Columns are loaded in compression.

Explanation:
Columns are structural members that are primarily designed to carry axial compressive loads. They are commonly used in buildings and other structures to support vertical loads and provide stability. When a column is subjected to an axial load, it experiences compressive stress, which is the internal resistance to deformation caused by the applied load.

Compression:
When a column is loaded in compression, the applied load acts along the longitudinal axis of the column and tends to shorten the column. This results in compressive stress being developed within the column, which is distributed uniformly along its cross-sectional area. The primary failure mode of a column under compression is buckling, which occurs when the compressive stress exceeds the critical buckling stress of the column material.

Other types of stress:
While compression is the primary type of stress experienced by columns, there can also be other types of stresses present, depending on the loading conditions and column design. These include:

1. Shear stress: Shear stress occurs when there is a tendency for one part of the column to slide relative to another part. It is induced by lateral loads or forces that are not aligned with the longitudinal axis of the column. However, shear stress is typically secondary in columns compared to compression.

2. Hydraulic stress: Hydraulic stress refers to the pressure exerted by a fluid on the column. It can occur in columns that are submerged in liquid or are in contact with fluid-filled containers. Hydraulic stress is also typically secondary and is considered in specific cases where fluid pressure needs to be accounted for.

3. Tension: Tension is the opposite of compression and occurs when a column is subjected to an axial load that tends to elongate the column. While columns are primarily designed to resist compression, they may also experience some degree of tension depending on the loading conditions. However, tension is not the primary type of stress in columns.

To summarize, columns are primarily loaded in compression, and the design of columns is focused on providing adequate strength and stability to resist compressive forces and prevent buckling.