The use of ceiling plenums as replacement air routes
27 February 2014
Authors: Richard Brooks, general manager, Advanced Smoke Group Ltd; and Eamonn Coll BSc (Hons) Fire Tech MIEI, fire engineer, H+H Fire
In more recent times, as a result of a number of factors (including improved construction methods, new technologies available to the construction industry along with developers desire to push the boundaries of building design still further), building designs are becoming more complex and thus require solutions that are not necessarily ‘standard’.
This statement can ring true for many aspects of design and construction but not least for smoke control on common corridors where buildings do not necessarily ‘stack’ in the traditional way.
This paper will concern itself with the design of an innovative smoke control solution for a common corridor mechanical ventilation system for a project in South London that incorporated many new features such as:
- Mechanical smoke ventilation shafts (MSVS) drawing smoke down to basement level to discharge at ground level instead of at roof level; and
- Replacement air provided to dead end areas of the common corridors via the use of ceiling voids created above the suspended ceiling constructed along the length of the corridors.
The need to explore the options listed above was as a direct result of a complex building design which was necessary (architecturally and commercially) to take the utmost advantage out of the riverside setting of the building.
As with many modern projects, there are many design limitations and restrictions present when designing a suitable means of smoke control in common areas of residential buildings.
One of the most common problems faced by a fire/smoke control engineer when attempting to develop a viable smoke control system for the common areas of a residential apartment building, is establishing a suitable smoke shaft location acceptable to the architect and developer. More often than not, the development of the project has moved to a stage in which the shaft locations are fixed and their relocation will cause significant issues in connection with apartment and building layouts.
Further difficulties can arise in connection with the smoke shaft discharge. The appearance of the roof line is frequently a consideration for the development of penthouses due to planning considerations and the desire for the associated terraces to be clear of obstruction with as little plant as possible visible from them. The termination of the smoke shafts and the location of the exhaust fans is one of the areas affected by the aspiration for clear roofs.
The limitations that faced us on the South London project included restrictions in both the shaft and discharge locations due to apartment layouts, floor space utilisation and limited roof space and roof space being overlooked.
The design of the building resulted in one staircore and common corridor terminating below the topmost level and apartments being constructed above it. This restricted any shaft from penetrating through to the roof level.
The second stair did continue through to roof level but connected to a number of extended corridors, which due to MSVS locations (adjacent to the stairs) required alternative means to get replacement air to the ends of the corridors to avoid compromising the layout of apartments adjacent to the dead end portion of the common corridor
The method to get air to the end of the stairs was by the use of ceiling plenum within the suspended ceiling of the common corridors. This presented a number of significant design issues which included the amount of M&E services within the ceiling void, the construction of the ceiling itself to ensure limited air leakage along its length and the resistance to airflow by services and grilles.
The design objectives for any mechanical ventilation system must satisfy the criteria as agreed with both the approving authority and the relevant regulatory controls.
The objectives of the smoke control systems are generally to:
- Provide sufficient airflow during the escape period in order to return the corridor to tenable conditions by adequately diluting and extracting smoke which will enters the common escape route as the occupants of the apartment on fire make their escape (temperature/visibility);
- Protect the ‘sterile’ area of the stairwell from smoke contamination;
- Provide aid to the fire service personnel during fire fighting operations;
- And more often than not nowadays, make provision for ‘comfort’ ventilation.
Regulators are now increasingly asking for mechanical smoke ventilation systems to be designed in accordance with the guidance given in the Smoke Control Association (SCA) guide  and are requesting that this guide is referenced in the technical submissions.
Traditionally, smoke control systems (MSVS system) serving the common areas of a residential developments comprise smoke extract fans mounted at roof level linked to a smoke shaft that has fire dampers located at each level served by the system.
The crucial part of the system, in order to affectively meet the above performance criteria is that no part of the common area should be stagnant. The problem often faced by the designer is the distance between the extract shaft or replacement air path and the end of a corridor.
The way in which such a difficulty was overcome on the South London project was to use the ceiling void as the supply air plenum.
When using the ceiling void as the supply air plenum, the void is linked to the stairwell via a fire damper and a ceiling mounted air transfer grille located at the “dead end” of the common corridor; thus allowing the free flow of air to an otherwise stagnant area.
The use of the ceiling plenums as a method for replacement air presented a number of technical issues notwithstanding the regulatory approvals issues that always come with new solutions. The ceiling void is often heavily ‘populated’ by mechanical and electrical services, it is therefore necessary to co-ordinate the design requirements with the appropriate bodies.
Co-ordination with the M&E designer and on site contractors is one of the crucial elements. Firstly ensuring that the M&E design is rationalised as much as possible in order to get the optimum amount of free area in the ceiling plenum, secondly ensuring that the on site operatives install the services as compact as possible to minimise pressure drop and ensuring the ceiling is installed as air tight as possible to eliminate extensive leakage along the ceiling length. In practice making the ceiling relatively air tight has not proved a particular problem the co-ordination of the services has been the biggest issue.
