This document summarizes structural design considerations for deflection and cracking in reinforced concrete beams. It discusses: 1) How deflection occurs when a structure carries a load and guidelines for limiting deflection to prevent issues. 2) How cracking develops in concrete when tensile strength is exceeded from beam deflection. 3) Codal provisions for maximum allowable crack widths depending on exposure conditions. 4) Methods for controlling crack widths, including bar spacing and calculating crack widths. 5) Codal provisions for limiting span-to-depth ratios to control deflections. 6) How to calculate short-term and long-term deflections, including effects of creep and shrinkage.

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