The case for precast staircases

Jeremy Milbank of Milbank Concrete Products looks at reasons for specifying concrete staircases and what factors to bear in mind when doing so.

Precast concrete staircases have the twin compelling benefits of being highly durable and fireproof, as well as being able to accept a variety of finishes. This is why they are a popular option for architects in a wide variety of settings, however there are a variety of factors to bear in mind if you are considering going for precast concrete as an option for your project.

One of the key practical plusses on site is that precast concrete staircases eliminate the disruption associated with the formwork and propping needed for in situ flights, also because they are manufactured in factory controlled conditions they tend to be of higher quality. Precast staircases can be installed in a day and provide immediate access to upper levels for following trades.

There is no such thing as a standard precast staircase however. All are bespoke products, with risers, goings (the horizontal distance between the face of the first and last risers) and widths tailored to meet the requirements of a particular section of a building. Flights can be supported at each end on cross landings spanning the width of the stairwell, or have attached top and/or bottom landings. Some straight flights are manufactured with integral intermediate landings. Where headroom is not an issue, winders can be accommodated by thicken- ing the intermediate landings, but where headroom is limited, full storey-height flights with swept soffits can be produced.

There are a plethora of choices, and which is right for a particular project will depend on the set of key parameters in each case. In the following paragraphs, some of the other important aspects of precast concrete staircase design and construction which need to be interrogated by specifiers are covered, to help with thinking around which option to pick.

Waist thickness is governed by the span to depth ratio together with the amount of cover to the main reinforcement needed to meet durability and fire resistance requirements.

The span to depth ratios, set out in Eurocode design codes, ensure that total deflection is limited to one two hundred and fiftieth span, and normally limits the part of deflection occurring after construc- tion of finishes to the lesser of one five hundredth span or 20 mm. As a result, flights spanning between cross landings can have a thinner waist than those with attached landings.

Normally 20 mm cover is sufficient for concrete stairs inside buildings with one hour fire resistance.

Most precast stair manufacturers will have a selection of moulds on which a flight may be manufactured. These may include:

• Flights cast the normal way up, on ramps which are adjustable in pitch; these flights will have trowelled treads and ex-mould finishes to other surfaces;

• Flights cast on their side, so that only the string is trowelled.

Alternatively, the flights can be cast upside down; these flights will have a trowelled soffit and ex-mould finishes to other surfaces.

Curved stairs and storey height dog-leg flights can be awkward to transport and will require extra strapping to ensure that the load is secure when being taken to site. Also, because of the flight shape, it may only be possible to carry one or two units, which may be considerably less than the vehicle’s maximum load capacity, hence haulage costs will be higher.

Each element of a precast stair may weigh over a tonne, so will require a
crane to offload and hoist into position. Most elements are simply supported, although bolted fixings can be provided to cater for progressive collapse details. In the absence of intermediate supports, curved stairs will require temporary propping before forming a moment connection to the supporting structure, either by means of in- situ concrete stitches or welded angles.

Jeremy Milbank is the technical consultant at Milbank Concrete Products