Air is supplied to the stairwell via an AOV, which opens at the head of the stairs in the event of fire. The air is then drawn into the stairwell due to the pressure differences created by the extract fans connected to the smoke shaft; it then passes through a fire damper located above the stairwell door, into the ceiling void and then into the corridor via an air transfer grille linking the ceiling void with the corridor.
To create a negative pressure within the corridor could increase leakage, but more importantly, if the suction is excessive, it could cause the force required to open doors within the escape routes to exceed that which some people are able to apply (100N being the maximum recommended under current guidance) or indeed it could collapse the ceiling.
In order to limit the level of pressure differential between the corridor and the ceiling plenum, a two stage system was adopted for the South London project; 50% airflow for the escape period and 100% airflow for the fire fighting phase.
As the fire service arrive to fight the fire, they will have the facility to switch the system to fire fighting mode, increasing the airflow rate, enabling the smoke control system to draw an increased quantity of air through the open stairwell door, protecting the sterile area of the stairwell from the migration of smoke. In the fire-fighting mode, the damper between the stair and the ceiling plenum was arranged to close as the ceiling plenum was not required as a critical component in fire fighting mode (thus it was not a fire-rated ceiling).
As mentioned above, the layout of the development in South London meant that some smoke shafts were unable to vent to the roof level; consequently the design of the smoke control system had to be innovative in terms of how to vent the smoke from the building.
In this case, the smoke extract fans were located at basement level and connected to Durasteel fire rated ductwork discharging via vents positioned within planters located at podium level.
The system was further complicated by the phased handover of the development; this demanded significant design input with the support of CFD modelling to validate the design.
There were four principle considerations when considering the downward extract of the smoke control system and phased handover:
- How the phased handover impacted on the use of ceiling void and air transfer system;
- Continued protection of stairwell with enhanced system performance;
- Impact of smoke buoyancy when inducing smoke from the fire floor down to basement level for discharge to planters via ductwork;
- Zoned smoke control system must continue to operate reliably on a phased handover.
In addition to CFD modelling undertaken on the South London project, calculations were also submitted to the regulatory authorities to support the design intent that hot dense smoke could in fact be drawn down into the smoke shaft and discharged via the ductwork to the planters, as this was a significant departure from the norm.
In addition to the use of calculations to provide verification of the system design, cold smoke demonstrations were used during commissioning to prove the airflow and effectiveness of the zoned system. Cold smoke was used in each fire zone on floors which were considered to provide the worst case scenarios (see Figure 2). These cold smoke tests, clearly demonstrated the results predicted by the CFD modelling.
In order to justify the design and gain initial approval of the design from both the Building Control body and the London Fire Brigade’s Fire Engineering group, it was necessary to conduct a number investigative CFD analysis based on various fire scenarios.
The CFD analysis was also used extensively in the design of the systems particularly in relation to the size of the ceiling plenums and the free area required. It also served well in determining the pressure differentials for above and below the ceiling. This was a significant point in the design as it determined the construction of the ceiling as it had to be ensured that over pressure would not fracture the ceiling thus altering the air flow.
The CFD analysis demonstrated that with the required extract rate for means of escape, the pressure differential in the ceiling plenum was found to be around 60 Pa. It was carried out using the CFD engine Fire Dynamic Simulator from the National Institute of Standards and Technology.
As with the majority of buildings todays, they will most likely a one time or another be subject to improvement works and may be adapted to include new technologies. This is particularly of concern in this project given that the ceiling plenums are so designed that the free area within them is of critical importance. Therefore provisions have to be put in place to ensure that the ceiling plenums and services are not altered without the proper design considerations. A suggested route is to have the building control put a condition on the building so that any alterations that affect the suspended ceilings in the common corridors are subject to Building Control approval.
Another route and perhaps a more enforceable route would be that the ceiling plenum be managed under the RRO along with all design information noting critical elements passed to the end user under Regulation 38.
Whatever route is chosen, great care must be put into the management and ongoing maintenance of the system to ensure future alterations do not negatively affect the smoke control system operation.
 Guidance on Smoke Control to Common Escape Routes in Apartment Buildings (Flats and Maisonettes) by the Smoke Control Association
Richard Brooks is general manager of Advanced Smoke Group Limited, a company specialising in smoke control within buildings. Has 30 years’ experience in the smoke control industry and was chair of the Smoke Control Association from 2002 to 2004. Brooks was also chair of the Smoke Control Association working group producing the SCA guide to the ventilation of Loading Bays, Service Yards and Coach Parks. He currently chairs BSI working group reviewing BS7346: Part 7: 2006 for car park ventilation.
Eamonn Coll is a fire engineer with H+H Fire. Further to graduating from the Letterkenny Institute of Technology with an Honours Degree in Fire Technology, Eamonn has spent to date 6.5 years within engineering consultancies. During this time, projects have included residential, high rise, mixed use office and retail, shopping malls, overground and underground rail stations throughout Ireland, the UK and Eastern Europe.