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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