Apart from the length as the obvious difference between a short column and a long column, other differences are there.
So, today’s post is about short columns vs. long columns.
Definition of column
A column is a vertical element of a structure that transfers compressive loading. Designers provide it in all frame concrete structures. It plays a vital role in the safe transfer of superimposed loading to the foundation.
Civil engineers provide columns in different types of structures like buildings, piers, and towers. Sometimes we name them piers, struts, or pedestals. However, the purpose and mechanics of work remain similar to a bigger extent.
Anyhow, at this stage, I think you should know the difference between a column, a strut, and a pedestal. But to proceed, we need to know about effective length in terms of a column.
Effective length is the length of the column that will be taking up the loading. It depends on the support types like hinge or fixed. A column with both ends hinged would have a different effective length than a column having both ends fixed.
|Sr. No.||Type of end supports||Effective Length||Remarks|
|1||Both ends pinned||Le = L = length of column|
|2||One end fixed other free||Le = 2 x L|
|3||Both ends fixed||Le = 0.5 x L|
|4||One end fixed and one pinned end||Le = 0.7 x L|
What is a column?
In terms of effective length, we call a compression member a column with its effective length more than three times its least lateral dimension. For example, if we have a compression member of 9 in x 9 in, we need at least a height of 27″ to make it a column. If its length is less than that, it would be a pedestal.
In columns, the load’s reaction force travels along the axial direction of the columns. The forces run parallel to the long axis of the columns.
What is a pedestal?
So, a pedestal is a structural compression element with an effective length less than three times its least lateral dimensions.
What is a strut?
A strut is a structural element that is inclined or horizontal but still carries axial loading. We use struts in trusses.
Anyhow, let’s now move on to our main topic, which is about the short column and long column.
Types of columns – Short Column vs. long column
The two of the most common types of columns we use in buildings include short and long columns. Designers need to identify the type of column based on the requirements of the building.
They’ll look into the factors like space availability, imposed loading, architectural style, and type of loading. Classification for long and short columns is based on the proportioning of length and cross-section or lateral dimension.
We use the ratio of the effective length of the column (explained above) with the lowest lateral dimension. Here’s the time when I think you should know about a term called “slenderness ratio.” So, what is the slenderness ratio?
The slenderness ratio distinguishes between short and long or slender columns. The former’s design is governed by the column size and material properties, whereas the latter’s design is determined by column slenderness.
- If the cross-sectional measurements of a column are smaller compared to the length, it is considered slender.
- Compared to a short column with similar cross-sectional proportions, the columns with a higher slenderness ratio will crumble underneath a compression load.
- As a result, the slenderness impact should be taken into consideration throughout the design of columns. The slenderness ratio helps us in understanding the tendency of the column to fail in the buckle.
- That’s because we don’t want the column to fail in the buckle. Because in such a way, we are unable to utilize the entire cross-section and length for load-carrying.
- A slenderness ratio is an aspect ratio between the effective length of the column and the least radius of gyration.
The least radius of gyration is defined as:
k=I/A where I am the second moment of Inertia while the A is the cross-section of the column.
So, we can conclude that the tendency of buckling of the column depends directly on effective length. But it has an inverse relation to the cross-sectional parameters like a moment of Inertia and cross-sectional area.
Do you know? We use the concept of slenderness ratio not just in columns but also in buildings and tall towers.
Long columns have a ratio of the effective length to the lowest lateral dimension as larger than 12. In simple words, it means the length is way too much than the cross-sectional dimensions. For circular columns, we take the length of diameter for the ratio.
Such a long column is slender. With such, it will eventually fail by buckling rather than by crushing. It typically has a length of 20 to 30 times the diameter. Most of the columns in our homes and offices fall under this category.
Characteristics of Long Column:
- Long columns are slender. They are prone to buckling and bending, which can lead to their failure.
- Euler’s concept is used for using longer columns.
- The lateral measurement will be considerably smaller compared to the column length.
- The slenderness ratio will be much more than 45.
- Although the usable length of the columns will be more than the minimum lateral dimension of the respective columns, the proportion of the actual lengths to the minimum lateral dimension of that specific column will be less than 12.
- The weight-bearing capacity decreases as column length increases.
Short columns are known as those with a ratio lesser than 12 between the effective lengths and the lowest lateral dimensions.
Characteristics of Short Column:
- Either crushing or axial pressure will cause small columns to cripple and collapse.
- Rankin’s approach will be applicable in these sorts of columns.
- The lateral measurements will be greater than that of the column length.
- There will be a slenderness ratio lower than 45.
- Although the available lengths of the column to the minimum lateral measurement of the columns would be much less than 12, the proportion of the reasonable length to the least lateral dimension of the respective columns will be lower than 12.
- The load-carrying capability of short columns will rise when the column length is reduced.
So, here’s how we can conclude the difference between long and short columns:
|Sr. No.||Property / parameter||Long column||Short Column|
|1||Ratio of lateral dimension with height||It is very small for long columns as height is more.||It is larger in case of short columns|
|2||Failure||It fails by buckling||It fails by crushing|
|3||Ratio of effective length with least lateral dimension||Greater than 12||Less than 12|
|4||Slenderness ratio||Greater than 45||Less than 45|
|5||Radius of gyration||Less||More|
|6||Load carrying capacity||Less||More|
|7||Stresses||Subjected to buckling stress||Subjected to compressive stresses|