How to design of singly reinforced beam in details

BEAM:-

A Beam is any structural member which resists load mainly by bending. Therefore it is also called flexural member. Beam may be singly reinforced or doubly reinforced. When steel is provided only in tensile zone (i.e. below neutral axis) is called singly reinforced beam, but when steel is provided in tension zone as well as compression zone is called doubly reinforced beam.

The aim of design is:

To decide the size (dimensions) of the member and the amount of reinforcement required.

To check whether the adopted section will perform safely and satisfactorily during the lifetime of the structure.

FEW DEFINITIONS

OVERALL DEPTH :-

The normal distance from the top edge of the beam to the bottom edge of the beam is called Overall depth. It is denoted by ‘D’.

EFFECTIVE DEPTH:-

The normal distance from the top edge of beam to the centre of tensile reinforcement is called Effective depth. It is Denoted by ‘d’.

CLEAR COVER:-

The distance between the bottom of the bars and bottom most the edge of the beam is called Clear Cover.

EFFECTIVE COVER:-

The distance between centre of tensile reinforcement and the bottom edge of the beam is called Effective Cover.

 EFFECTIVE COVER = CLEAR COVER + ½ DIA OF BAR.

END COVER:-

End Cover = 2X Dia. of Bar OR 25mm (Whichever is Greater)

NEUTRAL AXIS:-

 The layer / lamina where no stress exist is known as Neutral Axis. it divides the beam section into two zones, compression zone above the neutral axis & tension zone below the neutral axis.

DEPTH OF NEUTRAL AXIS:-

The normal distance between the top edge of the beam & neutral axis is called Depth of neutral Axis. It is denoted by ‘n’.

LEVER ARM:-

The distance between the resultant Compressive force (C)  and Tensile force (T) is known as Lever arm. It is denoted by ‘z’. The total compressive force (C)  in concrete act at the C.G. of Compressive stress diagram i.e. n/3 from the compression edge. The total tensile force (T) acts at C.G. of the reinforcement.

                              LEVER ARM = d-n/3

TENSILE REINFORCEMENT:-

The reinforcement provided tensile zone is called tensile reinforcement. It is denoted by Ast.

COMPRESSION REINFORCEMENT :-

The reinforcement provided compression zone is called compression reinforcement . It is denoted by Asc.

TYPES OF BEAM SECTION:-

 The beam section can be of the following types:

Balanced Section

Unbalanced Section

(a) Under - Reinforced Section

(b) Over- Reinforced Section

BALANCED SECTION:- A section is known as Balanced Section in which the compressive stress in concrete  (in Compressive zone) and tensile stress in steel will both reach the maximum permissible values simultaneously.

The neutral axis of balanced (or critical) section is known as Critical neutral axis (nc). The area of steel provided as economical area of steel . Reinforced concrete sections are designed as balanced sections.

2. UNBALANCED SECTION:-This is a section in which the quantity of steel provided is different from what is require for the balanced section.

Unbalanced sections may be of the following two types:

(a) Under - Reinforced Section

(b) Over- Reinforced Section

(a) UNDER-REINFORCED SECTION:- If the area of the steel provided is less than that required for balanced section, it is known as Under - Reinforced Section. Due to less reinforcement the position of actual neutral axis (n) Will shift above the critical neutral axis (nc)i.e. n< nc. In Under-Reinforced section steel is fully stressed and concrete is under stressed (i.e. Some concrete remains un-utilised). Steel being ductile, takes some time to break. This gives sufficient warning before the final collapse of the structure. For this reason and from economy point of view the Under-Reinforced sections are designed.

(b) OVER-REINFORCED SECTION:- If the area of steel provided is more than that required for a balanced section, it is known as Over- Reinforced Section, As the area of steel provided is more, the position of neutral axis will shift towards steel, therefore Actual axis (n) is below the critical neutral axis (nc)i.e. n > nc. In this section concrete is fully stressed and steel is under stressed. Under such conditions, the beam will fail initially due to over stress in the concrete . Concrete being brittle, this happens suddenly and explosively without any warning.