DIFFERENCE BETWEEN LONG COLUMNS AND SHORT COLUMNS

In our previous topics, we have seen some important concepts such as Assumptions made in the Euler’s column theory, Difference between column and strut and concept of eccentric loading with the help of our previous posts.

Today we will see here one very important topic in strength of material i.e. difference between long columns and short columns with the help of this post. 

We will first understand here the basic concept of columns and after that we will discuss the types of columns and also we will differentiate between long columns and short columns here.

So let us first understand here the meaning and characteristics of columns.

What is column?

Column is basically defined as a vertical member of a structure and it will be subjected with vertical compressive load. Line of action of compressive load will pass through the axis of the column or sometime also parallel to the axis of the column.

In simple, a member of structure will be termed as column if it is vertical and it’s both ends are fixed rigidly and also subjected with axial vertical compressive load.

Example

Vertical structural member between roof and floor could be considered as the best example of column.

Let us see here few important points in respect of concept of columns

Columns will be subjected with only axial vertical compressive loads

Columns are basically vertical members in structures

Column will be longer in length as compared to strut

Both ends of column will be fixed rigidly

Normally, columns carry heavy vertical axial compressive loads

Cross-sectional dimensions of columns will be usually large

Applications of columns are usually seen in concrete and steel buildings

Types of columns

In design phase of columns, we must have to determine what group the column falls under. On the basis of length and lateral dimensions of columns, columns are basically divided in two groups.

Long columns

Short columns

Let us see here the difference between long columns and short columns

Long column

Long column is basically defined as the column in which the ratio of effective length of the column to the least lateral dimension of the column is more than 12.

Let us consider the following figure which indicates a column and we will figure out here the condition to accept this column as long column. Let us consider that length of the column is L as displayed in following figure.

We have considered here the cross-section of the column is circular or rectangular. Cross-section of the column might be any section such as T section, hollow rectangular section, hollow circular section, I section etc. 

But we have considered here only two sections i.e. circular cross-section and rectangular cross-section. We will figure out here the condition to accept this column as long column for column with circular cross-section and also we will figure out here the condition for column with rectangular cross-section.

Effective length of the column i.e. length of the column which is bending, Le = L

Diameter of column of circular cross-section = d

Width of column of rectangular cross-section = B

Depth of column of rectangular cross-section = D

If we will first consider the case of column with circular cross-section, here we will have least lateral dimension as diameter of circular cross-section i.e. d.

So, we will have condition to accept this column as long column

For considering column as long column, (Le/d) > 12

If we will now consider the case of column with rectangular cross-section, width B is smaller than depth D in above displayed column of rectangular cross-section. Therefore we will have least lateral dimension as width of the rectangular cross-section i.e. B.

So, we will have condition to accept this column as long column

For considering column as long column, (Le/B) > 12

Important characteristics in long column

Long columns will fail only because of buckling or bending.

For long column, Euler’s theory will be applicable.

For long column, Lateral dimensions will be quite small as compared with the length of the column.

For long column, Slenderness ratio will be more than 45.

For long column, Ratio of effective length of the column to the least lateral dimension of the column will be more than 12.

For long column, Load carrying capacity will be reduced with increase in length of the column.

Short column

Short column is basically defined as the column in which the ratio of effective length of the column to the least lateral dimension of the column is less than 12.

Let us consider the above figure which indicates a column and we will figure out here the condition to accept this column as short column. Let us consider that length of the column is L as displayed in above figure.

If we will first consider the case of column with circular cross-section, here we will have least lateral dimension as diameter of circular cross-section i.e. d.

So, we will have condition to accept this column as short column

For considering the column as short column, (Le/d) < 12

If we will now consider the case of column with rectangular cross-section, width B is smaller than depth D in above displayed column of rectangular cross-section. Therefore we will have least lateral dimension as width of the rectangular cross-section i.e. B.

So, we will have condition to accept this column as short column

For considering column as long column, (Le/B) < 12

Important characteristics in short column

Short columns will fail only because of crushing or direct compression.

For short column, Rankine’s theory will be applicable.

For short column, Lateral dimensions will be quite large as compared with the length of the column.

For short column, Slenderness ratio will be less than 45.

For short column, Ratio of effective length of the column to the least lateral dimension of the column will be less than 12.

For short column, Load carrying capacity will be increased with decrease in length of the column.

Do you have suggestions? Please write in comment box.

We will now discuss failure of column in detail in the category of strength of material in our next post.

Reference:

Strength of material, By R. K. Bansal

Image Courtesy: Google

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