Structural Concrete - Hassoun

712 Chapter 18 Stairs
The stairs may be supported at some edges within adjacent walls or may be designed as a
free-standing helical staircase, which is most popular. The structural analysis of helical staircases
is complicated and was discussed by Morgan [1] and Scordelis [2] using the principles of strain
energy. Design charts for helical stairs are also prepared by Cusens and Kuang [3]. Under load, the
flight slab will be subjected to torsional stresses throughout. The upper landing will be subjected to
tensile stresses, whereas compressive stresses occur at the bottom of the flight. The forces acting at
any section may consist of vertical moment, lateral moment, torsional moment, axial force, shearing
force across the waist of the stairs, and radial horizontal shearing force. The main longitudinal
reinforcement consists of helical bars placed in the concrete waist of the stairs and runs from the
top landing to the bottom support. The transverse reinforcement must be in a closed stirrup form
to resist torsional stresses or in a U-shape lapped at about the midwidth of the stairs.
A study was made to determine the effect of the following parameters on the forces and
moments that develop on helical staircases. These parameters are:
1. The total arc subtended by the helix with an angle that normally ranges from 240 ∘ to 360 ∘ .
Referring to Fig. 18.17, for 16 equal runs at 20 ∘ pitch, the total arc equals 320 ∘ . If the arc
is increased from 240 ∘ to 360 ∘ the vertical moment may increase by about 1200% for a live
load of 100 psf. Other forces increase appreciably.
Reinforced concrete helical staircase.