The device of concrete and reinforced concrete structures provides additional reinforcement due to reinforcing bars. The latter, by the way, is one of the most popular segments of the steel industry, as evidenced by its widespread use in construction. For concrete columns, reinforcement plays a particularly important role due to the impossibility of using other supporting structures other than the lower and upper floors. Internal rod reinforcement with metal rods in different configurations is the optimal solution to the problem.
General reinforcement requirements
For columns, hot-rolled, thermomechanically hardened and cold-deformed metal bars of various profiles can be used. Diameter on average varies from 12 to 40 mm. If you plan to use cold-formed rods of a periodic profile, then a small diameter of 3-12 mm can also be used. In terms of tensile strength, classes A and B are allowed that meet guaranteed yield strengths with a coefficient of at least 0.95.
In special cases, when reinforcing monolithic columns, special requirements may be made with regard to ductility, weldability, corrosion resistance and fatigue strength. As a rule, this is due to the specifics of the concrete mix and cement used. Of key importance in almost every case of reinforcement is the nature of coupling with concrete. The lack of adhesion can be compensated by the design of the profile with grooves and ridges. The same hot-rolled and cold-deformed rods can have annular and sickle-shaped protrusions of different sizes. On the contrary, many brands of concrete with a fragile structure allow the use of only smooth cores - for example, class A240. Now it’s worth moving on to a more detailed consideration of the parameters of the reinforcement used in the reinforcement of the columns.
Reinforcement length
When laying a prefabricated column, the formwork parameters are carefully calculated, which should also be part of the reinforcing metal tooling. It is important that the ends of the working rods not connected to the anchor elements are at the following distance from the end of the part:
- 20 mm, if a monolithic column is arranged at least 6 m long.
- 15 mm, if the column has a length of more than 18 m. The same restriction applies to mast structures and supports.
- 10 mm if a prefabricated column with a length of less than 18 m is laid.
In each case, the reinforcing of the column involves the abandonment of the part of the bar, which must be protected with special anti-corrosion agents or additionally insulated with frame equipment.
Rebar diameter
In the case of longitudinal rods, elements with a thickness of at least 16 mm are used. Monolithic prefabricated structures can be strengthened with 12 mm rods. Small diameters are also allowed when using reinforcement made of structural steel with a protective coating. Consideration of the diameter is also important from the point of view of the configuration of its placement in the body of the column. So, longitudinal rods can be installed only in one row and preferably with exposure of equal diameter. If it is planned to reinforce the columns with rods of different thicknesses, then a maximum of two formats is allowed without taking into account structural reinforcing equipment. Rods of different diameters are usually used for reasons of economy, but it is not possible to use adjacent sizes in the same column. For example, laying rods with a diameter of 8 and 10 mm or 10 and 12 mm is not allowed.
Reinforcement Area
Calculation of the area is performed by sections of longitudinal reinforcement. As a result, it is estimated what percentage of the cross section of the rods is occupied on the surface of the column. A maximum of 5% is allowed, but only in the case of a measured arrangement of rods without overlap. The overlap connection doubles the cross-sectional area of the reinforcement at the joints, which does not always allow the correct assembly of the column. You should also maintain the symmetry of the placement of the rods relative to the cross-sectional area of the structure - especially when it comes to future operation of the structure with a high bending load. One way or another, the optimal percentage of column reinforcement will be 2-3%. In the section itself, it is necessary to take into account not only the basis of the bar, but also the protrusions in the form of ridges.
What should be the joining of reinforcing rods?
The connection and outlets of the fittings also determine the reliability of the structure. An important role of overlap has already been noted, which increases with the use of monolithic columns. At the same time, do not underestimate the effect of such ligaments on the structural integrity of the column. The fact is that, for example, a 25-millimeter rod (in diameter) must dock with an overlap of at least 140 cm in length. Moreover, if the docking is run up, this distance is doubled. Therefore, it is recommended to strive to minimize the connecting nodes when reinforcing the columns with longitudinal rods. If it comes to large spans and the implementation of transition zones is inevitable, then the joints are transferred to the places where the cross section of the column itself changes. Such configurations are found in stepped, double-branch and torn structures. Also, as an alternative, a welded joint with overlays is recommended.
Gaps between the rods
To begin with, it is worth emphasizing the importance of the balance between the strengthened mass and the voids in the body of the column. Oversaturation of working metal rods weakens the concrete structure, making it more sensitive to dynamic loads. Conversely, the lack of reinforcing equipment increases the risk of damage to the column when operating under static loads. Even if the floors and the reinforced column act on each other in moderate pressure indices, then after a while cracks will begin to form on weakened sections of the structure. It is possible to maintain balance by maintaining the standard distance between reinforcing bars of 400 mm. If this distance is not enough due to the minimum inclusion of crushed stone or stone in the solution, then large gaps are diluted due to structural thin reinforcement with a diameter of 12 mm.
