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For a reinforced concrete section, the shape of shear stress diagram is
The ratio of permissible shear stress in limit state method of design and working stress method of design is
Permissible shear stress.
In limit state method of design,
τc = 0.25√fck
in working state method of design,
τc = 0.16√fck
∴ Ratio = 25/16
The contribution of bent up bars towards shear resistance shall not be more than
Total shear strength = shear resistance of effective concrete area as a function of longitudinal bars + shear resistance of vertical shear stirrups + shear resistance of-inclined shear stirrups.
Tests have shown that inclined bars alone do not provide a satisfactory solution and their contribution is limited to 50% of net shear strength after deducting the contribution of concrete. The remaining shear resistance is provided by vertical stirrups.
If the nominal shear stress (ζv) at a section does not exceed the permissible shear stress (τc)
If the calculated shear stress ( ζv ) is
(i) less than allowable shear stress (ζc) but more than 0.5ζc, the minimum shear reinforcement in the form of stirrups shall be provided such that,
(ii) more than ζc, shear reinforcement shall be provided in the form of vertical stirrups or bent up bars with stirrups or inclined stirrups to resist
Vus = Vu - τcbd. Here τv < τc,max
(iii) If τv > τc,max redesign the section.
A beam of rectangular cross-section (b x d) is subjected to a torque T. What is the maximum torsional stress induced in the beam (b < d and α is a constant)?
Torsion constant of a rectangular section of width b and depth d (b < d) may be expressed as,
J = b3d
For T, L and / sections torsion constant,
where bi and di are the dimensions of each of the component rectangles into which the section may be divided.
Torsional shear stress for rectangular section
τ = T/αb2d
For T, L and / sections torsional shear stress may be calculated for each component rectangle by considering them subjected to torsional moment,
The minimum cover in a slab should neither be less than the diameter of bar nor less than
When shear stress exceeds the permissible limit in a slab, then it is reduced by
Shear stress in slabs is controlled by increasing the depth of slab and not shear reinforcement while in beams shear stirrups are provided to control shear.
Flexural shear failure in a reinforced concrete beam
1. occurs under large bending moment and less shearforce
2. results in cracks which are normally at 90° with the horizontal
3. occurs when shear span to effective depth ratio is less than 1
Which of these statements are correct?
For M 20 grade of concrete, the maximum shear stress shall not exceed