14 May 2003
Auditoria environmental design strategies
Auditoria 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 to design efficient, elegant solutions. Byron Stigge, Stuart Martin and Mark Owen of Buro Happold suggest some auditoria environmental design strategies
The criteria for auditorium design come in many forms dependent on the type of auditorium, the performance requirements and the space itself. Auditoria for concerts, opera, ballet and theatre all have different environmental criteria and the conditions can change from theatre company to theatre company. At the very least, new auditorium designs need to include an element of flexibility unless catering for a specific purpose.
Environmental conditions
A temperature of around 20°C is frequently used as the design standard for auditoria often with an allowance for this to rise gradually through a performance to around 23°C. From experience it is essential to build flexibility into the plant and to be able to vary the internal environmental temperature from simple controls located in the theatre manager’s office.
The control of humidity, although not particularly important in general theatres (providing it stays within a range of 40 to 60%), is more critical in concert and operatic halls where the level of humidity can have an adverse effect on the performance of the human voice or musical instruments. On stage humidification systems are often provided to cater for this.
The amount of air supplied to auditoria both in terms of ventilation rates and fresh air has been the subject of heated debate throughout the theatre community. Current guidelines provided by bodies such as the Association of British Theatre Technicians (ABTT) state minimum recommended standards. These are based primarily on the ventilation requirements of more permanently occupied spaces and often do not relate to the particular conditions found in theatre auditoria. Many owners and engineers believe that the way theatres are operated (long unoccupied sessions interspersed with short periods of high occupancy) coupled with the large volumes, fresh air and ventilation rates should be investigated individually and not based upon fixed data.
The subject of room noise criteria is much debated in auditorium design and again will depend on the use of the space. It is essential to seek advice from a specialist acoustic consultant in relation to the acoustic performance of the air systems, penetrations through acoustic enclosures and isolation of the air systems from the structure itself.
System design
Traditionally many theatre and concert hall auditoria were provided with ventilation schemes utilising downflow, mixed air (air from high level) principles. As this generally occurred as the ventilation was provided as a retrofit into an existing space, the only way to install the ductwork/diffusers was via the roof void/high level. However recent new theatre projects have used more efficient methods for providing comfort including displacement and natural ventilation. Advances in CFD (Computational Fluid Dynamics) modelling techniques have helped in the effective design of such systems and these tools should ideally be involved in the design of any ventilation system to predict air movement and comfort levels.
Downflow mixed air systems
These systems use air volumes of generally between 11 litres/second to 14 litres/second per person, (with at least 8 litres/second of fresh air) discharging and exhausting air at high level through high velocity diffusers. Due to the mixed air principle and thus reduced supply air condition this type of system often requires lower air volumes than its displacement system counterpart. As the major duct installation is at a high level, this negates large risers to auditorium floor level and varied seating and stage arrangements can also be accommodated.
However, without careful diffuser selection this reduced supply condition can result in air “dumping” onto the audience creating uncomfortable down draughts. Careful grille/diffuser selection is also required to provide the correct air throw without generating high noise levels. Air distribution can also be compromised in the heating cycle due to the natural buoyancy of warm air leading to the fresh air not actually reaching the audience. These systems provide poor audience comfort control as the point of monitoring is traditionally located in return air ducting and is subject to gains not associated with the occupants, such as lights. There is also poorer air quality control due to contaminants remaining at low level.
Displacement ventilation
This is an altogether more suitable approach to ventilation in auditoria as the discharge air only treats the occupied zone. The air terminals with discharge velocities of around 0.15 to 0.2m/s (velocities above 0.25m/s can cause discomfort) are located at low level and air is exhausted at high level. The principle is that the supply air “creeps” around the auditorium floor until it encounters a heat load at which time the air will begin to gently rise. The air volumes can be as high as 14 litres/second per person but design volumes of around 8 litres/second per person are proven to give suitable results, especially if utilising exposed thermal mass.
The advantages of this system include supply air directly treating the occupied zone providing good comfort control, low noise levels, and the removal of contaminants and pollutants. Lower operating costs can also result from the free cooling capacity of the external air condition. In small theatres of less than 200 seats the scheme can be implemented without mechanical cooling using the thermal mass/night purging ventilation strategies.
Disadvantages include the requirement of a riser space to distribute air to low level, large numbers of diffusers to produce low velocity discharge, and balcony front upstands to avoid air “dumping” on the audience below. The use of displacement ventilation can also impact on the flexibility of seating and staging options.