Limitations of the protective reinforcing layer
The maximum longitudinal reinforcement layer is 50 mm. This thickness includes the core of the rod, and its structural elements with a coating. The possibility of using rods with a diameter of 40 mm while maintaining technological 10 mm is determined by the fact that the reinforcing layer itself may require additional reinforcement. In particular, the reinforcement of columns with a cross section of 600x800 mm involves the inclusion of a welded mesh, clamps and ties. Large-format rods are additionally fastened together by reinforcing ligaments. Moreover, additional reinforcement elements of the reinforcement itself should not be confused with overlays during welding, which perform the responsible structural task of connecting two or more rods.
The main limitation concerns the thickness of the protective layer, which is due to the proportional increase in the risks of cracking of the column at the points of passage of the rods. The stress experienced by the concrete structure with foreign inclusions will be excessively high and will lead to destruction under dynamic loads. This factor is partly offset by the aforementioned grids and clamps, but it is best to initially observe the norms for the formation of the reinforcing layer.
Cross reinforcement requirements
In column constructions where the calculated transverse force cannot be provided only by the concrete structure, transverse reinforcement is also used. The step when laying it should be no more than 300 mm. If it is planned to carry out compressed reinforcement, then the calculation of column reinforcement according to the indentation is done based on the thickness of the rods - the step should be no more than 15 diameters, but fit into 500 mm. As for the interactions of transverse and longitudinal reinforcement, it will depend on the cross section of the column and its saturation with working rods. In principle, two configurations are possible. In one, mating is not allowed, since a layer of longitudinal rods is arranged closer to the edge, and the transverse rods are laid in the spaces left. In the second embodiment, joints are made if the longitudinal reinforcement is implemented in several rows from the edge to the central part. Basically, transverse thin rods are connected to structural rods with a diameter of not more than 12 mm.
Column Reinforcement Technology
Reinforcement methods vary in knitting techniques, formwork approaches and rod placement configurations. As for knitting, it can be done using wire or welded. In the first case, it is recommended to use a knitting construction gun for reinforcement, and in the second, an inverter welding machine for accurate connection. At this stage, the frame is formed. The reinforcement configuration for the columns may be different depending on the characteristics of the structure. It is optimal to use a combined version with the use of longitudinal and transverse reinforcement, in which adjacent knitting of two frames will be realized. The formwork structure is arranged using molding blanks into which the prepared metal skeleton is immersed and subsequently poured with concrete. Differences in the methods of formwork are reduced to the type of material used - wood, polystyrene foam or combined fibrous materials. In this choice, the main condition is the possibility of combining reinforcement and formwork by weight and technical loads in general.
Column foundation reinforcement
Construction columns are installed on the foundation, the so-called bearing glass, which is also subjected to reinforcement. To form part of the structural sole, grades of heavy concrete with a high strength class are used. Glass reinforcement is performed by hot-rolled rods with a periodic profile. Of key importance when reinforcing the foundation under the column will be the node for pairing the soles of the soles with the elements of the main longitudinal reinforcement. For this ligament, at the transition from the sole to the stem of the column, welding of rods with washers to the skeleton of hot-rolled glass rods is used. The difficulty lies only in the correct transition from one level to another, observing the symmetry of the reinforcing contours.
Features of spiral reinforcement
The most difficult, from the point of view of the device rods, is the strengthening of columns with a circular cross section. The problem is complicating the configuration of the reinforcing layer, which requires additional support. In such systems, indirect reinforcement with spiral metal bars is used. Features of the reinforcement of round columns is expressed in the fact that the longitudinal rods are additionally entwined around the perimeter with turns of laid-on wire. The diameter of the spiral is not more than 20 cm.
Column consoles reinforcement
Due to the lack of the ability to install supports for the column, builders often use cantilevered protrusions as an element of structural reinforcement. It is recommended to install such parts on a steel reinforcing frame, which can enter the upper floor or the lower foundation. The consoles are reinforced with metal rods of small diameter, clamps and a welded mesh, depending on the design parameters. The greatest effect of strengthening the columns in the composition with consoles can be achieved with a homogeneous ligament of overlap, the main frame of the trunk and sole.
Conclusion
Features of the use of reinforcement under the columns are determined by the structural insulation of this part of the structure. Of course, both floors from the upper and lower parts provide the necessary support, but overpressure with load can adversely affect the structure of the column. It is to prevent internal destruction processes that use longitudinal and transverse reinforcement. At the same time, the requirements give considerable freedom to designers both in the choice of rods and in the configurations of their bookmarks. Fundamental restrictions relate mainly to the selection of materials, the appointment of sizes and methods of installing the frame.