Test: Beams & Slabs - 3 - Civil Engineering (CE) MCQ

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.

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

The limiting values of depth of neutral axis depending on the different grades of steel is as follows:

Depth of balanced neutral section using working stress method is given by,
⇒ 

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.

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.

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 
where,
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
x_u = 12 cm
C = 50 kg/cm²
m = 18

For a balanced section, 
0.36 f_ck.b.x = 0.87 f_y A_st
⇒ 
As f_y increases A_st decreases.

x_u > x_u,critical : Over reinforced section
x_u = x_u'critical : Balanced section
x_u < x_ucritical : Under-reinforced section

The shorter span length is 3000 mm.
For two-way continuous slab with mild steel as reinforcement,

Hence, the minimum overall depth of slab for mild steel to satisfy the vertical deflection limit is 75 mm or 7.5 cm.

Minimum reinforcement (A_st)_min in either direction in slabs is given by, 

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).