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How do I define the Tributary Area that is used for columns?

Understanding the method of measuring the tributary area of columns is crucial when trying to determine the axial loads of the column. When making calculation by hand, the most important element in calculating column's load is the tributary space.

When we determine the loads on the vertical part of the structure, we take into consideration the tributary method to calculate columns but it's not restricted to columns. Axial loads on concrete walls can be calculated by this method.

The Tributary Area Method is the method of approximation and is also an exact method.

The majority of the time, we create an electronic model to aid in structural analysis. It is necessary to determine whether the results of the models are accurate.

The verification of computer models is also performed using the load calculations made manually employing the tributary area method.

Based on the position of the columns the area to be taken into account in calculation differs.

The figure above indicates three tributary zones for columns.

Area - 1 : A center column with one-fourth of the space from four panels that surround the column.

Area - 2 : An edge column that has one-fourth the area of two panels that surround the column.

Area - 3: A corner column that covers one-fourth the area of one side of the column.

Based on the size of the column and the column's size, we determine the axial load of the column.

If there is a load in an area that are not a problem, we can apply this technique directly. But, if there is an element of line weight on slabs we cannot apply it directly.

For instance, if you have a wall in the slab, it's situated on the one-fourth of a panel. In such cases we look at the distance ratios.

Based on the distance to that wall's location, the overall force of the column determined. If the wall isn't situated within the one-fouth of the space to be considered as a column the wall load won't be considered for that specific column, if we apply the tributary-area method. The load calculation is wrong.

Thus, we need to depart from the tributary region method used for these types of calculations. In these cases, the walls' distance is taken into consideration and the loads on columns surrounding it are calculated using the distances.

The column loads may show some variance from the computer model because of the lack of considering their stiffness. When using the finite element method the stiffness of structural elements is determined by the distribution of the loads.

Additionally, the deviation in the loads can be expected in structures with irregular shapes.

Also Read: Live Load Vs Dead Load

What are the functions of columns?

The fact that columns are all over in buildings might not be an issue for you. They play a vital part in building construction since their primary purpose is to transport different types of load from the structure down the ground. Once the loads are on the ground, they are taken care of by the foundations before being supported by the earth's surface. The weight that columns need to carry could be derived from various sources:

The structural weight it self (dead load) like beams and floors.

Things that fill structures (live load) like furniture, people and equipment.

The effects of earthquakes, snow and wind (environmental load).

The most common function of a column loning to one of the categories (dead weight) is to bear the slab's weight that is laying on it. But, slabs are often supported by just one column. Most of the time, you'll see many columns laid out in a structure grid underneath the slab.

If we want to know how much the load each column is carrying Then the obvious question is what part of the floor is occupied through which column? To answer this question the concept of tributary zones can be useful.

What are Tributary zones?

Tributary region, sometimes called influence area is the space around the column which absorbs the weight. A quick search on the term "Tributary" brings us to this definition from Wikipedia that describes a geographic phenomenon instead of the concept of structural engineering:

"A Tributary [..] is a river or stream that flows into a bigger stream , or its main branch (or parent) or lake."

It is interesting to note that the same principle could be applied to forces and columns, by substituting rivers for columns or water with forces. The force of gravity from the slab flow into the column and the tributary zones help determine the amount of slab each column catches.