By Ron Burns
Plugholing can have a devastating effect on a carefully designed smoke ventilation system.
The science of smoke control is relatively new, especially when compared to the Roman aqueducts. The readers familiar with my presentations will know far too well the association I constantly draw between the behaviour of smoke and water.
As urban renewal picks up momentum through our cities, more buildings require an upgrade, or in some cases their first smoke ventilation system does.
The term I use is “identically opposite” where smoke mimics water in the opposite manner: where water flows downwards, smoke rises. Analysing the behaviour of smoke using day-to-day life occurrences like a bath, or a milkshake at the local burger ranch often brings a wry smile and then a chuckle at the realisation of the similarity of fluids and flows, which are readily interpreted in simple practical solutions.
Since we are in the bath, plugholing is a reality which can have a devastating effect on a carefully designed smoke ventilation system. Before we climb into the subject, I want to look at the relevance in today’s society.
South Africa is littered with many buildings built prior to 1990. Many of these buildings carry significant landmark importance, a few are iconic buildings. With the urban renewal programmes being rolled out in major cities around the country, once these buildings enter the renovation stage, local metros have the opportunities to upgrade the designs, which are several decades adrift of the latest technology and scientific developments. We have, as a life safety fraternity, the responsibility to ensure we apply our skill and knowledge to the best of our abilities to save lives in the event of a fire.
If we look at a standard office building with a 2 600m2 floor plate and six storeys high, getting smoke off the floor seems simple enough. First, we are going to ignore any form of natural ventilation through the external façade of the building.
Secondly, the slab-to-slab dimension would characteristically be in the region of 3 800mm. Now if you subtract the 300mm thick slab for the clear space between floors, it leaves 3 500mm. But then there is also the service space of 500mm and 100mm for the ceiling, leaving a slab to the underside of the ceiling of 2 900mm. The designer only has 400mm of space to install a vertical venting device and maintain a clear layer of 2 500mm above finished floor level. It is unlikely any vertical ventilating device will be fitted above the ceiling. The volume of vertical ventilating devices and the cost of electrical mechanical operation will ensure the option is financially prohibitive.
The solution would be to use a powered exhaust system. For a standard office fire using the tables provided on EN 12010:5, it would be fair to assume an extraction rate of 13.6m3/s in a sprinkler-protected building along the coast in South Africa. This is only the starting point of the design.
Calculating the exhaust rate is a fairly simple task: getting the air out of the building a little more challenging. The first question, given that we have already established a single smoke zone (less than 2 600m2), is how we propose to extract the smoke and toxic fumes:
A series of perimeter fans with non-return dampers;
A central ducted system complete with vertical risers;
De-pressurisation of the atrium and dragging the smoke into the atrium and preventing smoke migration onto the non-fire floors using fire curtains.
Option 1
The series of fans on the external façade comes with a plugholing challenge. Assume 2-off Ø710 fans: the clear height required to install a fan of this diameter is a minimum of 900mm. The designer is still required to take into account the ceiling requirements, and often a suggestion to enclose the fan in a bulkhead is a proposed method. The building we described above had a 3 500mm clear space between the two slabs. This results in the centre line of an Ø710 fan being at 3 050mm above finished floor level and the bottom of the fan inlet being at 2 600mm above finished floor level, a snug fit by all accounts.
The unaddressed item is the 100mm between the fan inlet and the clear layer at 2 500mm, above finished floor level. This results in designers needing to install 12 fans each with a Ø710mm inlet and a duty of 7.8m3/s to achieve the 2 500mm cleat layer and overcome the plugholing effect. Failure to install this quantity of fans may not affect the correct volume of smoke being extracted, however the effect of plugholing results in the 2 500mm clear layer being lost.
Inlet air is required at each level. The maximum inlet air velocity is 5m/s, so there will be an extensive inlet air requirement of 19m2 per floor for the additional fans required to overcome the plughole requirement.
Option 2
The solution is similar to Option 1 with respect to the extraction rate. The challenge is to ensure the inlets remain above the
2 500mm clear layer. There are two ways to achieve this:
1. A two duct riser system installation.
a. The duct is fitted with a series of smoke dampers, one at each level.
b. The smoke damper is connected to a set of internal duct which has sufficient inlet area as determined by the plugholing calculation to ensure that the dispersement of exhaust air inlets are of sufficient area, quantity and spacing to ensure the effect of plugholing is negated.
2. A two duct riser system is installed.
a. The duct is fitted with a series of smoke dampers, one at each level.
b. The damper protrudes into a masonry duct created on the ventilated floor.
c. The perimeter of the duct equates to the area required to prevent the plugholing. This is similar to the duct installed above, yet requires no co-ordination with other services in the void.
d. The negative aspect of this solution is the loss of lettable floor area.
Inlet air is required at each level. The maximum inlet air velocity is 5m/s. The requirement shall be 3m2, significantly less than Option 1.
Option 3
This solution requires the same extraction volume as Option 2, of 13.6m3/s. If the building contains an atrium, the opportunity to install the inlet of the exhaust fan into the atrium while ensuring the exhaust fan remains hidden is normally easily achieved. The additional height available always negates the requirement for multiple exhaust air inlets.
Inlet air is simple for this installation. Only one inlet air path is required at the lowest level due to the atrium. The challenge is to prevent the early deployment of the fire curtains, which are a designed failsafe. Although this solution provides an elegant, cost-effective smoke solution, there will be associated costs related to the installation of the fire curtains.
I conclude with this thought: as urban renewal picks up momentum through our cities, more buildings require an upgrade, or in some cases their first smoke ventilation system does. Innovative and accurate application of the building code will present fresh and exciting challenges to the fire engineer. It is always the detail which provides the greatest challenges.
For conversations relating to sticky challenges please feel free to contact me through RACA Journal or via LinkedIn.
