17 September 2002
Atria design strategies
Atria can be high consumers of energy and difficult spaces to condition effectively. Large volumes with high occupancies, these spaces challenge both mechanical and fire engineers. Buro Happold engineers Byron Stigge, Stuart Martin MRICF and Mark Owen IEng ACIBSE, MASHRAE discuss how these challenges can be met with efficient and elegant solutions.
You’ve been assigned a four-storey atrium with skylights, a big south facing façade. Has the architect included any shading whatsoever? Usually not. Bridges and balconies overlook the space at high level. The tall volume encourages a temperature gradient to develop since ‘hot air rises’. And because of cost issues the skylight windows are inexpensive and not thermally broken, so condensation develops up where warm, moist air meets the cold thermal bridge of a skylight mullion. These issues make mechanical engineers striving for sustainability and indoor environmental quality cringe.
Displacement ventilation offers significant advantages for climate control in atria, by providing supply air volumes at low velocity directly to the occupied zone. Instead of working on the mixing principle (all air in a room is well mixed and at the same temperature) displacement ventilation works on the buoyancy principle (hot air rises). Lower supply air volumes are needed as unoccupied areas are allowed to float outside the comfort range, making ducts, air handlers and chillers smaller.
By natural thermal convection warm stale air, with its impurities, rises and is displaced by cool fresh air. As people, lights and other heat sources warm this cool, fresh air it then rises to be exhausted at high level. The result is that supply air constantly refreshes the space in a natural flow by displacing older air – at lower operating costs than a conventional system.
An exciting feature for architects is that the supply air grilles can be discreet and integrated with the architectural design. Displacement systems are not necessarily under-floor systems, but are any low velocity supply directly to the occupied area. The outlets can be design features, and don’t need to be pre-packaged diffusers. Any large surface with porosity or cracks that will allow air to seep out at about 30fpm will suffice. This gets architects involved in designing the ventilation system, thereby integrating it with the rest of the design and elevating its priority, making it easier to pass ducts and pipes.
Despite many advantages of displacement ventilation, however, there are several criteria that have to be carefully considered when designed. In high occupancy spaces the lower volumes of supply air and higher supply temperatures may not sufficiently remove the latent load, causing humidity to build up over time. Also, diffusers are usually placed near or on the floor, so supply air velocities should be less than 30fpm to avoid uncomfortable drafts around people’s feet and ankles. If a large porous plate is used as the diffuser, the porosity of plate of the displacement diffuser should be between 8% and 30%. Diffuser manufacturers have designed and tested low-velocity diffusers, for which dimensions, flow-rates, throw-distances, and velocities are all published. The properties of these manufactured diffusers can be used to benchmark custom designed diffusers.
Skylights bring in wonderful daylight to nourish plants and open peoples’ eyes. But they also bring in huge solar gains. Fixed louvers work effectively to block summer sun at high angles and accept some winter sun at lower angles. Motorised shades would be ideal, but maintenance is particularly difficult. For deep atria (taller than 10 storeys) sunlight will not penetrate down to the floor level, so a more sophisticated technique of using solar tracking mirrors can be used to bounce light evenly down straight to the floor level on all sunny days throughout the year.
Condensation on skylights is caused when the inner surface of the glass is below the dew point temperature of the air next to it. This is always a concern for cold weather climates, but particularly for atria skylights as the air at the top tends to be moister than room air as it collects moisture from more people, plants and water features. Sufficient high-level exhaust helps keep moist air from building up. But more importantly, thermally broken, double-glazed insulating glass will be the major element preventing condensation.
Balconies overlooking atria are wonderful circulation spaces that can connect organisations that occupy multiple floors of a building. They also protrude into the higher air zone that is warmer due to stratification.
Return air can travel through the atrium volume instead of dedicated ductwork if: 1) the floor plates open up to the atrium, 2) the space is open plan, 3) the return fans are located high in the atrium and 4) the floor plates are not deep (more than 9m). This technique works particularly well for 100% fresh air systems with fan coil units as small amounts of fresh air are supplied and all return air is exhausted.
Radiant floor heating is often an appropriate heating system as loads in atria are relatively even, low and predictable (8am-7pm).
Natural ventilationis often an appropriate strategy for atria. Tall spaces allow the stack effect to drive air pressures higher, so ventilators at high level can move a good deal of air. Atria usually have less stringent temperature and humidity requirements, especially when atria are physically separated from adjoining floor plates.
Computational fluid dynamics (CFD) models airflows and temperatures throughout a three dimensional space. This, albeit complicated, modelling technique verifies non-conventional strategies such as natural ventilation and displacement ventilation.
Atria send flags up to fire officers, so extra design and analysis for both evacuation and smoke exhaust are always required. The tendency to consider that a building with a large hole through the floors will be more dangerous than an equivalent non-atrium building is common due to the increased risk of fire and smoke spreading up and through the whole. To ascertain if there actually is an additional risk, several factors need to be considered. Firstly, what is the aim of the design? Is it to protect life during the escape period, to increase the safety of fire fighters who extinguish the fire, or is it to protect the building itself, the property inside the building, or the business activities (computer systems) inside the building. Each level of safety requires a different strategy to deal with the effects of fire and smoke spread/containment. It is essential at the start of any design process to define the goals so that a detailed design can be developed.
Means of escape only
Define the type of building and profile of the occupier so that the degree of risk to which they are exposed can be established. The degree of risk varies by a) those who are awake and familiar with the premises, b) those who are awake and unfamiliar with the premises, c) those who may be asleep, and, d) those receiving medical care. Clearly, the risk to people who are familiar with a premises and awake will be lower than those who may be asleep. The awake occupants’ response times to an alarm and the time taken to locate exits will be quicker. Also, if the proposed building’s fire compartment dimensions exceed those of the equivalent non-atrium building, there is a greater risk.
Often, though, the circulation zone passes along the atrium perimeter overlooking the atrium. Then smoke concentrations impede visibility and are dangerous to inhalation. If no balconies are present it is recommended in some codes that doors to escape stairs should be sited so that they are at least 4.5m away from any void edge.
Fire protection systems associated with the various occupier profiles can vary greatly. The inclusion of a particular protective system will be based on the ability of an uncontrolled fire to grow and spread through a space to the point where the conditions become untenable for escape.
Fire and smoke prevention systems
Automatic suppression, typically water based
More extensive fire detection and alarm systems, which may incorporate voice annunciation
Active fire shutters or doors to close off the atrium space or reduce the flow paths of flame and hot gases
Expensive fire rated glazing systems which incorporate radiant heat reduction properties where architects desire open aspects
Smoke control systems whether they are natural or mechanical in operation (smoke exhaust fans at high level).
Computational fluid dynamics (CFD) can model smoke generation, extraction and concentration levels in a three-dimensional space. Results from these models can be used as a tool to determine appropriate mechanical extract capacities, locations and area sizes. Then, the results can be presented to fire officials for variances to codes or value engineering reductions
Computational fluid dynamics were used to optimise the atrium design at the British Airways headquarters located next to London’s Heathrow Airport. Unoccupied areas near the edges are allowed to float beyond the normal comfort range.
Buro Happold engineers incorporate an effective HVAC and fire protection design scheme at the Poole Study Gallery, Dorset.
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