Core planning and standardized stairs

Lift and stair cores in commercial buildings can be grouped according to building use - hotels, offices, retail and special facilities (such as multi-purpose layouts that contain conference and exhibition space). Building Codes in Britain stipulate minimal tread-to-rise relationships that vary according to the 'use' category;1 these are dealt with in Chapter 10.

A wise designer ought to take the most generous proportion in multi-use buildings so that a common denominator in stair risers is maintained throughout. This means that standardized stairs can be made and a modular approach adopted to vertical dimensions within the total building section (Figure 3.1).

Under British Codes, the 180 mm riser x 280 mm tread provides the statutory minimum stair proportion for all categories of use within a multi-purpose design. Since 1999, however, there are new regulations on access for the disabled, and if a lift is not provided the stair has to be reduced to 170 mm maximum rise x 250 mm going. This ratio, however, does not match the comfort of the easy going 150 mm x 300 mm steps that served a similar role in the days of Sir Christopher Wren and which related to a brick module of 75 mm.

Today, commercial considerations play a dominant role in demoting stairs to a secondary place whilst lifts or escalators assume greater importance. Valuation surveyors run their rules over staircase dimensions to establish that minimal criteria have been applied since core plans do not count within the lettable area. The design process with hotels or offices must take this into account and every effort made to reduce wastage in core planning. In fact, larger scale layouts are worth developing at an early stage to establish options for floor-to-floor heights in relation to the size of stair shafts that have to be accommodated. A well 250 mm wide not only looks attractive but can absorb two risers at either end which in turn allows a 720 mm variation in storey height without increasing the overall size of the shaft (Figure 3.2). Floor areas and occupancy rates provide the traffic figures for lifts and stairs. Building Regulations allow a minimum of two steps, a single step being considered dangerous. Sizes for the former can be taken from installers' catalogues whilst the latter has to be approved by the fire brigade or licensing authority on means of escape in case of fire. The key dimensions relate to the maximum number of steps within an unbroken flight and the number and width of stairs needed for escape purposes.

Compactness with escape stairs can be achieved by stacking two sets of flights one above the other with a solid reinforced concrete wall separation (Figure 3.3a). A partial reinforced concrete fender wall, say 100-150 mm thick to infill the well, takes up less space than a

Poor relation to brick/block sizes

Figure 3.1 Modular heights for steps

Poor relation to brick/block sizes

Figure 3.1 Modular heights for steps conventional metal balustrade, particularly with those designs that embrace swept handrailing at well ends (Figure 3.3b). A half-step relationship across the well in dog-leg stairs will ease the geometry of handrailing and only add half a

Normal dog-leg layout

Figure 3-2 Steps at well end

Normal dog-leg layout

Figure 3-2 Steps at well end step in the length of the flight (Figure 3.3c). Chapter 8 details this construction.

The completion of the preparatory work can provide a matrix in establishing the repetitive upper floor plans whether hotel, office or retail. It is common practice to set out the main elements of the upper storeys in order to develop the 'footprint' of the buildings at ground level. The dichotomy that plagues core design rests with the attractions needed at entry level by comparison with the minimal qualities where immersed within lettable space at the upper floors.

The simplest solution is to unwind the principal flight into a more generous geometry at the entrance hall. A typical example is where a dog-leg design turns into an open well arrangement. Modern developments with fire resisting glazing can recapture transparency instead of stair shafts being hidden away in walled-off enclosures.

Figure 33a Solid wells: Stacked escape stairs

Northern Flight Dog LadderStair Core Arrangements
Figure 3 3b Solid wells: Fender walls

Figure 33c Solid wells: Half-step relationship

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