GATE 2017 CE (CE1) – Question 16

Concrete Structures · Working stress and limit state design concepts · 1 mark · Multiple choice

According to IS 456:2000, which statement about the balanced-section neutral-axis depth $x_{u,bal}$ of a reinforced-concrete section is correct?

It depends on the grade of concrete only.
It depends on the grade of steel only.
It depends on the grades of concrete and steel.
It depends on neither grade of concrete nor steel.

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Correct answer: (B) It depends on the grade of steel only.

Explanation

At the limiting balanced strain condition, concrete strain is 0.0035 and tensile steel strain is $0.002+0.87f_y/E_s$. Similar triangles give $x_{u,bal}/d=0.0035/[0.0035+0.002+0.87f_y/E_s]$. For fixed effective depth and steel modulus, this depends on steel grade $f_y$, with no concrete-strength term. Therefore B. The absolute depth also scales with section effective depth.

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