The calculated tension or
compression in any bar at any section shall be developed on each side of the
section by an appropriate development length or an anchorage or by a
combination thereof. So there must be sufficient length of bar beyond any
section to the total tensile force (or compressive force) in the bar at that
section. The term bond is used to define the cumulative effects of the
following factors
1. Adhesion
between reinforcement and concrete and this depends on the grade of concrete.
2. Resistance
to sliding owing to friction which depends mostly on the surface condition of
steel.
3. Gripping
of reinforcement consequent of shrinkage.
a) Development
length in tension
If the
diameter of a round bar is denoted by ‘d’ and the tensile stress be ft
then the total tensile force T = лd2/4 x ftx and this is
resisted by the bond resistance (лd x2L4) fb
where L4 = Development (i.e. bond) length in tension and fb
= average bond stress.
Therefore
(лd x L4) fb = лd2/4 x ft. Therefore L4 = ft/4fb x d.
Hence the
bond length is greater with higher tensile steel and will be lesser with richer
mixes.
b) Development
length in compression
For compression
bars as in columns, beams etc., the bond stress shall be increased by 25% than
the permissible bond stress in tension. So the development length of a steel
bar with the same grade of concrete is lesser by 25% in compression.
Therefore
(лd x Lc) x (fb x 1.25) = лd2/4 x fe
or Le = d x fc / 4 x 1.25 fb
= d x fc
/ 5 fb
Let, Lc/d = n = Compressive stress in the bar / 5 x
Permissible bond stress (in tension)
In no
case, value of n shall be less than 12. (For unknown stresses a bond length of
24d should be considered). No hook be provided for a bar in compression, but a
bend is desirable if the end of the bar is near an outer concrete face and it
shall not be accounted for anchorage purposes.
Development
Length For Plain and Deformed Bars – d = dia. Of bar
|
Working stresses Plain bar in tension
= 140 N/mm2 for bars up to 20 mm
dia.
= 130 N/mm2 for bars over 20 mm
dia.
|
Grade of concrete
|
For Plain Bars
|
For Deformed bars
|
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|
Tension
|
Compression
|
Tension
|
Compression
|
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|
Working stresses for compression
= 130 N/mm2 for all dia.
|
M – 15
M - 20
|
58 d
|
43.5 d
|
68.5 d
|
45.25 d
|
|
44 d
|
32.5 d
|
51.5 d
|
34 d
|
||
|
Working stresses for Deformed bar in tension =
230 N/mm2
In compression = 190 N/mm2
|
M –25
M - 30
|
39 d
|
29 d
|
46 d
|
30.20 d
|
|
35 d
|
26 d
|
41 d
|
27.20 d
|
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