presentation on rectangular beam design singly or doubly (wsd)

Ahsanullah University of Science and Technology

NAME: RAIHAN MANNANSTUDENT ID : 10.01.03.101

SEC : B

MY TOPIC NAMERECTANGULAR BEAM

DESIGN

Singly Rectangular Beam Doubly Rectangular Beam

Under Working-stress Design

Section of Rectangular Beam

Stress distribution in rectangular beam under working load

Singly Rectangular BeamA 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.

Assumptions for DesignofMembers by Working Stress Method

Plane sections before bending remain plane after bending.

Normally, concrete is not considered for taking the tensile stresses except otherwise specifically permitted. Therefore, all tensile stresses are taken up by reinforcement only.

The stress-strain relationship of steel and concrete is a straight line under working loads.

Singly Rectangular Beam stress distribution

Where,Jd =momnet arm=(1-k/3)Compressive force , C =(bkd/2)fc Tensile force , T = Asfs

neutral axis is determined by equating the moment of the tension area to the moment of the compression area

Moment calculation:Start by determining

Reinforcement calculation:

Design equation for singly reinforcement beam

Doubly Reinforcement beam

If a beam cross section is limited because of architectural or other considerations, it may happen that the concrete can not develop the compression force required to resist the given bending moment, in this case ,reinforcement is added in the compression zone, resulting in so called Doubly reinforced beam .i.e., one with compression as well as tension reinforcement.

Why Doubly reinforcement usedIf concrete can not develop the

required compressive force to resist the maximum bending moment, then additional reinforcement is provide in the compression zone.

Reinforcement is provided in both compression and tension zone.

Stress distribution in doubly reinforced beam

Development of momentIn stress distibution,the design

moment is more than the balanced moment of resistance of the section,

M = M1 + M2

The additional moment M2 is resisted by providing compression reinforcement As’

and additional tensile reinforcement As2

Moment calculationMoment is obtained by, M1=(fc/2)kjbd^2

= As1fsjd M2=As2fs(d-d’) =As’fs’(d-d’)

Reinforcement in BeamThe total tensile reinforcement

Ast has two components As1+As2 for M1 and M2

The equation of Ast,

Ast=As1+As2

Where, As1= (bd/100) And As2 = M2/[Ó (d-d’)] compressive reinforcement,

As’=M2/[fs’(d-d’)]

Clear cover for design

Not less than 1.5 in. when there is no exposure to weather or contact with the ground

For exposure to aggressive weather 2 in.

CLEAR DISTENCE OF REINFORCEMENT

Clear distance between parallel bars in a layer must not be less than the bar diameter or 1 in.

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