What is Chair Bars | Chair Bars Cutting Length Calculation (BBS)

Chair Bar-

Chair Bars or Chair reinforcement is tiny structural elements that holds the reinforcement bars placed properly in position and maintains space between top and bottom reinforcements.

Mainly used in slab and footing, it is the most significant part of raft foundation. Diameter of a chair bar should not be less than 12mm.

Usage of Chair Bars in Reinforcements-

• Proper holding of reinforcement in position.
• Maintaining the distance between the top and bottom reinforcements.
• Maintains the cover of top and bottom bars.
• Provides support vertically to the upper and lower cage reinforcement.
• Provides strength to the bars.
• Resist the weights of labor and fresh concrete.
• Prevents from collapse during vibration by holding the upper cage.
• Minimizes neatness of the work.
• Avoids reinforcement mess up.
• Increased tension property of the concrete thereby preventing the structure from collapse due to failure in the tension zone.
• Concrete surrounds the reinforcement from all directions including the bottom part as it holds reinforcements.
• A simple device, thus not require skilled labor for placement.
• Hardening of concrete does not create trouble as the chair remains trapped in the concrete avoiding further trouble in the future.

Chair Bar Segments:

There are three specific parts of Chair Bar-

Head of Chair: Horizontal part of chair on top, where the upper cage reinforcement rests.

Height of Chair: Vertical distance of the chair.

Leg of Chair: Vertical portion of chair bar in the bottom.

Evaluation of Cutting Length of Chair Bar:

Data provided-

Height of footing- 600mm

Diameter of top and bottom main bars-20mm

Diameter of top and bottom distribution bars-16mm

Clear cover(top and bottom)- 50mm

At first we need to know about to calculation of each segment of length of chair bars-

Height of Chair= Height of footing – (2 * clear cover) – ( dia of bottom main bar) – ( dia of top main bar + dia of top distribution bar)

=600 – (2 * 50) – 20 – (20 + 16)

=444mm

Head of Chair: Length of head = (2 * spacing of distribution bar) + (2 * 50)

= (2 * 150) + (2 * 50)

= 400mm

Leg of Chair= (2 * spacing of bottom main bar) + 50

= (2 * 150) + 50

= 350mm

Cutting Length of Chair Bar- Head of Chair + (2 *  Height of Chair) + (2 * Leg of Chair) – 4 bend of 90 degree

= Head of Chair + (2 *  Height of Chair) + (2 *  Leg of Chair) – 4 * 2d

= 444 + (2 *  400) + (2 *  350) – 4 * 2 * 12

=1848mm

=1.8m