Fire Safety Masterclass | Chapter One - Smoke control

Words by BSBG Media Team, Wednesday 04 October 2017

BSBG has teamed up with Design Confidence to provide a series of fire safety master classes aimed at deconstructing certain aspects of fire safety design. The series has a focus on the UAE, however the principles apply internationally. This technical study, written by Aaron Mc Daid, is related to smoke control for buildings in the Middle East, and the application of Fire Engineering Principles which can be used for design.

Notable projects

BSBG and Design Confidence have collaborated on countless innovative and holistic design solutions on a wide range of developments throughout the UAE and internationally. Some honourable mentions for projects where an advanced dynamic computer modelling approach has been undertaken, are Repton School in Abu Dhabi and the Dubai Festival City development. The advanced performance-based approaches outlined below have been adopted with multidisciplinary inputs in order to maximise efficiency and optimise the architectural aesthetic, while still meeting the prescriptive requirements of the Fire Code and fire safety objectives for the design. Both projects are currently in full operation and are approved by the relevant fire safety authorities.

Mechanical smoke control

Smoke control can be accomplished by using either an active or passive system. Typically, when we refer to smoke control we think of active systems, but control can be achieved with something as simple as the provision of physical separation, such as a fire barrier wall between two spaces to prevent the spread of smoke. Another example of a passive system is a natural ventilation opening which can be used to ventilate hot smoke. A natural ventilation system requires a buoyancy-driven flow (the difference between smoke temperatures and outside air temperatures), but these are generally ineffective within a humid desert environment, such as that of the UAE. More commonly in the region, active smoke control will be used. Examples of these are mechanical extractors that remove smoke, or the pressurisation of a space to prevent smoke ingress.

Code basis

In the Middle East, building codes are largely influenced by American codes and standards, such as the International Building Code (IBC) and the National Fire Protection Association. Put simply, a ‘code’ specifies where a type system is required, whereas a ‘standard’ sets out how to design that system. For example, the NFPA 5000 building code specifies that sprinklers must be present in a high-rise building. The sprinkler system must be designed to the NFPA 13 standard for sprinkler design. In Dubai the primary building code is the UAE Fire Code. This building code is the primary basis for the design of all buildings, and provides detailed design guidelines for where a given system is required and what standard it needs to be designed to. For smoke control systems, the UAE Fire Code requires systems to be designed to NFPA 92 standards.

Types of mechanical smoke control

There are two primary design methods for smoke control; first is a system required based on prescriptive criteria (such as air change rate per hour) or performance criteria (which maintains the smoke layer 1.83m above floor level). This section will provide a high level overview of the design methods:

Prescriptive based - Smoke control systems

Pressurisation and Corridor Smoke Control are examples of prescriptive design criteria. Pressurisation positively pressurises a space by filling it with fresh air, so that in the event a door is openened smoke or heat is prevented from entering the space. A great example of a space where pressurisation is provided is in stairways in highrise buildings (UAE Fire Code Chapter 10), where a 12.5 or 25pa pressure differential is required based on NFPA 92. Corridor smoke control is provided to maintain exit access, keeping it largley free from smoke by providing six air changes per hour in the event of a fire. It should be noted that these systems are not based on any specific fire scenario, they are a generalised requirement (see example illustration below):

Example of Smoke Control Systems provided based on simple criteria

Performance based - Smoke control design

For large open spaces where multiple floors are connected, smoke control needs to be considered in respect to occupant evacuation. This requires the provision of smoke control systems at a high level so as to maintain the smoke layer above the highest level open to the space. System design will specify the smoke extraction rate based on an a series of fire scenarios. See below:

Typical Atrium Smoke Control Fire Scenarios Assesment of the smoke flow within the space can be undertaken using Computational Fluid Dynamics (CFD) models to establish the available safe evacuation time. Examples of 3D CFD models are provided as follows:

Once the available safe evacuation time is established, evacuation modeling calculations or simulations can be conducted to establish if the required safe evacuation time is less than the available safe evacuation time, with a margin of safety considered.Example of CFD Model For the design of buildings within the UAE and around the world, the evacuation progression largely follows the the same principle. Evacuation from the space the occupant is located at the activation of the fire alarm (eg: office, apartment living room, classroom) to an exit (eg: enclosed staircase or final exit door out of the building). See illustration below:

Typical exit progression from a building

In the Middle East, buildings tend to be large, complex high rise structures. Buildings with high occupancies such as these require special fire safety management.

Typical sequence of alarm for a managed building

Further considerations

Smoke control is a key component in fire safety within the built environment. The design of the system will vary depending on the intent and based on different projects and requirements. Some key items to consider when designing smoke control systems are:

Door opening forces
Arrangement of supply and extract of ducting
Leakage due to construction
Testing and commissioning
Ongoing maintenance

To find out more about Design Confidence you can visit their website: www.designconfidence.com or follow them on LinkedIn.

BSBG Media Team

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