Design criteria

Principal criteria governing the selection of the structural systems include the climatic role to be played by the envelope, the visual character sought and the practicalities of cleaning and maintenance. Two further detailed issues - fire resistance and thermal movement - also need careful consideration.

Climatic role

The role of the atrium in relation to the internal and external climate will influence its form.

The role to be played by the atrium roof and outer walls as climate modifiers can be important to the choice of structure.

• Is the atrium to be fully climate controlled, or a freewheeling buffer space?
• Are the climatic location and building use ones which require a warming atrium (one which admits solar energy freely), a cooling atrium (an anti-solar shading structure) or a convertible atrium (one able to switch roles seasonally or daily)?

Warming atria will need to admit solar radiation but avoid overheating in summer.

A warming atrium will be oriented to collect sunlight, with its exact form related to latitude. Equator-facing glazed side walls and lantern lights would be expected, with roofs having a fully glazed surface in mild climates. Colder climates could need insulated lantern light sides, which additionally reflect low angle sun. Warm temperate climates could need saw-tooth roofs designed to admit winter sun but reflect summer sun angles.

Cooling atria need to admit light but minimise heat gain.

A cooling atrium seeks to admit light without significant solar penetration. Polar facing saw-tooth roofs and side walls might be needed, or solid roofs and fixed external shading features in tropical latitudes where all surfaces receive sun at some time.

Devices such as louvres can be used to change the characteristics of an atrium according to current conditions.

Convertible atria have to use moveable features to change from admitting sun to rejecting it. Banks of operable louvres on solar facing slopes are the most common approach. These are ideally external to the roof or wall but are often placed internally to increase reliability, though at the cost of increased thermal stress in the structure and louvre system.

If the internal space is climate controlled, 'ducts' may need to be integrated into the structure.

The frame may be asked to carry air or water for climate control of the atrium space. Box columns and beams at the Bank of Canada in Ottawa carry air supplies to the cavity between two glass skins. In other buildings air may be blown across glass to remove condensation, or hot water carried to combat downdrafts by radiation. The structure may be asked to carry these services rather than obstructing views with a further set of members.

Natural ventilation should account for the stack effect.

Natural ventilation, if provided, brings considerable stress as atria are natural chimneys with attendant stack pressure differentials. These must be accounted for in the design in order to keep doors and louvres operable.

Glazing type can also help in controlling the internal climate.

Choice of glazing materials is also a way of providing admittance, insulation and shading, but assigning a climatic role to the envelope gives pointers to the shape and support needs which the selected system should possess.

Visual character

Atria create a powerful visual impact, which is strongly influenced by the structural form used.

The atrium will probably be the part of the building with the strongest architectural character.

The structure of the atrium is very likely to be the most visible and expressive structure in the whole building. Choice of structural approach will thus be strongly influenced by the character sought. The examples which illustrate this chapter indicate the enormous range of effects which can be achieved. Some designers choose a solid atrium wall with windows, continuing the building's general frame, whilst others seek diaphanous fabric, with maximum uninterrupted transparency.

A wide variety of structural forms can be used.

Most use the rhythm of steel members to create scale and interest in themselves, usually in contrast to the expression of the surrounding building.

Frames can be light and close-centred or heavy and widely separated, shallow and sophisticated or heavy and industrial, with or without intermediate column support. Victorian forms can be selected to be consistent with historic character in the surrounding buildings or neighbourhood. Farm building shapes can be used in rural locations. Futuristic shapes are often found in resort or downtown buildings. Metaphor and allusion govern formal decisions on visible structure as much as in any other architectural element today.

Cleaning and maintenance

Provision for efficient cleaning and maintenance of the glass is an important design consideration.

An industry is developing in cradle systems to give access to external and internal atrium surfaces. Some of these systems are very complicated and overcome extremely difficult access problems set by designers. Others are ugly intrusions which spoil the architect's concept. In contrast, other atria are dingy and unkempt because the access problem could not be solved economically.

Incorporation of provision for feasible cleaning and maintenance gear into the design process is a powerful discipline.

Many roof and wall shapes are ruled out, but the final design retains its effect as the glass will be clean. Ease of cleaning favours extruded forms along which a gantry can run, rotational forms which can have a rotating gantry, and vertical planes serviceable from cradles. Tapering shapes are problematic, as are complex intersections. Where the structure allows integral catwalks this can make internal surfaces, lights and ventilation details accessible without gantries or cradles.

Fire protection

Atria do not generally require structural fire protection, although insurers may insist on sprinklers or intumescent painting.

Roofs and external glazed walls are not regarded as elements of structure, in the sense that no occupied space is supported by them. They do not therefore require fire protection for life safety, which is the limit of regulation requirements. Insurers are concerned, however, by the cost of replacing such large and expensive features in the event of a major fire, even if occupied space is relatively well protected from damage. Roof structures in steel may need to be fire protected, with the choices including intumescent paint or sprinklers.

Smoke control is a greater concern in relation to life safety.

Smoke removal strategies for atrium buildings sometimes involve use of the atrium, with fans or natural ventilation openings in the roof. Volume is needed below the roof and above inhabited levels, especially if open to the atrium, to accommodate a smoke cloud being evacuated. This requires a lantern roof, with height to match the calculated smoke handling need. BS 5588 part 7, issued in 1991, for the first time provided consistent fire safety standards for atrium design.

The fire safety plan for an atrium is influenced by a number of parameters.

The basic parameters of a fire safety plan are the height of the building, the extent to which the atrium enclosure separates it from the rest of the building and the function of the floor of the atrium. The LDSA has published design guidance which includes typical building cross-sections with deemed-to-satisfy solutions incorporating a variety of safety measures. With increasing height of building, the stringency of the fire safety requirements increases. Safety precautions that may be employed include alternative methods of evacuating the building, the extent of fire resistance of the atrium walls and the use of sprinklers, heat vents and smoke controls.

In future it will be possible to design atrium buildings without safety standards being over specified as they have been in the past. Also, mixed-use buildings will no longer be regarded unfavourably as long as alarm and sprinkler systems are designed to be adequate to the most onerous of possible conditions.

Thermal movement

High temperature differentials must be accounted for in relation to structural movement and thermal comfort.

Atrium envelopes are light, with large and rapid thermal movements as the framework and glazing pick up solar heat. In atria without air circulation systems a temperature gradient of 1° C for every metre of height is normal in the UK. Thus the top of a 20 m tall atrium may be 20° C above ambient temperature. The whole roof must move successfully but the point of maximum danger is the joint between envelope and occupied building fabric where differential movements can be pronounced. This is especially complex where atrium roofs intersect occupied building walls. Heat under such roofs is a problem for comfort in immediately adjacent accommodation.