Developed forms of reinforced concrete stairs

In situ reinforced concrete has the same limitation of span to thickness as floor slabs so that single span flights are limited to about 4 500 mm unless downstand spine beams or upstand balustrade beams are used. The spine beam version is usually pre-cast and implies pre-cast or metal-framed cantilever treads.

Dog-leg stairs are usually carried on cross beams at half landings or else on a thickened landing slab. Balustrade beams convert slab forms into slabs with twin upstand components which permit clear spans floor-to-floor with dramatic shapes as exemplified in the foyer to Sir Denys Lasdun's National Theatre (Figure 8.2$. Lasdun's use of concrete surfaces and texture celebrates the sculptural possibilities of a material that in monolithic construction can perform many roles. The foyer lighting enhances the surface treatment of board-marked concrete and accents the importance of the stair shafts rising through the extensive volume. The concrete profile echoes the tread-and-rise relationship while the stainless handrail-ing (bracketed off the solid balustrade) follows the awkward dictates of the bylaws without spoiling the overall form (as in Figure 8.2d).

Repetition in concrete stairs favours pre-casting the components. It is difficult to generalize but with high stair costs it is worth considering pre-cast manufacture that will simplify details into variants of Figures 8.2 b and 8.2c.

Pre-cast stairs can be made in site factories (Figure 8.3a) and completed at this stage with a range of finishes (Figures 8.3d-/ ). Spine beam stairs are more

Figure 8.2a Developed forms of reinforced concrete stair: Simple RC slabs (with and without landing beams)

Pre-cast edge

Pre-cast edge

Figure 8.2b Pre-cast treads and beams

Connected

Connected

Tube supports let in pre-cast holes

-Coping blocks

â– Vertical bar reinforced to concrete block balustrade

Tube supports let in pre-cast holes

-Coping blocks

â– Vertical bar reinforced to concrete block balustrade

Plate sleeve

Plate sleeve

-Bracket

Set screw

Sleeve

-Bracket

Set screw

Vertical bars grouted into holes

Standard height for landings and flights

Sleeve

Cellular blocks

Vertical bars grouted into holes

Cellular blocks

Height to follow basic geometry

Face plate set screwed to back plate

Varying height to accord with regulations

Face plate set screwed to back plate

Guarding follows regulation height

Varying height to accord with regulations

Guarding follows regulation height

Reinforced concrete upstand wall (in fair faced finish)

Upstand wall follows basic geometry of stairs

Solid balustrade

Reinforced concrete upstand wall (in fair faced finish)

Upstand wall follows basic geometry of stairs

Solid balustrade

Figure 8.2d Balustrade upstand beams efficient for long straight (Figures 8.3 b and 8.3c) or curved flights due to the lower weight per span compared with solid slabs. Pre-cast assembly increasingly depends upon steel bolt and plate fastenings which demand the accuracy and connection methods associated with structural steelwork. Pre-cast reinforced treads are often cast with the finishes integral with the component; terrazzo can be used as a trowel coat within the mould and as a wearing coat to the tread, the final grinding and finishing following the curing stage. A spine wall can serve as a dog-leg column with cantilever action in the treads; by contrast the shaft walls can have cantilever steps leaving the well open for a metal balustrade.

The profile chosen by Ralph Erskine at Clare Hall, Cambridge, allows the carpet to be wrapped around the concrete treads, the rounded corners preventing wear and permitting the carpet to be turned from time to time. The reinforcing is tied into the backing wall which forms the shell-like enclosure (Figure 8.2e).

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