Curtailment of Reinforcement in Concrete Beams and Slabs

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To economize the design of a flexural member, the tensile bars are curtailed at the section beyond which it is no longer required to resist flexure(bending) given that the remaining reinforcement of the flexural member will be able to resist the bending moment and shear created on the flexural member safely. The reinforcement will be provided to a certain length at the supports called the development length. The extra part that is removed from the reinforcement which forms the curtailed reinforcement. The curtailed bars in flexural members are extended for a distance equal to 12 times the diameter of the reinforcement bar or effective depth, whichever is greater, except at simple support or free end of the cantilever.

Curtailment of Reinforcements in Concrete Beams

The figure-1 below shows the curtailment of reinforcement in a simply supported beam. At simple support, positive moment tension reinforcement shall be limited to a diameter such that L_d computed for f_d does not exceed

where M1= Moment of resistance of the section assuming all reinforcement at the section to be stressed to f_d, f_d = 0.87 f_y, V= Shear force at the section due to design loads, L₀= Sum of anchorage beyond the centre of the support and the equivalent anchorage value of hook, etc., and = Diameter of bar. The value of (M1/V) in the above expression may be increased by 30% when the ends of reinforcement are confined by a compressive reaction. At least one-third of positive moment reinforcement in simple member shall extend along the same face of the member into the support, to a length equal to Ld/3.

Curtailment of Reinforcement Bars in Slabs

The bars that is curtailed in reinforced concrete slabs have to be fixed and anchored past the point where it is supported. The shear force and bending moment is initially calculated to determine the point where the curtailment has to be done. In slab design, we divide the member into two, middle and column strips in each direction as shown in figure-2. Wherever the moment is less, there is no need for reinforcement that will make curtailment necessary in those areas. The curtailment can be provided by cutting the bottom reinforcement meant for positive bending moment as shown in figure-2 which gives straight bars or it can be bent up which will be extended to the adjacent bars as shown in the figure-3.

Conditions for Curtailment of Flexural Reinforcement at Tension Zone

Flexural reinforcement shall not be terminated in the tension zone unless one of the following conditions are fulfilled:

• The shear at the cutoff point does not exceed two-thirds of that permitted.
• The stirrup area in excess of that required for shear and torsion is provided along each terminated bar over a distance from cutoff point equal to three-fourth the effective depth of the member. The excess stirrup area shall be not less than 0.4 bs/f_y, where b is the breadth of the beam and s is the spacing. The resulting spacing shall not exceed is the ratio of the area of bars cutoff to the total area of bars at the section.
• For 36 mm and smaller bars, the continuing bars provide double the area required for flexure at the cutoff point and the shear does not exceed three-fourths of that permitted.

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