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Question

In the design of beams for the limit state of collapse in flexure as per IS: 456-2000, let the maximum strain in concrete be limited to 0.0025, (in place of 0.0035). For this situation, consider a rectangular beam section with breadth as 250 mm, effective depth as 350 mm, area of tensioned steel as 1500 mm2 and characteristic strengths of concrete and steel as 30 MPa and 250 MPa respectively. The depth of neutral axis for the balanced failure is

A
140 mm
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B
156 mm
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C
168 mm
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D
185 mm
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Solution

The correct option is B 156 mm
(i) Maximum strain in concrete = 0.0025
(ii) As per the assumption of limit state design, maximum strain in tension reinforcement at the time of collapse should not be less than
0.87fyEs+0.002
Above two condition will result into maximum depth of N.A. or depth of N.A. for balanced failure.

Depth of N.A.= xu

xu0.0025=(dxu)0.87fyEs+0.002



0.87fyEs+0.002



xu(0.87fyEs+0.002+0.0025)=0.0025d

xu=0.0025×3500.87×2502×105+0.0045

= 156.6 mm

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