What type of reinforcement is only acceptable for structural use in spiral reinforcing?

The correct answer is plain reinforcement. In the context of spiral reinforcement used in concrete construction, plain reinforcement refers to smooth or round steel bars. This type of reinforcement is particularly important in applications like spiral columns, where it helps achieve a uniform confinement effect around the core concrete.

Plain bars can easily be manipulated and placed in a spiral configuration, which is essential for the structural integrity of certain designs, particularly in seismic zones. Additionally, the smooth surface of the bars allows for better consideration in terms of stress distribution within the concrete, providing the necessary confinement and enhancing overall performance.

Understanding the role of plain reinforcement in spiral applications is critical for ensuring that structural requirements are met, as its design allows concrete to achieve its full load-carrying capacity by mitigating potential buckling in columns under axial loads. The other types of reinforcement mentioned are not specifically suited for the nuances involved in spiral reinforcement applications.