Reinforced Concrete Structures MCQ Quiz in বাংলা - Objective Question with Answer for Reinforced Concrete Structures - বিনামূল্যে ডাউনলোড করুন [PDF]

Explanation:

Longitudinal Reinforcement:

(i) CI. 26.5.3 of IS 456:2000, specifies that the total area of longitudinal bars in a column section must NOT be less than 0.8% of the gross column area. This limit on minimum reinforcement is imposed because of the following reasons:

• In order to ensure that a minimum flexural resistance of the column exists due to unexpected eccentricities in the column loading.
• In compression members, creep under sustained loading is very predominant, especially at low percentages of steel. Thus, the resulting creep stress (due to creep strain) tries to yield the bars. 

(ii) Maximum Reinforcement: The maximum area of cross-section of longitudinal bars must NOT exceed 6% of gross column area. However, in practice, a maximum of 4% is recommended.

Additional Information

Diameter and Number of bar:

(i) The diameter of longitudinal bars in column NOT be less than 12 mm. These bars must NOT be spaced more than 300 mm apart on the column perimeter.

(ii) For rectangular columns. a minimum of 4 bars is provided.

(iii) For Circular columns, a minimum of 6 bars be provided.

Cover = 40 mm or bar diameter

Concept:

Values of the factor of safety (partial) for load combination:

Where, DL = Dead load, LL = Live load WL = Wind load EL = Earthquake load

Calculation:

Given: Dead load (DL) = 20 KN and Live load (LL) = 12 kN

Partial factor of safety = 1.5 (DL + LL) = 1.5 (20 + 12) = 48 kN

Additional Information

Values of the factor of safety (partial) for load combination:

For footing:

i) The thickness at the edge shall not be less than 150 mm for footings on soil and not less than 300 mm for footing on piles.

ii) The depth of the foundation should be a minimum of 500 mm.

iii) For reinforcement, footing is treated as an inverted slab. As per IS:456-2000, the minimum percentage of reinforcement of steel is 0.12% of the gross sectional area with HYSD bar and 0.15% of the gross area with plain bars of mild steel.

iv) Minimum clear cover should be 50 mm.

v) Permissible shear stress for footing, according to limit state method is τc = 0.25√fck and τc = 0.16√fck according to working stress method.

Concept: -

As per IS 14458 (Part 2), cl. 5.20

Note:- The live loads and imposed loads adding to the stability of the structures shall not be considered in working out factors of safety in (1) and (2).

Explanation:

Limit state of serviceability of prestressed concrete should satisfy cracking, deflection, and maximum compression also.

The crack width & deflection should not exceed the permissible limit and the maximum compressive force also should not exceed the strength of concrete.

Note: See article 19.2 & 19.3 in IS code 1343:1980

Minimum grade of concrete to be used in the design of prestressed concrete structure as per IS 1343 is as below:

1. For Post-tensioning minimum grade of concrete used is M-30.

2. For Pre-tensioning minimum grade of concrete used is M-40.

Hence it can be seen that grade of concrete used for prestressed member lies in the range of M30 to M60

Important Points

Cover to be used in the design of prestressed concrete structure as per IS 1343 is as below:

1. For Posttensioning minimum cover to be used is 30 mm.

2. For Pre-tensioning minimum cover to be used is 20 mm.

Confusion Points
As per IS 1343:2012, The limit state of serviceability deals with the deflection, cracking, and maximum compression. While As per 456:2000, The limit state of serviceability deals with deflection and cracking. Since the question is asking for prestressed concrete, we will go with IS 1343: 2012. 

The permissible design stresses for an RCC Column under different conditions are given below:

1. If minimum eccentricity is considered to be Zero i.e. e_min = 0

a. Permissible axial compressive stress in concrete = 0.45 f_ck

b. Permissible axial compressive stress in steel = 0.75 f_y­

2. If minimum eccentricity effect is considered: 

a. Permissible axial compressive stress in concrete = 0.4 f_ck

b. Permissible axial compressive stress in steel = 0.67 f_y­

Note:

In RCC beam subjected to bending, shear and torsion, the permissible values in steel and concrete are:

a. Design strength of concrete in flexure = 0.45 f_ck

b. Design strength of Steel in tension = 0.87f_y

The correct answer is longs spans.Key Points

• Post tensioning is a construction technique that involves the use of high-strength steel strands or cables to reinforce concrete structures.
• The suitability of post tensioning depends on various factors such as span length, design requirements, and construction constraints.
• Post tensioning allows for longer spans to be achieved without the need for intermediate supports.
• This is because the high-strength steel cables can provide the necessary tensile strength to counteract the weight of the structure and any applied loads.
• Longer spans can result in more open and flexible interior spaces, which can be beneficial for certain building types such as sports arenas, exhibition halls, and airports.
• Post tensioning can also help reduce the overall weight of the structure, which can lead to cost savings in materials and construction.
• The use of post tensioning can improve the durability and resilience of concrete structures, as the cables can help prevent cracking and deformation due to temperature changes, shrinkage, and other factors.

Additional Information

• End spans refer to the sections of a structure that are adjacent to a support, such as a column or a wall.
  • Post tensioning can be used for end spans, but it may not be as necessary as for long spans.
• Break spans are sections of a structure that are interrupted by an expansion joint or a construction joint.
  • Post tensioning can be used for break spans, but it may require additional design considerations and construction techniques.
• Edge spans are sections of a structure that are located at the perimeter, such as a balcony or a cantilevered slab.
  • Post tensioning can be used for edge spans, but it may require additional reinforcement to account for wind loads and other lateral forces.

Concept:

As per IS 456:2000 clause 26.5.1.1, Minimum tension reinforcement (Ast) in a beam  provided is given by:

\(\frac{{{A_{st}}}}{{bd}} = \frac{{0.85}}{{fy}}\)

Where,

b & d are width, effective depth of the beam

f_y is the yield stress

Calculation:

Given,

b = 450 mm, d = 550 mm

Grade of Steel is Fe 500

\({A_{st}} = \frac{{0.85}}{{fy}} \times bd = \frac{{0.85}}{{500}} \times 450\times550\\=420.75\; mm^2\)

Concept:

Effective Width of Flange of the different section:

Where,

bf = Effective width of flange

lo = Distance between points of zero moments in the beam

bw = Breadth of the web

df = Thickness of the flange

b = Actual width of the flange

Explanation:

Given,

lo = 6m = 6000mm, b = 1000mm, bw = 300mm, 

Walkway is the case of isolated T-Beam. Then,

\({b_f} = {b_w} + \frac{{{l_o}}}{{\frac{{{l_o}}}{b} + 4}}\)

\({{\bf{b}}_{\bf{f}}} = 300 + \frac{{6000}}{{\frac{{6000}}{{1000}} + 4}}\)

bf = 300 + 600 = 900mm

Explanation:

The shear strength of reinforced concrete with the reinforcement is restricted to some maximum value τcmax depending on the grade of concrete.

Table 20 of IS 456

Stipulates the maximum shear stress of reinforced concrete in beams τcmax as given below in Table. Under no circumstances, the nominal shear stress in beams τv shall exceed τcmax given in the table for different grades of concrete.

Important Points

 When the shear stress in concrete exceeds these values it leads to brittle failure due to diagonal compression.