Why Crank Bars are Provided in Slabs, Beams and Columns
The crank bars are the reinforced steel bars along with the hooks or bends at the specific angles, which are typically 45° or 30°. The crank bars have been placed strategically in the concrete structures, including slabs, columns and beams. This helps to increase the durability and strength through resisting the comprehensive and tensile forces.
Different types of shapes of the crank bars have been provided in the slabs and other structural elements. The maximum tensile strength is the positive moments which act within the middle of the slab. In this context, the maximum comprehensive stress is the negative moments which act at both ends of the support. The bottom steel has been required at the mid-span. In addition, the top steel helps to resist the negative moments at the support. The crank bars have been provided to the RCC slabs to save from the comprehensive stresses. While the crank bars have been provided to the slabs, the deformation capacity and strength of the slabs along with the bent up bars have been increased as compared to the slabs without any bent up bars.
Purpose of using crank bars in the construction process
The purpose of using crank bars within the construction process is as follows:
➢ These types of bars have been used for resisting the negative bending moments.
➢ The crank bars have also been used in the construction process for resisting the shear forces that are greater at the support.
➢ The crank bars have been used for the economization of the materials.
➢ The other purpose of using the crank bars is reducing the risk of the brittle failure of the slab-column connection.
➢ The crank bar is also used for reducing the amount of steel used in the construction.
Different types of crank bars
U-shaped Crank Bars
These types of crank bars have bends at both ends of the bar, which helps to form a U-shape. This U-shaped crank bar has been used in the foundation elements and slabs.
L-shaped Crank Bars
These types of crank bars have bends at one end of the bars and form an L-shape. This L-shaped crank bar has been used in the beams and columns.
The installation procedure of the crank bars
Proper placement of the crank bars
The crank bars have to be positioned on the basis of the design specifications, which helps to ensure the adequate coverage and spacing of the bars within the concrete members.
Secure anchoring of the crank bars
The bends of the crank bars should be embedded securely within the concrete, which helps resist the pull-out forces and ensure the effective transfer of the loads.
Standards for constructing crank bars
There are several standards that help to ensure that the crank bars meet the specific performances and safety criteria.
International standards
The crank bars, which are used in the construction process, have been made on the basis of the international codes, for example, Eurocode, BS (British Standards), or ACI (American Concrete Institute). These international standards help to specify the dimensions, spacing requirements, angles of bends and the placement requirements of the crank bars.
Specification of the materials
The high-strength steel has been used in the crank bars, which must adhere to the BIS standards for the mechanical properties. This material specification of the crank bar helps to ensure the structural integrity and durability.
Fabrication tolerances
The BIS standard also helps to define the permissible tolerance for the bends, ddimensions, and hooks which helps to ensure the reliability and consistency in the construction process.
Reason behind using crank bars in the slabs
There are several reasons for using crank in the slabs, which are:
➢ Crank bars help to prevent the negative bending moments in the slabs, which leads to the failure in the case of not handling properly.
➢ At the slab supports, shear forces are higher. In this conext, the crank bars help to spread the shear forces thus help to boost the structural stability of the slabs.
➢ Extra strength has been added in the slabs with the help of the crank bars, which helps to reduce the chances of sudden failure.
➢ The requirement for steel has also been reduced by using crank bars, which helps to save money in the construction process.
➢ The crank bars also help to make the slab more strong and flexible, which helps to keep the entire structure stable.
The process of ensuring the proper use of crank bars in the construction procedure
The proper use of the crank bars in the slab construction process can be ensured in several ways, which are as follows:
Correct bending
It is necessary to ensure that the crank bars are bent accurately at the required angles, which helps to match the specific design requirements.
Accurate spacing and placement
It is also crucial to have proper spacing between the crank bars. This proper spacing helps to ensure that the loads have been distributed evenly. In addition, the accurate spacing and placement of the crank bars also helps to prevent the weak point in the slabs.
Accurate lap length
The maintenance of the right overlap between the crank bars is also necessary for ensuring that the crank bars work together more effectively while providing the reinforcement.
To learn more, watch the following video tutorial.
Video Source: Engineering Infinity
Regular inspection
The placement of the crank bars needs to be inspected properly before pouring the concrete. The regular inspection helps to avoid misalignment in the crank bars that could weaken the structure.
