Con Tech & RCC Design

Interview Questions on RCC Structures Design


  1. Characteristic Strength of Concrete –

Characteristic strength is the compressive strength of concrete tested after 28 days of 150 mm size cube. Its represented as fck and not more than 5% of the sample should fail below this value.

  • Minimum Grade of RCC –
Environmental Condition Cement Content (kg) Maximum WC Minimum Grade of Concrete
Mild 300 0.55 M20
Moderate 300 0.50 M25
Severe 320 0.45 M30
Very Severe 340 0.45 M35
Extreme 360 0.4 M40
  1. Minimum Grade of PCC –
Environmental Condition Cement Content (kg) Maximum WC Minimum Grade of Concrete
Mild 220 0.6 M20
Moderate 240 0.6 M25
Severe 250 0.5 M30
Very Severe 260 0.45 M35
Extreme 280 0.4 M40
  • Balanced (Critical), Over-Reinforced and Under-Reinforced sections –

Balanced Section – Beam sections where the concrete and the reinforcement steel both reach their allowable stress at the same time is called a balanced section. The depth of Neutral Axis xu=xumax

Over Reinforced Section – Beams where concrete reach the failure stress earlier than steel is called an over reinforced concrete. NA falls below balanced NA xu>xumax

Under –Reinforced Section – The beams where steel reaches the allowable earlier than the concrete is called under reinforced concrete. Under reinforced beam is preferable as it gives warning before failure. The actual NA is above the balanced NA xu<xumax

  • Design constants – k,j,Q –

k is the neutral axis constant –

j is the lever arm. It’s the perpendicular distance between centres of gravity of tensile and compressive forces. It is given by j = d(1-k/3)

Q is the moment of resistance constant =  

  • Minimum and Maximum Steel in Slab Beam and Column, What are the minimum and maximum spacing of rods in slab and columns ?
Member Minimum Reinf. Maximum Reinf. Spacing of Main Spacing of Stirrups
Slab 0.12% for HYSD and 0.15% for Mild Min – Ast/bd = 0.85/fy   0.04bD ie 4% of Gross cs area. Max Size of bar = 1/8 th of Total Thickness of Member  Maximum spacing of main bar is restricted to 3 times effective depth or 300 mm whichever is less. For distribution bars the maximum spacing is specified as 5 times the effective depth or 450 mm whichever is less.
Beam Min – Ast/bd = 0.85/fy   0.04bD ie 4% of Gross cs area. Max Size of bar = 1/8 th of Total Thickness of Member Min distance = not less than (i) The dia meter of the bar if dia are equal (ii)The dia of the larger bar if dia are unequal (iii) 5 mm more than the nominal maximum size of coarse aggregate. HOWEVER when needle vibrator is used the distance may be reduced to 2/3 of the nominal maximum size of coarse aggregate The min .vertical distance between two bars should be 15 mm. Minimum of (i) Sv = 2.175fyAsv/b (ii) 300mm (iii) 0.75d –For Vertical Stirrups d –  For inclined where, d = effective depth of the section  
Column 0.8% of gross area of col. 4 Nos bar for Rectangular Column And 6 Nos Minimum Dia – 12 mm   6% when bars are not lapped 4% when bars are lapped   Max. Distance – 300 mm  Ties – ¼ Dia of main bar. 6 mm whichever is higher. Spacing – Least lateral dimension. 16xMain Bar. 300 mm.   Helical pitch Min of – 75 mm 1/6 Dc  & Should Not be less than -3 Dia of bar forming helix. 25 mm.  
  • Minimum diameter of rod in Slab, Beam and Column –

Slab – 8 mm

Beam -12 mm

Column – 12 mm

  • Maximum size of aggregates – 40 mm.
  • What is 24 SWG wire or Black Annealed wire? What is its quantity per Quintal of steel?

Wire used for binding reinforcement. Required Quantity 0.8 kg to 1.4 kg depending upon the dia of bar.

