Sometimes, a Fire Engineer gets the task of increasing the occupancy in a building. This task can be for already existing buildings or for new developments.
The new developments would require a new Fire Strategy from a Fire Engineer. One of the first tasks for a Fire Engineer is to assess the Occupancy Load throughout the building and to discuss it with Relevant Stakeholders: Client (to confirm it aligns with the Client), Architect and the Design Team (to coordinate occupancy number with other disciplines’ requirements such as a number of toilets or ventilation system), and AHJs (during the pre-lodgement meetings in order to de-risk the consent submission).
When analysing an already existing building, very often occurs that there is no existing Fire Strategy in place. This may happen if the building is very old, or if the correct processes were not followed when it comes to building design and records, during the building’s previous refurbishments. If that is the case, a new Fire Strategy needs to be developed according to the most up-to-date standard (e.g. BS9999). If there is a Fire Strategy in place which is reasonably new, a Fire Engineer would analyse the existing Occupancy Assessment noted in the existing base-build Fire Strategy.
Very often, the occupancy numbers are determined by the floor space factor which is just an estimate of the occupancy in the building (e.g. ‘occupancy density’). The occupancy density is the number of people per unit floor area that is recommended for different spaces in the building (e.g. 30 m2/person for a storage area, 6 m2/person for office area, 2 m2/person for shop sales areas). It is important to remember that this is not the maximum possible number of people who can be in the building, but only the occupancy recommendation. The maximum occupancy number in a building should be calculated according to the building’s accurate vertical and horizontal escape route widths (widths of the stairs, door clear widths, corridors widths) – known as the ‘escape routes capacity’.
Ways to Increase the Occupancy Numbers
As noted above, the occupancy number in the building should be calculated as per the escape routes capacity, where each horizontal and vertical escape route serves a certain amount of people according to its escape route width. Where two or more protected stairways are provided, it should be assumed that one of them may be obstructed during an evacuation due to fire or smoke. When determining the total capacity of all the stairways in the building, one stair (preferably widest, to be conservative) should be discounted in the capacity assessment. The occupancy number of the building is therefore calculated as per capacity of the remaining stairway(s).
In order to increase the occupancy, one of, or several of the measures below, can be applied:
- Increasing the vertical escape capacity by adding a new protected staircase – with an additional stair, the total escape route width for vertical evacuation will be increased, and therefore, the maximum number which is permitted to be in the building will be increased as well. The new stair will only increase the maximum permitted number of people on the floors which are served by these stair (e.g. not a ground floor). Note that the number of horizontal escape routes may need to be increased as well to align with the increased vertical escape capacity.
- Increasing the horizontal escape capacity by adding more doors to specific spaces, into protected stairs/corridors, increasing widths of the protected corridors or final doors – Note that this goes ‘hand-in-hand’ with the above-mentioned increase of the vertical escape routes capacity. The final occupancy assessment is based on the worst-case scenario relating to all limiting factors and ‘bottle-necks’ throughout the building. Therefore, to get the maximum occupancy number, the overall escape route analysis for the whole building, including both vertical and horizontal escape routes, should be undertaken.
- Provide protected lobbies to all protected escape stairs on each storey – If protected lobbies are provided, the likelihood of a stair not being available is significantly reduced. In those cases, one stair does not need to be discounted in the capacity assessment. Therefore, the overall escape routes capacity increases, as the total width of the available escape route is also increased. A protected lobby need not be provided on the topmost storey for the exception to still apply.
- Install the sprinkler system throughout the building – Similarly as above, if the building is fitted with the sprinkler system, the likelihood of a stair not being available is significantly reduced, and therefore, there is no need in discounting one stairwell while assessing the overall capacity in the building. Also, as per BS9999, ‘The provision of an automatic sprinklered installation permits a reduction in fire growth rate, allowing larger travel distances, smaller doors, larger compartments, reduced fire resistance periods and other provisions recommended in this standard’. The smaller doors can be provided for the building, in other words, if the same number of doors and same door widths are used in unprinklered and sprinklered building, the total capacity of the sprinklered building is higher than for a unsprinklered building.
- Provide a smoke control system designed in accordance with BS EN 12101-6:2005 to all protected staircases – A pressure difference controls smoke movement, preventing smoke infiltration into the protected stairs.
- Change the evacuation strategy from simultaneous to phased evacuation – Phased evacuation is an approach where the first people to be evacuated are all those on the storey most immediately affected by the fire, and the adjacent floors (or floors with disabled occupants, basements and similar), while the remaining floors are been evacuated at phased intervals. Such approach provides significant economies in the plan area occupied by the protected stairways, meaning that the total occupancy of the building can be increased as the protected stairs would be occupied only by the occupants of couple floors rather than the whole building, during an evacuation. Note that additional fire protection measures are required for the buildings with phased evacuation (e.g. each floors shall be a compartment, additional passive and active fire protection measures, as well as additional management arrangements).
- If the existing Fire Strategy is determined by the size of the floorplate and not by the escape widths, it is to be rewritten based on escape widths – As noted above, the Occupancy Assessment should be based on the building’s escape routes widths and their capacities and not on the size of the floorplate and occupancy density recommendations.
- If the existing Fire Strategy is written in ADB, rewrite it in BS9999 – Building Regulation Approved Document B (ADB) has more onerous requirements as it is written for more common building solutions. As a result, it can prove unacceptable for larger, more complex buildings. By taking a more holistic view to fire safety, BS 9999 allows compensatory measures to offset, e.g. travel distances and/ or total escape door width.
- Fire Dynamics Simulations and Crowd Modelling – Similarly as above, the performance-based approach (fire modelling) would provide more tailored results compared to the prescriptive solutions (standards such as BS9999, ADB etc). This is due to the fact that all prescriptive solutions simplify the design and do not assess all building’s actual features. The fire modelling tools do take into account these features in more depth, and therefore, the results are ‘custom-made’. As a result, the occupancy numbers in the building are often higher when using the fire modelling approach, compared to the more generalised results obtained by the prescriptive-based methods.
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