The impact of door size on smoke control design

Posted by Conor Logan on 24/06/20 10:00

One of the biggest challenges in the design of smoke control and pressurisation systems currently is the relatively recent trend towards installing full height doors which extend all the way to the underside of the ceiling, and/or very wide single leaf doors.

While there are no regulations governing the height of the doors which separate corridors, lobbies and stairs in a commercial or residential building, from a smoke ventilation point of view, full height or “oversize” doors can be problematic.

The issue with full height doors

Full height doors are an issue for all forms of smoke control. When smoke spills from the fire room into the corridor or lobby, or from the lobby into the stair, there is normally a buoyant, high level ceiling jet of smoke travelling under the ceiling. The fixed panels above doors have the positive effect of reducing the effect of the ceiling jet by creating a barrier to its’ momentum. If there aren’t any high level barriers or downstands, then the protection to the stair is drastically reduced.

Where the building layout is ADB compliant it is unlikely that CFD analysis or smoke testing will take place and the dangers can go unnoticed – in this instance the issue is compounded as a natural vent system is less likely to protect the stair in this condition.

The two CFD images show a staircase with full height doors on the left, and without full height doors on the right.

With a fire in the apartment, smoke is able to spill past the mechanical smoke extract system and flow straight into the stair, unless a downstand is installed. If the downstand is only shallow, it may also be necessary to increase the ventilation rate from ‘standard’ configurations that might be applicable to door arrangements with a downstand.

This can lead to issues with approvals with local authorities and fire departments but also specification issues when fans need to be uprated or re-configured.

Oversize Door Issues with mechanical extract systems e.g. Colt Shaft Systems

Where an oversize door i.e. a significantly high and/or wide is used, and particularly with mechanical smoke control, it is important to remember that the force being applied to the door by the ventilation system is a function of the area of the door. Therefore it is important to consider the effect of the system on door forces when designing and commissioning systems that create pressure differential.

Where pressurisation systems are installed

Pressurisation systems are designed to achieve a specified open door velocity through the stair and/or lobby doors. Large doors, whether full height, extra width or both therefore increase the required flow rate and the size and cost of the pressurisation system.

If the accommodation door opens into the area to be pressurised, the additional area of the door means an increased door opening force is needed. As the door size increases it becomes harder to achieve both the required 50 Pa pressure differential and the 100N maximum door opening force. Ultimately it may prove physically impossible.

Keeping smoke control in mind

I doubt if smoke control is in the forefront of an architect’s mind when full height doors are chosen. Nevertheless, like any other design decision, specification of full height doors has consequences and in this case one consequence may be reduced fire protection for occupants and fire-fighters or the need for larger smoke control systems, or a delay in getting building sign off from the approving authority.

And of course, if you decide to use full height (or wide) stair or lobby doors, do make sure that the information gets passed down the line as early as possible to the people who need it, the smoke control designers. Find out more.

Conor LoganConor Logan CEng FIMechE FCIBSE is Technical Director of Colt UK. Conor designs innovative smoke control and HVAC systems, represents Colt on many UK and EU standards committees and was Chairman of the Smoke Control Association for over 9 years.

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Topics: Smoke Control, CFD, Pressurisation