Test: RCC- 2 - Civil Engineering (CE) MCQ

Taking Moment of area of tension and compression zone for a singly reinforced beam:

Where,

X_c = critical depth or economic depth of neutral axis

m = modular ratio

A_st = area of steel required for balanced section

m = 18

c = 50 kg/cm2

t = 1400 kg/cm²

For all cases, Effective depth of the beam cross-section is to be taken as:

According to IS 456: 2000, minimum nominal cover to all bars in the slab is 20mm. According to new code, nominal cover does not depend upon the diameter of bar used.

The principle behind prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service.

A prestressed concrete member is one in which there have been introduced internal stresses of such a magnitude, and also distribution, that the stresses resulting from external loading are counteracted to a desired degree. By doing this, Nearly the full capacity of the concrete in compression can be used over entire depth under full loading.

It is necessary to impose a check on the magnitude of deflection in a structural member:

a) To ensure that the extent of deflection does not adversely affect the appearance or efficiency of the structure or finishes or partition etc.

b) To prevent structural behaviour of the member being different from the assumption made in the design. As per IS 456:2000, for beams and slabs, the vertical deflection limits may be assumed to be satisfied, provided that the span to depth ratio are not greater than the values obtained as below: 

Working stress method: working stress method is based on elastic theory. Concrete and steel are assumed to act together elastically and follow hook’s law. This method follows a deterministic approach as it assumes that the loads, factor of safety and permissible stresses are accurately known. In this method material strengths are not fully utilised in designing the member.

Limit state method: This method follows a non-deterministic approach as it adopts probable loads and probable strengths of materials as per actual or based on experience or observations depending upon the situations. Material strengths are fully utilised in designing the member.

As per Bureau of Indian standards,

Period of structural adequacy under fire: The time (t), in minutes, for the member to reach the limit state of structural inadequacy in a standard fire test.

As per IS 456:2000,

1. Lap splicing of reinforcement bar more than 36 mm in diameter should be avoided. In case such bars have to be lapped then they should be welded. (Reference IS456 clause 25.2.5).

2. Where there are two or more rows of bars, the bars shall be vertically in line and the minimum vertical distance between the bars shall be 15 mm, two third the maximum nominal size of aggregate or maximum size of bars, whichever is greater.

3. The minimum nominal cover for footing shall not be less than 50 mm.

As per IS 456:2000, slenderness limit for lateral stability of beam shall be:

The critical section for shear is act at a distance of d/2 from the face of the column. So, the total resisting area which resist that punching shear will be 

= 4(bd + d²).

As per IS 456 : 2000 In the Design of compression members the ultimate load on the short axially loaded column is:

P_u = 0.4f_ck A_c + 0.67f_y A_sc

where,

f_ck = Characterstics strength of concrete

f_y = characterstic strength of steel

A_c = Area of concrete in column cross-section

A_sc = Area of steel in column in compression

Dead load is the self-weight of the structure and the load which is permanently attached to the structure and will remain throughout the lifetime of the structure.

So at the beginning of design it is known to the designer.

Correct Answer :- c

Explanation :  For high strength deformed bars of grade Fe415:

For mild steel reinforcement.

Continuous slab 32 ≥ L/d

Simply supported slab ≥ L/d

Continuous slab 40 ≥ L/d 

where, L is the shorter span of slab

d is overall depth of the beam

32 ≥ 3000/d 

d ≥ 9.37 cm = 10 cm

Counterforts are firmly attached to the face slab as well as the base slab. The earth pressure acting on the face slab is transferred to the counterfort which deflects as vertical cantilever. The back of rear counterfort comes under tension and the front face is under compression. So the inclined (back) face of the rear counterfort should be provided with main reinforcement.

As per IS 456:2000:

1. Side face reinforcement in a RCC beam of breadth b and overall depth D exceeding 750 mm should be 0.1 %.

2. Maximum area of tension reinforcement should not exceed 4%.

Deep beams are structural elements loaded as simple beams in which a significant amount of the load is carried to the supports by a compression force combining the load and the reaction. As a result, the strain distribution is no longer considered linear, and the shear deformations become significant when compared to pure flexure.

In view of ample shear strength, deep beams are primarily recommended as transfer girders. These members transfer loads through-loading face to supports in the transverse direction. The deep horizontal members predominantly fail in shear rather than flexure. These beams are characterized with small span-to-depth ratio. Pile caps, corbel, brackets, foundation walls and off-shore structures are few examples of RC deep beams.

According to IS 456-2000 a beam shall be deemed to be a deep beam when the ratio of effective span-to-overall depth, l/D is less than:

1) 2.0, for simply supported beam; and

2) 2.5, for a continuous beam.

Though different codes define deep beams in different clear span-to-depth ratios, as a general rule deep beams are recognized by their relatively small span-to-depth ratio.

Hence, Deep beams are designed for bending moment and checked for shear.

According to the Code (CI. 34.1.2):

Code restricts the minimum thickness at the edge of the footing to 150 mm for footings in general (and to 300 mm in the case of pile caps).

IS 456:2000 limits the maximum diameter of reinforcing bars to 1/8^th of the total thickness of the slab.

D_max = 128/8 = 16 mm; Hence use 16 mm diameter bars.