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