The usual form of stairs can be classified into two types: (1) those spanning horizontally in the transverse direction, and (2) those spanning longitudinally.
8.8.1 Stairs spanning horizontally
Stairs of this type may be supported on both sides or they may be cantilevered from a supporting wall.
Figure 8.17 shows a stair supported on one side by a wall and on the other by a stringer beam. Each step is usually designed as having a breadth b and an effective depth of d = D/2 as shown in the ligurc: a more rigorous analysis of the section is rarely justified. Distribution steel in the longitudinal direction is placed above the main reinforcement.
Details of a cantilever stair are shown in figure 8.18. The effective depth of the member is taken as the mean effective depth of the section and the main reinforcement must be placed in the lop of the stairs and anchored into the support. A light mesh of reinforcement is placed in the bottom face to resist shrinkage cracking.
Light mesh h-
The stair slab may span into landings which span at right angles to the stairs as in figure 8.19 or it may span between supporting beams as in figure 8.20 of example 8.10.
The permanent load is calculated along the slope length of the stairs but the variable load is based on the plan area. Loads common on two spans which intersect at light angles and surround an open well may be assumed to be divided equally between the spans. The effective span (/) is measured horizontally between the centres of the supports and the thickness of the waist (h) is taken as the slab thickness.
Stair slabs which are continuous and constructed monolithically with their supporting slabs or beams can be designed for a bending moment of say Fl/10. where F is the total ultimate load. However, in many instances the stairs are precast or constructed after the main structure, pockets with dowels being left in the supporting beams to receive the stairs, and with no appreciable end restraint the design moment should be Fl/8.
Design of a stair slab
The stairs are of the type shown in figure 8.20 spanning longitudinally and set into pockets in the two supporting beams. The effective span is 3 m and the rise of the stairs is 1.5 m with 260mm treads and 150 mm risers. The variable load is 3.0 kN/nv and the characteristic material strengths are /ck 30N/mnr and/>* = 500N/mnr.
Try a 140 mm thick waist, effective depth. <1 115 mm. This would give an initial estimate of the span-effective depth ratio of 26.1 (3000/115) which, from table 6.10. lies a little above the basic value for a 'lightly stressed' simply supported slab.
Stairs supported by beams
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