BSBG has teamed up with Design Confidence to provide a series of fire safety master classes aimed at demystifying certain aspects of fire safety design. The series has a focus on the UAE, however, the principles largely apply internationally. The third part of the series is a technical discussion written by Murali Ramaiyan. This study is to be used for the design process and is related to fire safety in super high-rise buildings, with a focus on the different options available for evacuation strategy.
The number of super high-rise buildings increases almost day by day, as the construction industry progresses and technology enables us to build even higher. The demand for space, a luxurious view of the whole city and the appeal of avoiding city noise means super high-rise structures are on the increase in the UAE, as well as around the world. But this gives rise to problems in terms of design and construction, especially related to means of evacuation in emergencies and fire safety design. Since fire can occur at any time, the fire safety for a super high-rise building should consider the following issues:
– Long evacuation time
– Exterior firefighting is impossible, as the height is beyond the reach of firefighting ladders
– Stack effect
– Water supply limitations
– Challenges due to mixed occupancy
– Iconic nature of the structure.
As mentioned in the UAE Fire Code, buildings with habitable areas 23m to 90m in height from the lowest level of fire service access are classified as high-rise buildings, whereas the height limit for super high-rise structures is 90m and above.
In order to design a suitable fire strategy for a super high-rise building, it is essential to understand the Fire Safety Concepts Tree (NFPA 550). A Fire Safety Concept Tree (Figure 1) has two main branches, for example, in this case: ‘Prevent Ignition’ and ‘Manage Fire Impact’. Fire safety measures included in the ‘Prevent Ignition’ branch mainly require continuous monitoring and maintenance of the environment to ensure it remains fuel/ignition source free. Hence, this is more the responsibility of the building maintenance team/operator/owner.
Figure 1 – Fire Safety Concept Tree (NFPA 550):
The ‘Manage Fire Impact’ branch falls under the responsibility of a Fire Engineer. The role for the Fire Engineer here is to design a building that meets the required fire and life safety levels. Under ‘Manage Fire Impact’, a building must be designed to a prescriptive code requirement to either limit the amount of combustible/flammable products in the building, design a suitable fire suppression system to limit the fire growth and limit the risk to a building, or provide structural stability as per the code.
A suitably designed fire detection and notification system, properly designed egress system and well-documented fire safety management provides a better design for the “Manage Exposure’ branch of the fire safety of a building.
As shown in Figure 1, there are many potential ways to address the fire safety objective. The main objective of all fire codes (including the UAE Fire Code) and regulations is to reduce the risk of fire occurring through prevention. But prevention of fires depends on human actions addition to planned maintenance of building equipment and systems, and as such prevention cannot be guaranteed. It is therefore imperative to implement active systems (such as a sprinkler system) in conjunction with building elements (fire rated structure, exit stairway fire separation) to limit the effects of a fire, should it occur, while providing options for protection and evacuation of the occupants.
The development of an appropriate and effective evacuation strategy is considered one of the key fire safety aspects of any very tall building’s design. As a building increases in height, so does the vertical travel distance from the building to the point of discharge, and the time taken to evacuate the building. Above certain heights, the traditional method of evacuation using staircases will impact the occupants physically (fatigue) as well as psychologically (long vertical distance). In such instances, traditional evacuation strategies require re-evaluation to consider other strategies, such as relocation to refuge floors, or investigating the option of using elevator evacuation.
The application of a strategy other than that of the traditional evacuation strategy requires evaluation in collaboration with the design team, the stakeholders and the approving authorities. The different types of evacuation strategies which can be considered for a super high-rise evacuation are listed below:
– Simultaneous evacuation
– Phased evacuation
– Delayed evacuation
A Simultaneous Evacuation Strategy aims to evacuate all the building occupants at once to a place of safety outside the building (such as an Assembly Point). The evacuation process is single-staged, and initiated by the building fire alarm and notification system. The notification system is activated throughout the building on every floor. Evacuation of the entire building is the simplest strategy to implement in terms of notification system designing.
In this strategy, multiple exit components can be used individually or in combination with other elements. The strategy could involve exit by staircase alone, by elevator alone, or a combined evacuation system of staircase and elevator, with/without the use of refuge floors.
The detection and notification system is required to be programmed in such a way that if an activation occurs on any floor in the building, the notification system will activate for the entire building. The simplest form of the simultaneous evacuation is highlighted below.
The flowchart provided for this strategy explains the sequence of operation for the fire alarm system to be programmed. In order to avoid a single false alarm activation to evacuate an entire building, a Positive Alarm Sequence (PAS) is the best option for the fire alarm system operation. We will cover the detail of the PAS sequence of operation later.
Figure 2 – Simultaneous Evacuation:
There are scenarios where a single-staged Simultaneous Evacuation Strategy is not practical. If this is the case, a Phased Evacuation Strategy can be implemented in the building design. In a Phased Evacuation Strategy, occupants in the most critical areas such as anywhere on the fire floor and the floors above and below the fire floor will be evacuated immediately once the fire is confirmed. In addition to the evacuation messages on the immediate danger floors, an alert message will play on the floors above and below the evacuation floors to alert the occupants about the danger, but instructing the occupants not to evacuate. The below flowchart and the drawing provides the sequence of operation of the strategy. This option can be further customised to evacuate progressively based on the building management’s preferences. In progressive design, if the fire is not under control within a certain time period the alert floors move into evacuation mode and the floors above and below move to alert mode until the entire building evacuates or after a pre-determined period, full building evacuation is activated. This progressive design is subject to building fire safety management procedures and fire safety training for staff.
Figure 3 – Phased Evacuation:
As the building is very tall in nature, occupants who are using the staircases may not be able to continuously ascend the staircase due to muscle fatigue and stress. Hence, dedicated refuge areas are provided for occupants to wait for other means of exit (such as elevators or a rescue team), or wait for the building fire safety management team/authority’s instructions to evacuate the building.
In essence, the Delayed Evacuation Strategy is the same as the Phased Evacuation Strategy; however, occupants do not proceed to the exit level, but to the refuge floor to be given further instructions – evacuate (if fire is not under control), stay on the refuge floor (if fire is manageable), or go back to apartment (if fire is put out and the floor is back to normal).
Figure 4 – Delayed Evacuation:
In order to avoid false activation of the building detection system leading to occupant evacuation, a time delay is introduced in the PAS. This delay gives a trained responder up to three minutes to investigate the cause of an alarm signal before the evacuation signal is activated in the building. The time limits to acknowledge the alarm signal and reset the system are designed to help eliminate total reliance on human intervention to actuate the alarm, especially when personnel are not available to acknowledge, investigate and reset the alarm. AHJ approval is required for implementing this design in the building fire alarm system. The below flow chart provides the PAS sequence of operation.
Figure 5 – Positive Alarm Sequence:
On recent projects, the new UAE Fire Code has been specifically applied – given its mandatory nature within the UAE. Design Confidence works closely with BSBG’s design teams to provide a balance between visual aesthetic and fire life safety requirements. This integrated approach ensures that the design maintains its signature look and feel while still achieving the fire safety objectives of the design and prescriptive requirements of the codes. This holistic, total design approach is integral to the design process as late changes can have unnecessary cost and visual impact.
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