What is Doubly & Singly reinforced Beam-why we provide it

Beam can be defined as a structural member which carries all vertical loads and resists it from bending. There are various types of materials used for beam such as steel, wood, fibers etc. But the most common material is reinforced cement concrete

The loads carried by a beam are transferred to columns, walls,or girders, which then transfer the force to adjacent structural compression members.

Simply supported are mainly two types are describe as following

SINGLY REINFORCED BEAM:

The beam that is longitudinally reinforced only in tension zone, it is known as singly reinforced beam. In Such beams, the ultimate bending moment and the tension due to bending are carried by the reinforcement, while the compression is carried by the concrete.

But it is not possible to provide reinforcement only in the tension zone, because we need to tie the stirrups. Therefore two re bars are utilized in the compression zone to tie the stirrups and the re bars act as false members just for holding the stirrups.

DOUBLY REINFORCED BEAM:

Doubly Reinforced Beams have Compression Reinforcement in addition to Tension Reinforcement and this compression reinforcement may be on either side of beam (Top or Bottom face) depending upon type of beam i.e. Simply supported or Cantilever respectively.

In other words, Beams reinforced with steel in compression and tension zones are called doubly reinforced beams. 

Why we provide Doubly reinforced Beam

• This type of beam is mainly provided when the depth of the beam is restricted. If a beam with limited depth is reinforced on the tension side only it might not have sufficient resistance to oppose the bending moment. In order to increase the moment carrying capacity of the section.
• The member is subjected to a shock or impact or accidental lateral thrust.
• Doubly reinforced beam is provided to increase the moment of resistance of a beam having limited dimensions.
• Minimum compression reinforcement is provided to hold the Shear Reinforcement (stirrups) in position and for increasing the ductility of beam.
• For Safety reason we always provide doubly reinforced beam to counter Wind Forces, Seismic Forces and temperature stresses.

Design procedure of Doubly reinforced Beam

Determine the limiting moment of resistance

Mulim = 0.87.fy.Ast1.d [1 – 0.42Xumax]

Step 2

If factored moment Mu > Mulim, then doubly reinforced beam is required to be designed for additional moment.

Mu – Mulim = fsc.Asc (d – d’)

Step 3

Additional area of tension steel Ast2

Ast2 =Asc.fsc/0.87fy

Step 4

Total tension steel Ast, Ast = Ast1 + Ast2