what is the basic difference between a singly reinforce
section & a doubly reinforce section?
Answer Posted / aziz ul hakeem
Singly Reinforced Beams:
Ductile failure is always desirable and brittle failure should be always avoided.
Most beams in real life structures are Singly Reinforced Beams where steel bars are provided only in the Tension Face (hanger bars in the compression zone to hold the stirrups are not for structural reasons).
An upper limit on the amount of steel in Tension Zone is imposed to make sure that the failure should be always ductile. This is generally called As-max (based on pmax) which is about 75% of the balanced steel ratio or steel reinforcement.
Doubly Reinforced Beams:
However in real life situations where it is not possible to increase the beam depth and the applied bending moments are greater than the beams capacity with the As-max then we go for doubly reinforced beams. First the total amount of steel reinforcement is calculated and then the steel in excess of the As-max is provided both in the tension and compression zones. i.e. if 3 no. 8 bars, in excess of the As-max, are required in the tension zone then 3 no. 8 bars should be provided in compression zone and As-max+3 no. 8 bars should be provided in the tension zone.
Example:
In tension face/zone – Reinforcement Steel = As-max+3 no. 8 bars
In compression face/zone – Reinforcement Steel = 3 no. 8 bars
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