Question

The moment of resistance of a rectangular section depends upon
1. ultimate strain in concrete
2. area of high tensile tendons
3. tensile strength in concrete
4. compressive stress in concrete

Answer 1

Correct Answer - Option 2 : area of high tensile tendons

Concept:

Moment of resistance:

• It is defined as the algebraic sum of moments of internal forces about the neutral axis of the section.
• For the safety of the structure/section, the externally applied moment should be less than or equal to the moment of resistance of the section.

We know,

For prestressed concrete:

As per IS 1343:2012, cl. D.1.

Moment of resistance of rectangular section is given by -

\(M_u = f_{pb} A_{ps}(d-0.42x_u)\)  

Here,

f_pb - tensile stress in the tendon at failure

A_ps- the area of pre tensioning tendons in the tension zone

d - effective depth to the centroid of the steel area

x_u - neutral axis depth.

Explanation:

Moment of resistance of rectangular section is given by -

\(M_u = f_{pb} A_{ps}(d-0.42x_u)\)   

The moment of resistance of the section depends upon -

• Area of tensile reinforcement
• Tensile stress in the tendon at failure
• Depth of the section

For reinforced concrete:

As per IS 456:2000, cl. G.1.1.

For singly reinforced concrete section,

Limiting moment of resistance of the rectangular section is given by -

In terms of concrete compressive strength (i.e. f_ck)

\(M_u = 0.36f_{ck}bx_u(d-0.416x_u)\)

In terms of steel tensile stress (i.e. f_y)

\(M_u=0.87f_yA_{st}(d-0.416x_u)\)