Structural Concrete - Hassoun

18.2 Types of Stairs 711
For a symmetrical stair flight, fixed at both ends and subjected to a number of concentrated
loads at the node of each run, the moment at the fixed end can be calculated as follows:
M (fixedend) = 1
12 PS (n2 − 1)
where
P = concentrated load at node of run
S = width of run
N = number of runs
When n = 2, then
M (fixedend) = 1
12 PS (4 − 1) =1 4 PS
which is the same result obtained earlier.
If a landing is present at one or both ends, the load on the landing may be represented by
concentrated loads at spacing S.
8. Helical stairs (open-spiral stairs): A helical staircase is a three-dimensional structure, which
usually has a circular shape in plan (Fig. 18.17). It is a distinctive type of stairs used mainly
in entrance halls, theater foyers, and special low-rise office buildings. The cost of a helical
stair is much higher than that of a normal staircase.
Figure 18.17
Plan of a helical staircase (16 equal runs at 20 ∘ pitch).