If R and T are rise and tread of a stair spanning horizontally, the steps are supported by a wall on one side and by a stringer beam on the other side, the steps are designed as beams of width A. R + T B. T – R C. $$\sqrt {{{\text{R}}^2} + {{\text{T}}^2}} $$ D. R – T

The correct answer is $\sqrt {{{\text{R}}^2} + {{\text{T}}^2}}$.

The width of a beam is the distance between its two supports. In the case of a stair, the two supports are the wall on one side and the stringer beam on the other side. The rise and tread of a stair are the vertical and horizontal distances between each step, respectively.

The width of a stair step is therefore equal to the square root of the sum of the squares of the rise and tread. This is because the width of a stair step is the hypotenuse of a right triangle, where the rise and tread are the legs of the triangle.

For example, if the rise of a stair step is 6 inches and the tread is 8 inches, then the width of the stair step is $\sqrt {{{\text{6}}^2} + {{\text{8}}^2}} = \sqrt {100} = 10$ inches.

The other options are incorrect because they do not take into account the rise and tread of the stair.