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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