Length or Span to Depth Ratio for beam and slabs - Printable Version

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Length or Span to Depth Ratio for beam and slabs - Manish Jain - 12-20-2023

For RCC Structure 
According to clause 23.2.1 of IS 456-2000 

The vertical deflection limits may generally be assumed to be satisfied provided that the span to depth ratios are not greater than the values obtained as below:

a) Basic values of span to effective depth ratios for spans up to 10 m:

Cantilever    :    7
Simply supported  :   20
Continuous  :   26

b) For spans above 10 m, the values in (a) may be multiplied by 10/span in metres, except for cantilever in which case deflection calculations should be made.

c) Depending on the area and the stress of steel for tension reinforcement, the values in (a) or (b) shall be modified by multiplying with the modification factor obtained as per Fig. 4.

d) Depending on the area of compression reinforcement, the value of span to depth ratio he further modified by multiplying with the modification factor obtained as per Fig. 5.

For two way slab of shorter span (up to 3.5 m) with mild steel reinforcement, the span to overall depth ratios is given below may generally be assumed to satisfy vertical deflection limits for loading class up to 3 kN/m2 

Span                     span/depth ratio for Mild Steel    span/depth ratio for HYSD bars 
Simply Supported   35                                             28 
Continuous Slab     40                                             32


For Steel Structure : 

It is convenient to remember that serviceable steel section depths are in the range of ½” of depth for each foot of span (L/24). Some people might find it
easier to remember the following simplified rule where the length is expressed in feet and the depth of the member in inches:

Depth of Roof Beams, Roof Joists = 0.5*Length
Depth of Floor Beams, Floor Joists = 0.6*Length
Depth of Composite Beams = 0.55*Length

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