Tributary Area Method

[Howard Roark]

Hi there, 

I have recently joined a design firm and was assigned a task of designing wall footing.  My senior told me to use "Tributary Area Method" for this specific task. He gave me a list of formulas for various cases such as load transfer from slab to exterior short, exterior long, interior short, interior long beams etc. For rectangular slab sections, these formulas are quite easy to implement, but in cases, where slab isn't rectangular anymore, I don't know how to distribute load in order to get approximate the results. 

I have attached a pic below, in which I have to design wall footing for bath and kitchen as well. Kindly help me in understanding how this method works. 

Edited April 12, 2020 by MuhammadUsman

[Simple Structures]

Tributary Area Method - The area of slab above supported by the wall.

This is a little "trick' assignment I would give to a fresh graduate engineer -  but a good example for you to understand the concept, of loads onto walls or columns from slabs they are supporting.

In your case, the residential building has the "main load bearing" walls. These will support the slab above. The walls you have highlighted seem to be to be "minor" bathroom walls.

If these 'minor walls' are built at the same time as the 'main walls', then the concrete slab will impose load on these too. If these walls are built afterwards, once the slab has cured and deflected, and the load is taken by the main walls then there will be very little nominal load on these walls - from deflection of the slab under super (finishes, furniture, people etc) loading.

Also, in the plan you have shown the geometry of slab would mean that in majority of cases, the concrete slab would be spanning "one-way" and not "two-way". Nevertheless, see illustration below on how the load would distribute onto walls from such cases.

Now using the same logic work out load on your slab. Does this help?

 

[Simple Structures]

Once you have the loading on the wall from its self weight, and slab its supporting above (check how many storeys, 1 or 2 or 3), and then apply this as a line load on your foundation or slab at ground floor level.

1. With light loading we would thicken the ground floor slab and distribute load thus, at 45 degrees, and ensure allowable bearing pressure is not exceeded;

2. For heavy loading you would provide a foundation below the wall, below the slab.

[Howard Roark]

JazakAllah "Simple Structure" for explaining in detail. 

Your explanation of difference between Major load bearing walls and Minor load bearing wall did the trick for me. 

It would be great if you can share the source of these images, where you have highlighted how using certain formulas, load can easily be approximated on slab sides.

[Simple Structures]

Try this lecture link - it gives all info you need - 

http://web.engr.uky.edu/~gebland/CE 382/CE 382 PDF Lecture Slides/CE 382 L3 - System Laoding.pdf

[Zain Saeed]

The images below are self explanatory for the tributary area method. I use this method to find the size of the column in a multistory building

 

 

 

 

Edited July 1, 2020 by Zain Saeed