Showstopping concrete stairways have been the star turn in a number of recent high-profile projects. Elaine Toogood, architect at The Concrete Centre outlines the main considerations facing designers.
Concrete is a practical and cost-effective material for fire escape and accommodation stairs – but it also provides plenty of opportunity for the creation of highly individual feature stairways through its versatility of form, texture and colour. Concrete’s inherent solidity brings both a sense of permanence and effective acoustic dampening of footfall. The robustness and fire resistance of concrete stairs permits installation early in the construction phase with potentially little or no need for additional protection. This provides means of access and escape throughout the build process, with associated temporary works savings and health and safety benefits.
In the UK, the manufacture of prefabricated concrete stairs and landings is well established, with a substantial industry producing bespoke systems or standardised ones that can be tailored to suit project requirements. Elements are often cast in whole or half flights using pre-existing high-quality moulds designed to be reused multiple times. The moulds are adjustable to suit project specific tread and riser dimensions and the number of steps required. Alternatively, custom-built moulds can create more individual stair designs. The choice of formwork depends largely on how many stairs are required, their shape and finish. Concrete stairs can also be cast in situ to produce seamless, free-flowing forms that are integrated into the surrounding structure. They offer a bespoke solution where the practical installation of precast elements might be difficult.
Many concrete stair manufacturers offer a design and manufacturing service, and designers are wise to discuss stair proposals with them at an early stage to allow the manufacturing process to inform detail development. This is especially important if the concrete stair is intended to be left exposed.
It should be noted that responsibility for ensuring that the general layout and detail of the staircase configuration satisfies Building Regulations remains with the project architect or designer.
Described below are a number of areas that require special consideration when designing concrete stairs.
Span and depth
The depth of concrete for stairs refers to the flight waist thickness and depth of the landings.
A good rule of thumb for flights is span/25 (for simply supported spans). This can be reduced to span/30 if classed as continuous, as is often the situation for in-situ concrete flights and landings. Allowing a 200mm-deep zone for structural precast landings should be sufficient to accommodate either a halving joint detail or a screed topping to cover fixing brackets. Structural design will verify exact final dimensions.
A useful tool for structural engineers is the RC spreadsheets available from The Concrete Centre, which enable rapid production of design calculations to Eurocode 2 and for BS 8110. Spreadsheet TCC 71 relates to stair design.
Standardised precast concrete stairs are produced to a high quality of finish and tolerance, usually in grey concrete and not necessarily designed to be left exposed. Specifiers should establish pre-tender which of the finishes offered by the manufacturer will meet the project requirements. One surface of the concrete stair will be a trowelled finish, depending on the orientation of manufacture. For example, stairs precast upside-down have formed tread and riser faces and a trowelled soffit to the flight. In all in-situ concrete stairs, the face of each riser and the underside of the concrete stair will be “formed” finishes and each tread will be the unformed trowelled surface. Such exposed stair treads require skilful execution to minimise the difference in finish to the adjacent riser.
Specialist suppliers can provide concrete stairs in a range of colours and textures using blends of aggregates and pigments. Finishing techniques for bespoke stairs include acid etching, abrasive blasting, polishing, and exposing aggregate through surface retardant. All except polishing can be used to improve slip resistance and are best specified through reference to samples.
Fixings and junctions
A commonly overlooked junction on exposed concrete staircases is the underside, where stair flight meets landing or half landing. There are many solutions for fixing stairs to landings depending on the floor and frame construction.
Halving joints, where the stair and landing interlock using overlapping nibs of concrete, can provide a neat solution but are not practical for in-situ concrete landings. Factory-cast steel brackets or plates may be recessed or surface mounted, with differing implications for the width of joint visible on the underside. Integrated or attached half or full landings avoid this issue, but the waist of the stair increases due to the extra span. Although this form can facilitate very quick installation of the whole stair structure, it is less efficiently stacked and transported.
Nosings and other inserts
Where contrasting nosings are required, these are most simply applied to the concrete surface after manufacture. Simple recesses can be cast in the concrete to receive inlaid materials, though this is very difficult to achieve on the unformed faces. Carborundum non-slip inserts can be cast into the precast concrete flights.
Lifting eyes are needed so that precast elements can be manoeuvred within the factory and also for placing on site. For many applications they are typically left or grouted up after installation, unseen against the side of the wall or under a stair covering. Their installation needs to be controlled on staircases featuring exposed concrete, especially since some of the eyes are likely to be located on the face of the treads. A neat solution is to cast in threaded sockets. The lifting eyes can then be replaced by decorative discs, for example, in stainless steel or a matching concrete finish.