Concrete in an Axially Loaded Spirally Reinforced Short Column

Introduction:
Axially loaded short columns are structural members that primarily resist compression forces. These columns are reinforced with steel bars to enhance their load-bearing capacity. In the case of a spirally reinforced short column, steel bars in the form of helical spirals are used to provide additional strength and confinement to the concrete core.

Concrete Behavior:
When an axially loaded spirally reinforced short column is subjected to an external axial load, the behavior of the concrete inside the core can be analyzed. The concrete experiences different types of stresses due to the interaction between the axial load and the spiral reinforcement.

Biaxial and Triaxial Compression:
In an axially loaded spirally reinforced short column, the concrete inside the core is primarily subjected to triaxial compression. This means that the concrete experiences compressive stresses in three perpendicular directions. The axial load causes compression in the vertical direction, while the spiral reinforcement provides confinement and induces compression in the radial and tangential directions.

The confinement effect of the spiral reinforcement increases the compressive strength and ductility of the concrete core. The spirals prevent lateral expansion and bulging of the concrete, resulting in a more efficient load transfer mechanism. The concrete is confined to a smaller area, leading to higher stress levels and improved strength.

Bending and Compression:
Unlike the options mentioned, the concrete inside the core of an axially loaded spirally reinforced short column is not subjected to bending and compression. Bending occurs when a structural member is subjected to a combination of axial and lateral loads, resulting in tension on one side and compression on the other. However, in an axially loaded column, the primary load is axial, and the concrete experiences compression in all directions due to the confinement provided by the spiral reinforcement.

Therefore, the correct answer is option 'C' - triaxial compression. The concrete in an axially loaded spirally reinforced short column is subjected to triaxial compression due to the axial load and the confinement provided by the spiral reinforcement. This triaxial compression enhances the strength and performance of the column.