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

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- 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
- 6. Deflection(codal provision)
- 7. 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.
- 8. 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.
- 9. 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.
- 10. For flanged beam the values of a) or b) modified as per fig 6 in IS:456 pg.39.
- 11. 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.
- 12. SHORT-TERM DEFLECTION
- 13. 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
- 14. Long-term deflection Cantilever 0.5 Simply supported members 0.125 Members continuous at one end 0.086 Fully continuous member 0.063
- 15. Deflection due to creep
- 16. Control of deflection on site

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