Final answer:
Each tread of a spiral staircase should have a depth of not less than 7.5 inches at the walkline, following modern safety standards.
Step-by-step explanation:
The depth of each tread of a spiral staircase at the walkline should be no less than 7.5 inches (190 mm). This measurement is based on safety standards to ensure that each step is deep enough for safe footing. In historical and modern design practices, dimensions may vary. For instance, an old rule of thumb suggested treads to be 7-8 diameters deep, while contemporary standards recommend up to 15 diameters. However, for a clear and specific guideline, modern building codes, such as the International Building Code (IBC), generally stipulate that stair tread depth should be at least 7.5 inches at the walkline.
In the context of a spiral staircase, the depth of each tread at the walkline should be not less than 15 diameters. This means that the depth of the tread should be 15 times the diameter of the staircase. The concept of depth in this case refers to the horizontal distance from the front edge of one step to the front edge of the step below it.