In order to provide stage and seating flexibility it may be necessary to provide a hybrid solution for an auditorium. This can still be accommodated, for example using the principles of displacement with the inclusion of sidewall discharge points in areas where seating is removable.
Natural ventilation
Natural ventilation schemes have been successfully incorporated into recent new theatre designs but are generally limited to theatres with seating capacities of fewer than 300 seats. Air is drawn in at low level or below the seating and discharged at high level through the auditorium roof utilising the natural stack effect (generated by gains within the space). These schemes are often provided with supplementary mechanical ventilation to assist when the natural effect is not sufficient.
Advantages as with mechanical displacement schemes include supplying air directly treating the occupied zone, good comfort control, low noise levels, and low operating costs as the potential capacity of external air condition can be utilised for a greater percentage of the year.
However, the natural ventilation approach is not suitable for large theatres and theatres located in areas with high road traffic or low level pollutants. Also, the acoustic problems associated with large openings in the building fabric can preclude the use of natural ventilation in areas with high external ambient noise levels. There is also a limited response to high point loads, and reduced flexibility for seating and staging options. Requirements include chimneys or stacks to enhance stack effect, under seat plenums, large areas of external louvres for the intake of air, and large numbers of diffusers to produce low velocity (thus low resistance) discharge.
The most effective method for conditioning an auditorium space if achievable is a displacement system (either naturally or mechanically driven). However, each auditorium has its own identity and the solution to servicing will obviously relate to its usage and the type of space.
Auditoria fire design
Theatrical performances are played in one of two configurations: either end stage or thrust stage. In the end stage, the players and their props are viewed through a proscenium opening in the wall separating players from the audience. In thrust stage, where the audience becomes more intimate with the performers, the fire risk potential increases and more controls are demanded on the design. These two functional layouts have very differing fire performance requirements based on the risks the audiences are exposed to from the props and scenery.
To address the differences in risk two options are available for the design of the auditoria and stage areas. Both are simplistic and are generally rigid in application
End stage solution
This is a simplistic and rigid design approach following several rules. The auditorium is designed as one fire compartment and the stage and side stage areas are designed as a second fire compartment. There is no fire or smoke ventilation provision in the auditorium. The fly tower is provided with ventilation, and the fire control of materials on stage is considerably more flexible than a thrust stage configuration as the levels of pre treatment of the materials used is lower. The fire safety curtain is either a single fire rated element such as a fire shutter or is a water drenched curtain. The water drenching system is activated by either a mechanical link or manually by stage manager. Sprinklers are not generally provided on stage areas or within auditoria. A suitable fire detection and alarm system is provided to both spaces.
Thrust stage solution
Where the audience become more intimate with the performance the fire risk potential increases and more controls are demanded on the design. Scenery tends to be minimalist so that the audience, potentially seated on all sides of the players, can see the performance. The auditorium and stage areas are considered as one fire compartment. There is no fly tower as there is no flying scenery, therefore no stage ventilation is required. There are more stringent controls on the types of materials that can be used in the construction of scenery and sets. Pre treatment of materials is used to reduce the rate at which a fire can develop and spread through the scenery/props. Uncontrolled materials on side stage areas must be fire separated from the thrust stage/auditorium. There is no safety curtain as no proscenium opening exists.. A comprehensive fire detection and alarm system is provided to the one ‘combined space’.
In some design cases when these requirements are tested against the form, functionality and flexibility requirements of an auditorium, conflicts in the performance requirements may be derived at the line of the proscenium wall and the side stages. This may be the case when the theatre wishes to operate a mixture of End and Thrust stage performance types. Consequently, either the performances must adapt to one form of stage arrangement or an alternative solution to the fire safety curtain must be derived. Such a solution must provide adequate and appropriate measures that compensate for the lack of fire separation between the areas.
By establishing the design’s core performance requirements, the risk can be assessed and a set of goals defined for which solutions must be developed. An example of such an approach could be to provide an escape strategy based on the codes requirements, plus additional active fire control measures in fire risk areas by providing sprinklers and a smoke and heat exhaust ventilation system designed to maintain safe conditions in the space for not less than a defined escape time frame. This latter provision could also be beneficial to firefighters. If a clear layer smoke control solution is adopted firefighters will be able to establish the seat of the fire more clearly than in a room or
space without such ventilation systems.
Fire engineering has a key role to play in atria and auditoria design and ideally should be integrated into the design process rather than treated in isolation from other engineering disciplines.
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