Staircase proportioning rule: in stair design, the commonly used comfort relationship between the rise (R) and the going/tread (G) is kept approximately equal to which value?

Introduction / Context:
Designing comfortable and safe stairs involves proportioning the rise and going so that users can walk naturally. Rules of thumb relate R and G to a target value based on human gait.

Given Data / Assumptions:

• Rise R and going G are measured in millimetres.
• Use of standard ergonomic proportions.
• Question expects the widely taught comfort rule.

Concept / Approach:
The classic Blondel formula is 2R + G ≈ 600 mm (often within 600–650 mm). This places steps within a comfortable stride range. Other variants like R + G ≈ 450 mm appear, but 2R + G is the most recognized basis for proportioning.

Step-by-Step Solution:
Identify the intended ergonomic relationship → Blondel's rule.Select the benchmark value commonly rounded to 600 mm.Therefore, the answer is 600.

Verification / Alternative check:
Building codes and handbooks endorse ranges like R between 150–190 mm and G between 250–300 mm, which satisfy 2R + G around 600–640 mm.

Why Other Options Are Wrong:

• 350/420/450/500 do not represent the standard 2R + G comfort target for most occupancies.

Common Pitfalls:

• Confusing the rule as a product R*G; the common comfort relation uses a sum with 2R.

Final Answer:
600