GATE Civil Engineering (CE) 2027 Test: Beams & Slabs - 1 Free Online Test

In doubly reinforced beam we provide some steel in compression zone to reduce the depth of beam.

Since steel is 10 times costlier than concrete, the beam cost is going to be increased.

In general, doubly reinforced beams have a dimensional constraint, as a result of which larger quantities of steel reinforcements are required.

As per clause 37.1.1 of IS: 456-2000 the ultimate moment of resistance at any section should not be less than 70% of the factored moment at that section as obtained from the elastic moment envelope (considering all loading combinations).

Steel Is produced in factory in which controlled environment is possible. However, concrete is produced at site itself leading to more chances of carelessness. Thus, higher factor of safety for concrete in comparison to steel.

The partial safety factor for concrete (1.5) is higher than that of steel (1.15) because of higher variability associated with concrete i.e. quality control of concrete is not as good as that of steel.

Torsional Reinforcement detailing in Slab:

Torsional reinforcement is provided in the form of a grid or mesh both at the top and bottom of the slab. IS 456: 2000 recommends that the torsional reinforcement grid should extend beyond the edge for a distance not less than 20% of a shorter span. The total area of torsional steel provided in each of the four layers should not be less than:-

No torsional reinforcement is required if both the meeting edges are continuous.

The diameter of the bars shall not exceed one eight of the total thickness of the slab. 

∴  Maximum size = 75/8= 9.375 mm
So the diameter will be 8 mm.

Balanced Design means both concrete and steel will fail at the same time. Thus, it gives optimum smallest concrete section for a given reinforcement and maximum reinforcement for a given concrete section. 

As per IS 456:2000 for lateral stability, the clear span of
Simply supported beam should be less than 60b or 250(b²)/d 
Cantilever Beam should be less than 25b or 100(b²)/d

A slab is always designed for a shorter span.

As per clause 24.1 of IS 456:2000,

The shorter of the two spans should be used to determine the span to effective depth ratios (A-value).

For spans up to 3.5 m and with mild steel reinforcement, the span to overall depth ratios satisfying the limits of vertical deflection for loads up to 3 kN/m² is as follows:

The horizontal distance between parallel main reinforcement bars shall not be more than three times the effective depth of solid slab or 300 mm whichever is smaller. The total reinforcement in the slab should remain same. By replacing 10 mm bars by 12 mm bars, the spacing will increase as