Design Concept of Rectangular Doubly Reinforced Section

Double reinforced section is provided when moment acting on section is more than limiting MoR. Generally, double reinforcement section is design as balanced or under reinforcement section.

if BMu > MoR,lim

Provide doubly reinforced section,

BMu = MR1+MR2

MR1 = MoR,lim

BMu = MoR,lim+MR2

a) MoR,lim = 0.87 fy Ast₁ (d-0.42 Xu,lim)

Ast,lim =Ast₁ = MoR,lim/0.87 fy Ast₁ (d-0.42 Xu,lim)

b) MR₂ = BMu -MoR,lim

MR₂ =0.87 fy Ast₂ (d-d’)

Ast₂ = MR₂ /0.87 fy (d-d’)

also, MR₂ =A_sc (f_sc – f_cc )(d-d’)

Note : In IS code value of f_cc is neglected, but may be assumed to be 0.45fck.

Value of f_cc is read either from curve or from given stress-strain table, but if are not mentioned in the problem, it may be taken as follows:

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Numerical Example to find Ast and Asc of given section.