Design procedure for designing doubly reinforced section

7 step procedure for “Design of Doubly reinforced sections”

In our article series for “Design of Doubly reinforced sections”, we covered the following:

What are doubly reinforced sections?

Methods for determining Neutral Axis?

Solved numerical examples for determining Neutral Axis

Numerical examples for practice (Find Neutral axis)

Methods for calculating Moment of Resistance

Numerical example for calculating Moment of resistance

Types of problems in Doubly reinforced sections

6 step prodecure for determining stresses in steel and concrete 

Numerical example | Stresses in steel and concrete

In our previous article, we discussed a detailed 6 step procedure for determining stresses in steel and concrete followed by a numerical example. Now we shall move on with the “design procedure for doubly reinforced sections”.

Generally the following data are given:

Breadth of the beam = b

Effective depth of the beam = d

Permissible stress in concrete = σcbc

Permissible stress in steel = σst

Modular ratio = m

Bending moment = M

To solve a problem, the following procedure may be followed.

Design the beam as a singly reinforced one (balanced section)

Step One:

Find xc by

σcbc/ (σst/m) = xc/(d – xc)

Step two:

Find Ast by:

C = T

b xc σcbc /2 = σst.Ast1

Step three:

Find the Mr of singly reinforced balanced beam

Mr = b xc σcbc /2[d – (xc/3)]

Step four:

Find the remaining bending moment ‘M1’

M1 = M – Mr

Step five:

Find Ast2 for M1

M1 = T x lever arm

= Ast2. σst x (d – d’)

= M1 / σst x (d – d’)

Step six:

Ast = Ast1 + Ast2

Step seven:

Find Asc

Equating moments of equivalent area of tensile and compressive steel about Neutral axis(N.A)

mAst (d – xc) = (1.5m – 1) Asc (xc – d’)

Asc = mAst (d – xc)/ (1.5m – 1)(xc – d’)

 In our next article, we shall proceed with a numerical example using the same procedure.

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