A singly reinforced beam only has steel reinforcement on the tension side (along the bottom of the cross section) where as a doubly reinforced beam has steel reinforcement on both the tension and compression sides, ie. the top and bottom of the cross section.
when our demand is more than capacity then designer prefers to choose doubly reinforced beam .for example your maximum moment is greater than nominal momentthen doubly reinforced beam is used.
Singly reinforced beams have steel reinforcement in tension zone but in doubly reinforced beam steel reinforcement is provided in the tension zone as well as in compression zone
if reinforcement is provided in tension zone in a beam, then that beam is called singly reinforced beam.if the reinforcement is provided in top & bottom of the beam i.e., at tension zone as well as compression zone also then that beam is called doubly reinforced beam.
Under Reinforced Section
A doubly reinforced beam is provided reinforcement in compression side also as to take care of extra load due to big section and to take care of tension in compression side ie compressive stresses so the section is safe.
Singly reinforced beams have reinforcing only on the tension face. Doubly reinforced beams have reinforcing on the tension and compression face. Doubly reinforced beams can increase section capacity, but are primarily used to increase the ductility of the concrete beam. In certain codes, if the concrete section is controlled by the concrete failing in compression before the tension steel rebar yields (called a compression-controlled or brittle section), the calculated section capacity must be reduced to account for the brittle and sudden/unpredictable nature of a compression controlled (also called a brittle) failure. In order to make the section controlled by the yielding of the tension reinforcement (called a tension-controlled or ductile section), the ductility must be increased. This can be accomplished by adding steel in the compression zone, which gives strength to the compression area, thereby delaying the failure of the concrete. Enough compression steel can cause the section to switch from a brittle mode-of-failure to a ductile mode-of-failure, thereby permitting the designer to not decrease the section capacity from its calculated value. When identifying a doubly- reinforced beam, be sure that the steel in the compression area is meant to be for strength. Oftentimes, all faces of a beam will have some reinforcing for temperature/shrinkage protection and to tie the stirrups to. Steel that is only there for temp/shrinkage or to facilitate tying the rebar will usually be small and not much of it. In buildings, they will be often #3 bars @ 12" centers. In bridges, they may be #4 bars. These small bars should not be counted as compression-zone reinforcing. Look for bars in the compression zone that are of similar size to the main tension reinforcing.
A number is even if it is divisible by 2. It is doubly even if it is divisible by 4.
Reinforced concrete is made by placing steel bars in the tension zone of the beam x-section.
Bi-truncated conic section, or doubly-truncated conic section
Surely its centrally located if the section is symmetric
When the maximum stresses in steel and concrete simultaniously reaches allowable value the section is called balanced section when the %of steel in a section is less than that required for a balanced section it is under reinforced section when the %of steel in a section is more than that required for a balanced section it is over reinforced sectionover reinforcement is as per section design I.e Ast1+Ast2>Ast Ast1+Ast2