When designing, the ultimate or highest bending moment is taken into consideration. Even though the bending moment fluctuates, the reinforcement employed stays the same. It is an inefficient use of reinforcement and might be seen as such.
It can be done theoretically where MoR>BM (TCP).
TCP =Theoretically cutoff point
Reinforcement is not curtailed at theoretically cutoff point (TCP), it is done at actual cutoff point (ACP) due to following reasons:
- Stress concentration by increased stress because of shear reinforcement.
- Slight deviation/increasing in BM calculation.
Distance between TCP and ACP should not be less than “d” and “12 x diameter” (Whichever maximum) except end of the simple support or cantilever.
Exception Cases:
Flexural reinforcement should not be terminated in tension zone unless any one of the following condition is satisfied:
- The shear force at ACP should not exceed 2/3 rd of shear capacity of section.
- If shear force is more than 2/3rd of shear capacity at ACP then excess shear r/f is provided upto the distance of 0.75d from ACP.
3. For 36 mm or smaller diameter bar is the curtaining bar should provide at least double the area required for flexural at the ACP & SF should not more than 3/4 of shear capacity.
Example.
Positive Tension Reinforcement : At least 1/3rd of tension R/F of simple support and 1/4th of positive tension R/F of continuous support should be extended along the same face of members into the support for a distance of not less than Ld/3.
At Simple support and at point of inflection (contraflexure) positive tension R/F should be limited to diameter such that following condition must be satisfied.
Where,
Ld = Development Length
M1 = Moment of Resistance (MoR) at Mid depth of simple support and at the point of inflection respectively.
V= Shear force at mid depth of simple support & at point of inflection respectively.
Lo = for simple support, end anchorage beyond centre of support including equivalent anchorage value for hook and bend.
for point of inflection d or 12 x diameter (whichever maximum)
- Factor 1.3 (i.e. increase of value M1/V by 30 %) is used for simply supported end to account for confinement of concrete due to compressive end compression.
- At least 1/3rd of total reinforcement provided for total negative moment at support, should be extended beyond the point of inflection for distance not less than “d” or “12 x diameter” or “1/16th of clear span” whichever is greater.
