negatively affects the characteristics of the metal. However, if the symbol “C” is present in the marking of the reinforcement, it can be welded together. For example, if you have reinforcement grade A500C, you can use electric welding to connect it. In all other cases, the reinforcement inside the foundation or concrete slab is tied with tying wire.
In addition, when reinforcing strip foundations, it is necessary to remember that the ends of the steel rods should not protrude beyond the formwork and come into contact with its walls. It is recommended to maintain a distance from the reinforcement to the edge of the concrete structure of at least 50 mm. This issue is further discussed in the added video.
Reinforcement of foundation corners
When pouring strip foundations, the angles of the structure play an important role. The fact is that it is at the corners that the greatest stress occurs. As shown in the video, the manufacture of reinforcement for intersections and corners requires the use of pre-bent blanks from A3 reinforcement. It is important to remember that the usual crossing of steel rods in the corners of strip foundations is unacceptable, since this method of reinforcement will not ensure the solidity of the structure. You will end up with several reinforced concrete beams that are not connected to each other.
When building private houses with your own hands, an unacceptable method of tying reinforcement bars that intersect in the corners is often used. The drawing below shows two options for reinforcing the corners of a strip foundation, which are executed incorrectly.
The diagram shows that the connections of the longitudinal rods are made in the form of ordinary crosshairs and do not have additional reinforcement. In addition, there are no additional clamps at the intersections of the reinforcement.
The usual crossing of reinforcement should also not be allowed during the reinforcement of sections that are T-shaped structural elements. All these places need to be further strengthened.
Sometimes a house design includes bay windows, which are protrusions on the facade of the building. From the inside, bay windows represent part of a multifaceted room. To reinforce the foundation under the bay window, it is necessary to bend the reinforcement at an obtuse angle. In this case, the longitudinal reinforcement bars of the strip foundation must pass through the frame and communicate with the external reinforcement. In addition, the design contains additional clamps and L-shaped reinforcement.
The step-by-step instructions for reinforcing the corners of a concrete foundation are generally aimed at obtaining a monolithic frame connected into a single whole, which additionally leads to. Clamps are used to connect its elements. It must be remembered that all corners and joints of the reinforcement must contain additionally installed L-shaped or U-shaped elements. Vertical and transverse reinforcement clamps for strip foundations must be located in increments of at least 3/8 of the height of the foundation structure. In this case, the distance between the clamps should be at least 25 cm. In corners, at crossroads and under bay windows, the step between the clamps should be half as large as in conventional longitudinal elements of the foundation structure. Good luck with your construction!
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