  • Values of covers in Slabs, Beams and Columns and Column Footings. –

The cover depends upon the exposure condition of concrete for Mild Condition the values are –

Slab : 20mm

Beams :25mm

Columns : 40mm

Footings : 50mm.

  1. Point to be supervised before the casting of cement concrete in RCC structures?

Weather the reinforcements are provided as per the drawing.

Proper cover is provided or not.

Alignment conform to the proposed drawing.

Sufficient support for formwork is provided or not.

Shuttering oil is applied or not.

The surface is free from any kind of soil or debris. Etc.

  • Sizes of vibrators Oscillations, Maximum time of needle vibrators, and its range of vibration at one spot.

The size of needle vibrator vary from 40 to 100 mm in diameter. The period of vibration required vary from 30 seconds to 2 minute.

The normal radius of effect of needle vibrator is 0.50 to 1.0m. It would be preferable to immerse the vibrator into concrete at intervals of not more than 600 mm or 8 to 10 times the diameter of the needle.

The frequency of vibration varies up to 15000 rpm. A range between 3000 to 6000 rpm is suggested as a desirable minimum with an acceleration of 4g to 10g.

The concrete should be placed in layers not more than 600mm high.

  • Why Binders and Stirrups are used in columns and Beams? Where are they close and wide?

In columns ties are used to tie the main bar thus preventing them from buckling and to hold the main bar in position. The tie bar reduces the unsupported length of main bar thus acting as short column.

The stirrups are used in beams and they resist diagonal shear in beams and also hold the main bar in position.

They are placed close near the support and towards the centre the spacing may increase.

  • What is the minimum diameter of binders or lateral ties? Why longitudinal rods in columns are provided along the circumference and not at the center.

The minimum dia of binder in column is– ¼ Dia of main bar. 6 mm whichever is higher.

If the bars would be placed in centre the integrity of the concrete shape could not be maintained, crack would from due to bending stress also placement of concrete would become difficult for constructing a homogeneous column. Further he column would become ineffective in carrying eccentric load.

  • Value of lapping of roads and their ideal locations in Slabs, Beams and Columns.

For flexural tension Ld or 30d whichever is greater.

For direct tension -2Ld or 30d whichever is greater.

The lap length in compression is 24d.

The top reinforcement of the beam is lapped at center of the beam and the bottom reinforcement is lapped near the support. Basically where there is minimum bending moment the lap should be preferable placed there.

  • How turbid water can be used for concreting?

The turbidity may be removed by adding coagulant like alum also the water may be kept in tanks to reduce the turbidity by sedimentation process.

  • Why stirrups are not provided in slabs? –

The dead load are carried by the beam the beam would carry most of the shear also due to in sufficient space available the stirrups could not be placed for holding the bars in position in slab.

  • One Way and Two Way slabs. – When the length of the slab is greater equals to 2 times the width then it’s called one way slab and when the ratio is less than 2 its two way slab.

Stripping time of shuttering and Centering’s. Locations from Where Centering are to be stripped off in simply supported and cantilever structures?

Type of formwork Minimum period before striking formwork
a) Vertical formwork to columns, walls, beams             16-24 h
b) Soffit formwork to slabs (Props to be re-fixed immediately after removal of formwork)             3 days
c) Soffit formwork to beams (Props to be re-fixed immediately after removal of formwork)             7 days
d) Props to slabs:   Spanning up to 4.5 m  Spanning Over 4.5 m               7 days             14 days
e) Props to beam and arches:    Spanning upto 6 m    Spanning over 6 m               14 days             21 days

In simply supported structure the formwork should be stripped from the support towards mid span.

In Cantilever structure the formwork should be stripped from end towards support.

Where Rectangular and T-Beams are recommended? Value of bf ?

The rectangular beams are recommended along the periphery of the structure. T-Beams use concrete more economically hence require less steel.  

The value of bf =  for T beam

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