Wednesday, April 11, 2018

BASIC DETAILING RULE IN RCC



Basic detailing rule in RCC


Introduction to RCC
      RCC stands for Reinforced Cement Concrete
      RCC is generally reinforced with Steel because steel is stronger in tension but concrete is weaker in tension
      Both steel and concrete has almost equal coefficient of expansion/contraction
      Coefficient of expansion for steel is 0.000012 per degree centigrade
      Coefficient of expansion for Concrete is 0.000011 per degree centigrade

Code referred
      IS 456: 2000(reaffirmed 2005) Plain and Reinforced Concrete – Code of Practice

Nominal cover (ref. cl.26.4)
Exposure condition
Minimum cover (mm)
Remark
Mild
20
Moderate
30
Moist condition
Severe
45
Frequent wet and dry
Very Severe
50
Sea water
Extreme
75
Chemically affected area

Nominal cover
      It is the distance measured from the face of the member to the outermost face of the reinforcement including Stirrups or links.
      It prevents buckling of steel bar in column
      It provides protection to reinforcement from corrosion.
      It provides fire resistance to reinforcement.
      It provides sufficient embedded depth so that reinforcement develops the requisite stress.

FIGURE SHOWING NOMINAL AND CLEAR COVER

Nominal cover
      For column, nominal cover is 40mm or diameter of bar
      For column 200mm or less in size, nominal cover is 25mm
      For footing, it is 50mm but generally 75mm is taken

Beam
      Tension reinforcement: Ast,minimum = 0.85*b*d/(fy); where fy is grade of reinforced bar.
      Ast, maximum = 0.04bD
      Compression reinforcement: Asc, maximum = 0.04bD
      Side face reinforcement is provided when D>750mm
      Ast of side face >= 0.1% of web area of beam with spacing of bars <=300mm and <=width of web
      Minimum grade of concrete used is M20


Figure of beam cross section


Slab
      Ast, minimum = 0.15% of bD for mild steel
      0.12% of bD for high tensile steel
      Maximum diameter = 1/8 * thickness of slab
      Thickness of one-way slab (ly/lx >2) shall not be less than 125mm and when earthquake force is considered, it shall not be less than 150mm.
      Thickness of two-way slab (ly/lx <=2) shall not be less than 125mm and when earthquake force is considered, it shall not be less than 150mm.
      Minimum grade of concrete used is M20

Figure of slab


Spacing requirement of bars
      Minimum spacing between two bars = diameter of bar or nominal diameter of aggregate +5mm
      Minimum vertical spacing between two bars = 15mm or diameter of bar or 2/3 *maximum size of aggregate
      Maximum distance between bars
       180mm in beam for 0% redistribution for Fe415
       3d or 300mm for main bars in slab
       5d or 450mm for distribution bars in slab

Reinforcement detailing of column (ref.cl.26.5.3)- For Longitudinal reinforcement

      Area of steel, minimum = 0.8% of gross area of concrete
      Area of steel, maximum = 4% of gross area of concrete when congestion of bar is expected
      Area of steel, maximum = 6% of gross area of concrete when congestion of bar isn’t expected
      Minimum diameter of longitudinal bar = 12mm
      Minimum number of bars = 4 in rectangular/square column
      Minimum number of bars = 6 in circular column
      Minimum number of bars = 8 in octagonal column
      Maximum spacing of longitudinal bars = 300mm (to prevent shrinkage, temperature effect, etc.)

Reinforcement detailing of column (ref.cl.26.5.3)-Transverse bar
      Diameter of tie >= 6mm and >= ¼*maximum diameter of longitudinal bar
      Suppose 4 nos. of 25mm dia. And 4 nos. of 28mm dia. Of bar is provided then
      Diameter of tie >=6mm and >= ¼*28=7mm
      So, adopt diameter of tie greater than or equal to 7mm
      Spacing of tie must be minimum of following
      <=300mm
      <= least lateral dimension of column
      <= 16 * minimum diameter of longitudinal bar
      From previous example of (4 nos. of 25mm and 4 nos. of 28mm diameter of bars in column of dimension 400mm x 500mm)
      Spacing is minimum of-
      <=300mm
      <=400mm
      <=16*25 i.e 400mm
      Hence, spacing of tie is minimum of (300mm,400mm,400mm) = 300mm
      Provide additional tie when spacing between longitudinal bars is greater than 75mm
      When spacing of tie element <= 48*diameter of tie, provide open tie
      When spacing of tie element > 48*diameter of tie, provide close tie

Figure showing tie in column


Functions of transverse bar
      Provide lateral support to longitudinal bars
      To confine core of concrete by preventing bursting of concrete under compression
      To provide shear reinforcement

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