Limit State Method MCQ Quiz in मल्याळम - Objective Question with Answer for Limit State Method - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Explanation:

Creep of concrete:

(i) Deformation of structure under sustained load. Basically as long term pressure or stress on concrete can make it change shape. This deformation usually occurs in the direction the force is being applied. Like a concrete column getting more compressed, or a beam bending.

Following factors influence the creep of concrete:

(i) High cement content increases the creep of concrete.

(ii) High water cement ratio increases creep of concrete.

(iii) Creep increases when aggregate content is low.

(iv) It increases when air entrainment is high in concrete.

(v) Low relative humidity increases creep.

(vi) Small size/thickness of members show large amount of creep.

(vii) Early loading of concrete members increases creep.

(viii) Long term sustained loading increases creep.

Concept:

The ratio of limiting depth of neutral axis to the effective depth of the beam is given by

\(\frac{{{x_u}}}{d} = \frac{{{\rm{Max\;strain\;concrete}}}}{{\frac{{{\rm{Grade\;of\;steel}}}}{{{\rm{Modulus\;of\;Elasticity\;of\;steel}} × {\rm{F}}.{\rm{O}}.{\rm{S}}}} + 0.002 + {\rm{Max\;strain\;concrete}}}}\)

Note:

We know the standard values of the ratio of limiting depth of neutral axis to the effective depth (k) of the beam for different steel sections as following:

 xu = k × d

∴ For Fe415 grade limiting value of depth of neutral axis is 0.48×d

Concept:

As per IS 456 : 2000, Table 16;

Cement content should not exceed 450 kg/m3.

Explanation:

(i) In the design of reinforced concrete structures, the reinforcement provided is embedded in the concrete up to a particular distance from the face of the member because of the following main reasons:

• To provide protection to reinforcement from corrosion.
• To provide fire resistance to reinforcement.
• To provide sufficient embedded depth so that reinforcement develops the requisite stress.

(ii) This distance is measured in different ways and known by different names:

• Clear cover: This is the distance from the face of the member to the outermost face of the reinforcement including shear or torsion Stirrups or links.

• Nominal cover: This is the same thing as clear cover albeit with a different name. This term is used by the code. It is the distance measured from the face of the member to the outermost face of the reinforcement including Stirrups or links. It is the dimension shown in drawings and detailing.

• Effective cover: This is the distance measured from the face of the member to the center of the area of the main reinforcement, that is tension or compression reinforcement. This is the dimension usually used for design calculations.

Effective cover = Clear cover + (Diameter of Stirrups/links) + 0.5 × (Diameter of main reinforcement bars).

Also,

Effective cover = Overall depth - effective depth.

Concept:

According to IS 456: 2000, the Minimum Cement Content, Maximum water-cement Ratio and Minimum Grade of Concrete for different exposures with Normal Weight Aggregates of 20 mm Nominal Maximum Size

Additional Information

• It can be seen in the above table, that minimum cement content for various concrete work is dependent on exposure conditions, and the type of concrete i.e plain concrete or reinforced concrete, and it is independent of the grade of cement used.
• Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions such as fly ash or ground granulated blast furnace slag that may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolana and slag specified.

Concept:

The ratio of limiting depth of neutral axis to the effective depth of the beam is given by

\(\frac{{{x_u}}}{d} = \frac{{{\rm{Max\;strain\;concrete}}}}{{\frac{{{\rm{Grade\;of\;steel}}}}{{{\rm{Modulus\;of\;Elasticity\;of\;steel}} \times {\rm{F}}.{\rm{O}}.{\rm{S}}}} + 0.002 + {\rm{Max\;strain\;concrete}}}}\)

Note:

Without getting into the tedious calculation,

We know the standard values of the ratio of limiting depth of neutral axis to the effective depth (k) of the beam for different steel sections as following:

Calculation:

From the above table we can say that Limiting depth x_u is function of grade of steel and Effective Depth

Given, Grade of steel = Fe500, Effective cover = 50 mm,

Effective depth (d) = 450 - 50 = 400 mm

Reinforcement 4 - 18 mm diameter bars. 

∴ For Fe 500, xu ≈ 0.46× 400 = 184 mm.

Explanation:

As per CI. 3.1 of IS 875 (part 2), Imposed floor loads for different occupancies:

Concept:

According to IS 456: 200, the Minimum cement content and maximum water-cement ratio based on exposure conditions for plain cement concrete and reinforced cement concrete are given below:

Extreme exposure condition: When the surface of member in the tidal zone and Members in direct contact with liquid/ solid aggressive chemicals.

Important Points

A s per IS 456: 2000, Table 5;

Explanation:

As per IS 456: 2000:

For design purposes, the compressive strength of concrete is assumed to be 0.67 times the characteristic strength of concrete. The partial factor of safety of 1.5 is also to be applied in addition to this.

Characteristic Strength of concrete Cube = fck

Characteristic strength of concrete in actual structure = 0.67fck

Design strength of concrete in flexural compression = 0.67fck/ partial FOS = 0.67fck/ 1.5 = 0.45f­ck

Explanation :

As per IS 456:2000

• In the 4th Revision of IS, 456-2000 i.e. Amendment-4 modified the Grade of concrete.
• This modification happened in May 2013. As per this Amendment, the Grade of concrete is classified into three groups i.e. Ordinary Concrete, Standard Concrete, and High Strength Concrete.
• In Ordinary Concrete, there are Grades such as M 10, M 15, and M 20.
• ​There is some modification in Standard Concrete Grade i.e. Number of Grades that comes under Standard concrete are M 25, M 30, M 35, M 40, M 45, M 50, M 55, and M 60. 
• High Strength Concrete includes Grade Designation such as M 65, M 70, M 75, M 80, M 85, M 90, M 95, and M 100.

Note: For the Concrete Grade above M 60, design parameters given in the standard may not be applicable and the values may obtain from the specified literature and experimental results.