A reinforced concrete member is subjected to combined action of compressive axial force and bending moment. If εc is the least compressive strain in the member, fy, the yield stress of steel and, Es, the modulus of elasticity of steel, the maximum permissible compressive strain in concrete member will be
Maximum permissible compressive strain in concrete under, axial , compression is taken as 0.002
The maximum compressive strain at the highly compressed extreme fibre in concrete subjected to axial compression and bending and when there is no tension on the section shall be 0.0035 minus 0.75 times the strain at the least compressed extreme fibre.
The span to depth ratio limit is specified in IS: 456-2000 for the reinforced concrete beams, in order to ensure that the
Basic values of span to effective depth ratios.for spans upto 10 m to satisfy vertical deflection limits are for
Cantilever beam - 7
Simply supported - 20
Continuous - 26
As per the provisions of IS : 456-2000, in the limit state method for design of beams, the limiting value of the depth of neutral axis in a reinforced concrete beam of effective depth 'd' is given as
The limiting values of depth of neutral axis depending on the different grades of steel is as follows:
If the permissible stress in steel in tension is 140 N/mm2, then the depth of neutral balanced section using working stress method is
Depth of balanced neutral section using working stress method is given by,
A continuous beam is deemed to be a deep beam when the ratio of effective span to overall depth (l/D) is less than
According to IS 456 recommendations,'the maximum depth of stress block for balanced section of a beam of effective depth d is
For a continuous R.C. beam, match List-I (Condition) with List-II (Placement of live load) and select the correct answer using the codes given below the lists.
A. For maximum sagging moment in a span
B. For maximum hogging moment at a support
C. For maximum hogging moment in a span
1. The span adjoining the span as well as alternate span
2. The same span as well as alternate spans
3. The adjacent spans on both sides of this support as well as spans alternate to these
4. Spans next to the adjacent spans of the support plus alternate spans
In a doubly reinforced rectangular beam, the allowable stress in compression steel is
The stress in the compression steel depends on the strain, ∈sc, the level of compression steel. The strain at the level of compression steel under loading does not reach the yield strain value of steel. Therefore, allowable stress is less than the permissible stress in tension in steel. The maximum permissible compressive stress in steel are 130 MPa and 190 MPa for Fe 250, Fe 415 and Fe 500 grades respectively.
The side face reinforcement, if required, in a 7-beam’will be
For beams exceeding overall depth of 750 mm side face reinforcement is provided. Such reinforcement shall be not less than 0.1 percent of the web area and shall be distributed equally on two faces at a spacing not exceeding 300 mm or web thickness whichever is less.
Deep beams are designed for
The effective depth of a singly reinforced rectangular beam is 30 cm. The section is over reinforced and the neutral axis is 12 cm below the top. If the maximum stress attained by- concrete is 50 kg/cm2 and the modular ratio is 18, then the stress developed in steel would be
The limiting value l/D for simply supported and continuous span deep beams are as follow: For simple span deep beam, l/D < 2.0
For continuous span deep beam, l/d ≤ 2.5
l = Effective span taken as centre to centre distance between the supports of 1.15 time the clear span, whichever is smaller,
D = Overall depth
d = 30 cm
xu = 12 cm
C = 50 kg/cm2
m = 18
Which one of the following statements about the percentage of tensile steel required to produce a balanced reinforced concrete section is correct?
The required percentage of steel
For a balanced section,
0.36 fck.b.x = 0.87 fy Ast
As fy increases Ast decreases.
Minimum clear cover (in mm) to the main steel bars in slab, beam, column and footing respectively are
Beam sections designed in accordance with LSM as compared to sections designed in accordance with WSM will have
If the depth of actual neutral axis in a beam is more than the depth of critical neutral axis, the beam is called:
xu > xu,critical : Over reinforced section
xu = xu'critical : Balanced section
xu < xucritical : Under-reinforced section
For a continuous slab of 3 m x 3.5 m size the minimum overall depth of slab to satisfy vertical deflection limits is
L/d ≤ 28 simply supported slab
L/d ≤ 32 continuous slab
L is the short span of slab
∴ 3000/d ≤ 32
d ≥ 9.37 cm = 10 cm
The minimum reinforcement using mild steei in slab should not be less than
Minimum reinforcement (Ast)min in either direction in slabs is given by,
In the reinforced concrete slab, the spacing between main reinforcement should not exceed
Maximum diameter of reinforcing bars in slabs is limited to one-eight of the total thickness of slab and the maximum spacing of main bars is limited to 3d or 300 mm (whichever is less).
The reinforced concrete beam curved in plane is designed for
Flexural collapse in over-reinforced beam is due to
Over-reinforced beam undergoes brittle and sudden failure and is therefore not preferred for designing load bearing members.