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Monday, 25 June 2018

Dabun Pina | Decrease the depth and Manage

Slab Steel & Formwork

I visited a site in Narhe as a site trainee and observed the RCC detailing on the site. Generally, we observe the steel, formwork and the previously casted RCC work. I was on the 3rd floor and was checking the slab and beam reinforcement and I observed that the beam in the drawing was 18" but actually they'd prepared the formwork for 17" only and so was the reinforcement in which the stirrups were just 15". This appeared a bit weird to me but upon asking the charge on site I found a few things quite interesting!

Small builders cannot invest too much in making the formwork and so they use the same ply and plates a number of times to save their investment and get maximum works done. Generally, most contractors have the formwork in specific dimensions such as 9", 12", 15", 21", 24", etc. and they adjust the shuttering within this sides.

They use this common set of Shuttering material for multiple repetitions. 

But then, there are a few sections which are of different dimensions and so they have to make a few modifications to this Shuttering material. Generally adding a plank or patti does their work. 

Now let's return back to the slab and beam level. Consider the filling things suppose you have to cast a beam of depth 18" now the arrangement that I observed was as follows.

Slab Formwork Setup

1. Bottom Patti
2. A side of 12" but 1" is consumed in nailing it to the bottom Patti so clear span for covering is 11"
2. Shuttering plates 2"
Total here is 13" 

Now 13" can be covered of the beam but the beam is of 18" next thing that comes to aid is the slab thickness which is around 4-4.5" and most carpenters flush the beam top with the top surface of the slab. So they leave the remaining 4-4.5" as it is and then put sides on the ends where the slabs end.

Here's a graphical representation of the complete setup:
Slab formwork detailed section
Now let's so the math :

  1. Side 11"
  2. Shuttering plate thickness 2"
  3. The remaining cover from the slab concrete is around 4"
So the total shuttering/covering depth amounts to 17" which is 1" short to fulfill the design depth of 18"  and under such situations, the carpenters and the barbenders on the site fix the difference by changing the depth of the beam by 1". so the beam now becomes of 9"X17" and the stirrups become of 6" X 15". 

So this is a type of adjustment that is done on site but I'm not sure if it is structurally sound. I know that there's a lot of factor of safety taken while designing the structure even then it's not ethical to make such modifications to the structure as a lot of investment as well as lives of the users depend on the stability of the structure.

I thought it was some kind of unusual phenomenon but this practice has a name, its called "Dabun Pina" in regional Marathi Dialect.

These were a few of my observations on the site, would love to read your experiences and opinions!

Thanks for Reading
-Sayyed Shadaab
Engineer for the Digital Civilization

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