The correct answer is A. 10.
A long column is a column whose effective length is greater than 10 times its lateral dimension. This means that the column is slender and is more likely to buckle under load.
A short column is a column whose effective length is less than 10 times its lateral dimension. This means that the column is stocky and is less likely to buckle under load.
The effective length of a column is the length of the column that is actually resisting bending. It is calculated by taking into account the end conditions of the column.
The lateral dimension of a column is the dimension of the column that is perpendicular to the direction of the load.
The ratio of the effective length to the lateral dimension is a measure of the slenderness of the column. A slender column is more likely to buckle under load than a stocky column.
The following are the options of the question:
A. 10
B. 15
C. 20
D. 25
Option A is the correct answer. A column is regarded as long column if the ratio of its effective length and lateral dimension, exceeds 10.
Option B is incorrect. A column is not regarded as long column if the ratio of its effective length and lateral dimension, is less than or equal to 10.
Option C is incorrect. A column is not regarded as long column if the ratio of its effective length and lateral dimension, is less than or equal to 15.
Option D is incorrect. A column is not regarded as long column if the ratio of its effective length and lateral dimension, is less than or equal to 20.