The device of concrete and reinforced concrete structures provides for additional reinforcement due to reinforcing bars. The latter, by the way, is one of the most demanded segments of ferrous metallurgy, which is confirmed by its wide use in construction. With regard to 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 bars in different configurations is the optimal solution to the problem.
General Rebar Requirements
Hot-rolled, thermomechanically hardened and cold-formed metal bars of various profiles can be used for columns. The average diameter varies from 12 to 40 mm. If you plan to use cold-formed rodsperiodic profile, then a small diameter of 3-12 mm can also be used. In terms of tensile strength, classes A and B are allowed, corresponding to guaranteed yield strengths with a coefficient of at least 0.95.
In special cases, the reinforcement of monolithic columns may have special requirements regarding ductility, weldability, corrosion resistance and fatigue strength. As a rule, this is due to the specifics of the concrete mixture and cement used. Of key importance in almost every case of reinforcement is the nature of the bond 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-formed rods can have annular and crescent-shaped protrusions of different sizes. Conversely, many brands of concrete with a brittle structure allow the use of only smooth rods - for example, class A240. Now it is worth moving on to a more detailed consideration of the parameters of the reinforcement used in strengthening the columns.
Rebar length
When laying a prefabricated column, the parameters of the formwork are carefully calculated, which should organically include reinforcing metal equipment. It is important that the ends of the working rods that are not connected to the anchor elements are at the following distance from the end of the part:
- 20 mm if a monolithic column with a length of at least 6 m is arranged.
- 15 mm if the column is longer 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 is laidm.
In each case, the reinforcement of the column involves leaving a 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 also be reinforced with 12 mm rods. Also, small diameters are allowed when using reinforcement made of structural steel with a protective coating. Accounting for the diameter is also important from the point of view of the configuration of its placement in the body of the column. So, the longitudinal rods can be installed only in one row and preferably with a shutter speed of equal diameter. If it is planned to reinforce the column 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 in order to save money, but adjacent sizes cannot be used in the same column. For example, it is not allowed to lay rods with a diameter of 8 and 10 mm or 10 and 12 mm.
Reinforcement area
Calculation of the area is performed according to the sections of the longitudinal reinforcement. As a result, it is estimated what percentage of the section of the bars occupy 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 sectional area of the reinforcement at the joints, which does not always allow for the correct assembly of the column. You should also maintain the symmetry of the placementrods relative to the cross-sectional area of the structure - especially when it comes to the future operation of a structure with a high bending load. One way or another, the optimal percentage of column reinforcement will be 2-3%. In the section itself, one should take into account not only the base of the bar, but also protrusions in the form of ridges.
What should be the joining of reinforcing bars?
Connection and rebar outlets also determine the reliability of the structure. The important role of overlap has already been noted, which increases with the use of monolithic columns. At the same time, the impact of such ties on the structural integrity of the column should not be underestimated. The fact is that, for example, a 25-mm rod (in diameter) must be joined with an overlap along a length of at least 140 cm. Moreover, if the docking is done in a run, then this distance is doubled. Therefore, it is recommended to strive to minimize the connecting nodes when reinforcing the column with longitudinal bars. If it concerns large spans and the implementation of transition zones is inevitable, then the joints are transferred to the places where the section of the column itself changes. Such configurations are found in stepped, two-branch, and break designs. Welding with pads is also recommended as an alternative.
Spaces between rods
To begin with, it is worth emphasizing the importance of the balance between the reinforced 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 disadvantagereinforcing 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 indicators, then after a while cracks will begin to form in the weakened sections of the structure. Balance can be maintained by maintaining a standard distance between reinforcing bars of 400 mm. If this distance is not enough due to the minimal inclusion of crushed stone or stone in the solution, then large gaps are diluted with structural thin reinforcement with a diameter of 12 mm.
Restrictions of the protective reinforcing layer
The maximum layer of longitudinal reinforcement is 50 mm. This thickness includes both the base of the rod and its coated structural elements. The possibility of using rods with a diameter of 40 mm while maintaining technological 10 mm is due to the fact that the reinforcing layer itself may require additional reinforcement. In particular, the reinforcement of columns with a section of 600x800 mm provides for the inclusion of a welded mesh, clamps and ties. Large-format rods are additionally fastened together with reinforcing ligaments. Moreover, additional elements of reinforcing the reinforcement itself should not be confused with overlays during welding, which perform the important structural task of connecting two or more rods.
The main limitation concerns the thickness of the protective layer, which is due to a proportional increase in riskscracking of the column in the places where the rods pass. The stress experienced by a concrete structure with foreign inclusions will be excessively high and, under dynamic loads, will lead to destruction. This factor is partly offset by the aforementioned meshes and clamps, but it is best to initially comply with the norms for the formation of a reinforcing layer.
Requirements for transverse reinforcement
In column structures, 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 perform a compressed reinforcement, then the calculation of the reinforcement of the column by offsets 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 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 transverse rods are laid in the gaps left. In the second option, 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 differ in tying techniques, formwork approaches and bar placement configurations. As for knitting, it can be done with wire orwelded way. In the first case, it is recommended to use a knitting construction gun for fittings, and in the second, an inverter welding machine for precise connections. At this stage, the frame is formed. The configuration of the reinforcement under the columns can 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 also be implemented. The formwork structure is arranged with the help of molding blanks, into which the prepared metal skeleton is immersed and then it is poured with concrete. Differences in the methods of creating formwork come down 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 in terms of weight and technical loads in general.
Reinforcement of column foundations
Building columns are installed on the foundation, the so-called carrier glass, which is also reinforced. To form part of the structural sole, heavy concrete grades with a high strength class are used. Reinforcement of the glass is performed by hot-rolled rods with a periodic profile. When reinforcing the foundation for the column, the junction of the sole bars with the elements of the main longitudinal reinforcement will be of key importance. For this ligament, at the transition point from the sole to the column shaft, rods with washers are welded to the skeleton of hot-rolled sleeve rods. The difficulty lies only inthe correct transition from one level to another, observing the symmetry of the strengthening contours.
Features of spiral reinforcement
The most difficult, from the point of view of the arrangement of rods, is the reinforcement of columns with a circular cross section. The problem lies in the complication of the configuration of the reinforcing layer, which requires additional support. In such systems, indirect reinforcement with spiral metal rods is used. Features of the reinforcement of round columns are expressed in the fact that the longitudinal rods are additionally wrapped around the perimeter with coils of overhead wire. In this case, the diameter of the spiral is no more than 20 cm.
Reinforcement of column consoles
Because of the lack of options for installing column supports, builders often use cantilever ledges as an element of structural reinforcement. It is recommended to install such parts on a steel reinforcing frame, which can be included in the upper ceiling or in the lower foundation. Consoles are reinforced with metal rods of small diameter, clamps and welded mesh, depending on the design parameters. The greatest effect of strengthening columns in the composition with consoles can be achieved with a homogeneous bunch of overlap, the main frame of the trunk and the 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 overlaps at the top and bottom provide the necessary support,but overpressure with load can directly affect the structure of the column. It is to prevent internal processes of destruction that longitudinal and transverse reinforcement is used. At the same time, the requirements give considerable freedom to designers both in the choice of rods and in the configurations of their laying. Fundamental limitations relate mainly to the selection of materials, the appointment of dimensions and methods of installing the frame.
