deflection & cracking criteria for RCC structures as given in IS:456 (2000)

1. Structure Design
DEFLECTION AND CRACKING
PREPARED BY : PRATIK PATEL( L.D.C.E)

2. Introduction
DEFLECTION :
A structure member DEFLECTS when it carries load.
It should not adversely affect the appearance or efficiency or finishes of the
structure apart from the structural considerations.
CRACKING:
 The deflection of RC beam is associated with cracking of concrete.
Crack develop at the region where the tensile strength of concrete is
exceeded.

3. Cracks in beam

4. Cracking (Codal Provision)
As given in Is-456(2000) cl.35.3.2 pg. 67. The surface width of crack should not
exceed the following value.
Exposure Condition
Surface Widthe Crack ( Limiting Value )
Mild
0.3mm
Moderate
0.2mm
Severe
0.1mm
To assure the crack width does not exceed the above values IS:456 Suggest Two methods.
BAR SPACING CONTROL: cl.26.3 IS:456
CRACK WIDTH CALCULATION: Specific attention is required to limit the designed crack width to
a particular value , calculation can be done by formula given in annex F of IS:456

5. Bar spacing control(cl.26.3.2)
The horizontal distance between two parallel main reinforcing bar should not
be less than
 Diameter of the bar if diameter are equal
 Diameter of the larger bar if diameter are unequal
 5mm more than the nominal size of coarse aggregates
When there are more than one row of bar
• The bar should be vertically in line
• The min vertical distance between the bars shall be greater of
1.
15mm
2. ⅔ of the nominal max size of aggregate
3. Max size of bars

7. Deflection(codal provision)

8. Span/effective depth ratio
The vertical deflections limit is assumed to satisfy if the span/depth ratio are not grater than
a) Basic values up to spans of 10m
Span
Span/effective depth ration
Cantilever
Simply supported
Continuous
7
20
26
b) For spans above 10m the values in a) may be multiplied by 10/span in meters except for
cantilever(calculation should be made)
c) Depending on area and stress of tension & compression reinforcement the values in a) or
b) shall be multiplied by modification factor obtained as per fig 4 and fig 5 respectively
given in IS:456 pg.38 and pg.39.
d) For flanged beam the values of a) or b) modified as per fig 6 in IS:456 pg.39.

9. DEFLECTION IN SIMPLE BEAM
For slabs spanning in two directions the shorter
of two spans should be used for calculating the
span/effective depth ratio.
For 2 way slab span up to 3.5m with mild steel
reinforcement ,the span/effective depth ratio to
satisfy vertical deflection limits for loading class
up to 3KN/m2
Simply supported slab
35
Continuous slab
40
 For hysd bar of grade 415,the above values
should be multiplied by 0.8.

10. Depending on are and stress of tension & compression reinforcement the values in a) or b)
shall be multiplied by modification factor obtained as per fig 4 and fig 5 respectively given in
IS:456 pg.38 and pg.39.

11. For flanged beam the values
of a) or b) modified as per fig
6 in IS:456 pg.39.

12. TOTAL DEFLECTION ( annex C)
The total deflection shall be taken as the sum of the short-term deflection and
the long-term deflection.
1. Short term deflection: It is due to loading
2. Long term deflection: It is due to the effect of creep and shrinkage .It is rapid
at the initial period of loading and then slows w.r.t. time. Within 2-3 years the
long term deflections are largely completed.

13. SHORT-TERM DEFLECTION

14. For continuous beam
Deflection shall be calculated using the values of Ir , Igr and Mr modified by the following equation:
Where
Xe = modified value of X
X1,X2 = values of X at the support
Xo= values of X at mid span
K1= coeffiecient
X= value of Ir ,Igr or Mr as appropriate

15. Long-term deflection
Cantilever
0.5
Simply supported members
0.125
Members continuous at one end
0.086
Fully continuous member
0.063

17. Deflection due to creep

18. Control of deflection on site