Saturday, August 15, 2020

Design of One-Way Slab Part-2: Calculating the aspect ratio of effective spans

 Now that we understand all the modification factors. Let’s learn how to calculate the aspect ratio. If you have not read my previous blog on how to determine the effective depth of one way slab please go through that one. 

For Basic Values for span < 10m

Cantilever = 7

Simple Supported = 20

Continuous = 26

For Basic Value for span >10m

Cantilever = (7*10)/span length

Simple Supported = (20*10)/span length

Continuous = (26*10)/span length

CALCULATIONS 1

Our Slab rested on one simply supported and continuous at other side

So,

  Basic Value = (20+26)/2 = 23

  K1 = 1

  K2 = 1.4

  K3 = 1

  K4 = 1

   => l/d < 1*1.4*1*1*23

   => 3400/d < 32.2

   => d > 3400/32.3

   => d = 105.6


Step 3 : To Calculate Overall Depth 

Overall Depth > Eff. Depth + (Dia of bar/2) + Nominal Cover

CALCULATION 2

Assume 12mm Dia Bar

Nominal Cover = 30mm For Moderate Exposure

(Ref Table 16, Page 47, IS456)

Overall Depth > 105.6 + 12/2 + 30

      > 141.6

      = 150mm (multiple of 5mm)

Now,

    Effective Depth , d =150-12/2-30

=114mm


Step 4 : To Calculate Effective Span

From Clause 22.2(a), Page-34, IS456 2000 

Minimum of ..

a) Clear Span + Eff. Depth of slab

b) Centre to Centre of Supports

CALCULATION 3

a) Clear Span + Eff. Depth of slab

  =3400 + 114 = 3514mm

b) Centre to Centre of Supports

  =230/2 + 3400 + 300/2 = 3665mm

  For shorter effective span, we get = 3514 mm

   Longer Effective span =230/2 + 9000 + 230/2 = 9230 mm

  Now,

    Check Aspect Ratio = longer effective span/shorter effective span

      = 9230/3214

      = 2.626 > 2

So We can say , It's a one way slab .

To have a deeper understanding of this concept you can visit : https://youtu.be/Q5ROilapCls


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