Fire Engineering - Steel Structures.

Background
The fundamental principle of atrium fire safety design is to ensure that the design standard is not compromised by the presence of the atrium; the standard of the building with the atrium should not be less than the standard of an equivalent building without the atrium. This section describes the considerations and processes involved in the development of the atrium fire strategy, concluded by summarizing the implications for the structural design.

The primary objective of the fire strategy is to achieve an acceptable standard of life safety. BS 5588: Part 7 is the relevant ‘life-safety’ guidance for the incorporation of atria into buildings, and is applicable in all but limited circumstances.A package of fire safety provisions is developed to address the following requirements:

• Means of escape
• Compartmentation
• Structural fire resistance
• External spread of fire
• Facilities for the fire service. Smoke clearance is relevant for this situation.

This package depends on the size and use of the building.As examples:

(1) In a small office building, the design is for simultaneous evacuation and there is no need for compartmentation between storeys. Thus in this situation the atrium can be open, as the consequences of a fire anywhere in the building with an atrium are no different (in terms of life safety) than those in the same building without an atrium.
(2) In tall office buildings, storey-to-storey compartmentation is defined to enable phased evacuation; this vertical compartmentation needs to be maintained in the building with an atrium.
(3) In a hospital, as well as vertical compartmentation there is a need for horizontal compartmentation.This enables progressive horizontal evacuation of the fire compartment, with all other compartments remaining in place.
(4) With an atrium it is necessary to consider thermal radiation across the atrium, to ensure that this does not create untenable conditions in a non-fire compartment on the same storey adjacent to the atrium.

In situations where occupants remain in the building, the psychological effects of smoke in the atrium must also be considered, even though there may be no other risk posed by the fire.

The property protection and business continuity implications must be carefully considered. Even in situations where there is an acceptable standard of life safety, the atrium provides the opportunity for substantial spread of smoke, especially  with an unenclosed atrium. These additional requirements are usually specified by the client, operator, or insurer and not regulated under legislation. However, it should be noted that there are Local Government Acts (such as Section 20 in London) and national legislation outside England which have property protection implications.

The design procedure of the atrium can be generalized by consideration of:

• Use of the accommodation adjacent to the atrium
• Use of the atrium base
• Performance of the atrium façade
• Control of smoke within the atrium.

Effective smoke control is usually the key to the success of the atrium fire strategy.

This can be achieved either by preventing smoke entering the atrium (e.g. an atrium façade with fire-resisting performance and a fire-sterile atrium  base) or, alternatively, by managing any smoke within the atrium. The atrium can be divided into three zones:

(1) Atrium base
(2) The smoke reservoir at the top of the atrium, where any hot smoke will collect and can be extracted
(3) Remainder of the atrium.

Different design considerations will apply for these different zones.

The smoke management will normally take advantage of a smoke extract system.

The design of this system depends on the quantity of smoke within the atrium and the extract capacity. The quantity of smoke is dependent on the fire size and width of spill, where the smoke spills into the atrium. Complex spill geometries create large quantities of smoke. Smoke extract can be by either mechanical extract or natural systems. In either case it is necessary to arrange for low-level make-up for replacement air.Wind-effects on natural systems must be addressed.The extract can serve either to limit the depth of the smoke reservoir, or to limit the smoke reser- voir temperature, or both. In either case the fire size must be controlled.

The fire size is normally controlled either by use of sprinklers or by management control. Management control defines islands of fire load, the size  of the island dependent on the amount of combustibles, separated to ensure that the fire does not spread between islands. Typical rules are 10m2 islands separated by 4m.

Sprinklers are usually assumed in smoke control calculations to limit the fire size to a maximum area equal to the sprinkler-grid area. In practice, sprinklers may extinguish the fire or control it to a substantially smaller area.As an alternative to sprinklers or management control, a fire-engineered approach can  be used to account for the likelihood of a particular fire size occurring, given the use of the accommodation (‘natural fire’ based design).

It is important also to account for cool smoke, which cannot be assumed to rise (and may even drop).Where the smoke cannot be assumed to be hot enough to rise to the reservoir by buoyancy, it will be carried by the airflows that exist within the atrium. CFD analysis may be required to design the cold smoke system. Careful coordination with the building services engineer will enable the best value solution, particularly with natural ventilation systems, as the cold smoke is primarily carried by the environmental flows. An upward flow of air is beneficial for both the fire strategy and the environmental strategy.

Use of the atrium base can be enabled, provided that the rules for smoke and fire spread are respected. It is difficult to use sprinklers for controlling the fire size in the atrium base, as there is no ceiling above it (the atrium roof is usually too high to enable effective operation of sprinklers), and thus fire load control is either by management control or by an engineered approach.

Where the atrium facade is required to have a fire performance, this is defined either to prevent the fire breaking into the atrium, or to prevent the smoke breaking back in.Where an enclosure is required to have a fire-resisting performance, this can either be achieved by a formal fire-resisting system, or alternatively a smoke-retarding construction may be used in combination with an extract system to limit smoke temperatures. There is currently no standard for smoke-retarding construction, and thus it is often preferable to specify fire-resisting construction to avoid complicated specifications and enhanced site control.

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