By Ron Burns
Windows: the beauty and the beast of smoke ventilation. Undoubtedly, the most versatile ventilation invention ever and a critical element in smoke ventilation, yet often incorrectly applied.
Windows will always be a hot topic of discussion. They offer so many alternatives to a building. Many a designer rushes in wanting to maximise windows, in so doing neglecting simple fundamentals. Before we discard the window option, we need to appreciate the advantages of using windows as part of an automated ventilation system, smoke or not. Windows are designed into buildings to supply natural light and ventilation to the building. Windows are building elements that are tried and tested weather resistant elements.
There are general engineering considerations that need to be applied when selecting windows for any application. The wind loading on the facade is a critical factor, as is the elevation and height of the building. You can imagine the wind loading on a typical building of, say, 25 floors in Midrand. The wind load increases by approximately 1 000Pa at the top floor of this building. This amounts to additional static load bearing on the external facade when the window is closed. Serious diligence is required when selecting the windows on the facades of these buildings. Weather seals and the movement of the window when in the closed position require consideration, as the window is not a static building element and modern designs allow the window to “shift” when in the closed position to allow for pulsating and gusting winds.
The advantages, especially in dry climates, definitely make it worth automating the windows. If the building designer installs a weather station, it would be possible to monitor the ambient conditions and open the facade of the building, allowing the building to drop its core temperature into equilibrium with the ambient conditions. An abundance of cool fresh air entering the building would result in the structure absorbing and thermally storing the lower temperature. The advantages of purging stale air and replacing it with fresh air at no additional cost to the building cooling system bring untold advantages. The weather station operated to the pre-programmed conditions only opens the windows when a low temperature condition allows. The control allows the windows to remain closed if high winds or rain occurs. All these are good and healthy advantages of the automated window system. Time is insignificant, the building is unoccupied for at least eight hours and that is sufficient time for the ambient air to percolate into the building. Even the air-conditioning plant appreciates the help from nature once the window automation is calibrated and commissioned accurately. The energy-savvy designer will also calculate the options to use this system to prevent the early start of the major plant until later on in the day, particularly when the autumn and spring solar loadings are low.
“Why not use these windows for smoke ventilation” is the natural progression in the designer’s mind.
I have many concerns around this application for smoke ventilation. Smoke is far too theoretical and the practicalities are long forgotten once installation of the required Aerodynamic Free Area (AVCV) is calculated. In last month’s article we discussed the inherent problems of vertical venting. As we are all aware windows are installed in the vertical. This means that the AVCV needs to be doubled. The second challenge is the size of the window. Assuming this is a building with a 3.4m slab to slab, then a 300mm thick slab leaves us with a 3.1m soffit to soffit. The smoke free clear layer for an office space is 2.5m above the highest occupied level. This leaves only 600mm space for the window. Once the framing of the window system and the framing of the glazing is taken into account, it is common to only have a 400mm high operable section. For ease of discussion, we are going to assume the architect is able to offer a ceiling design that allows full access to the entire soffit along the perimeter to facilitate the smoke ventilation.
The myth of top hung windows versus bottom-hung windows has long since being dispelled by the EN 12101 code. Part 4 of this code offers a section that deals with calculating the AVCV of a window opening. The co-efficient is calculated based on opening angles of the windows. The co-efficient ranges from 65% for a window that opens 90 degrees, to a co-efficient of 30% for a window that opens less than 30 degrees. Before the purist grasps frantically for a Prozac: these are co-efficient for air inlets.
For smoke ventilation windows, the correct product to use is an EN 12101:2 certified product. To the best of my knowledge, only one product of this calibre is readily available in South Africa. It is an imported item and the price tag is often prohibitive. As South Africans, we all too readily select cost over certification and resort to calculation of the AVCV based on our own theoretical reasoning. Keeping with general practice, let’s continue with the analogy of the 400mm high window in this facade. The window is 400mm high, so we will use a 400mm stroke actuator — this will achieve an opening angle of approximately 45 degrees resulting in a co-efficient of 40%.
I have installed a few window actuated systems in my time and although there are suppliers who advocate a single actuator opening a window width of 1 200mm, I have lost my appetite for a window this wide. I would recommend a window width of 1 000mm. The geometric area of the window is 0.4m2 and applying the co-efficient of 40% leaves us with an AVCV of 0.16m2.
The AVCV required for this example, to keep a smoke-free clear layer of 2.5m with a centreline opening of 2.8m, will require an AVCV of 29.4m2. The building has a sprinkler system. With the windows selected, each floor would require 184 windows. This will equate to 73.6m2. Assuming a floor plate of 1 900m2 this equates to 3.87%. We should not forget that doubling the area is also a requirement to overcome the head wind condition.
A common mistake made is to apply the portion of SANS 10400:2011 Part T 42, which speaks about the openings being in the top third of the building. This means the bottom of the window could be 930mm below the soffit of the slab, resulting in a clear layer of 2.17m. This is now in contravention of the 2.5m smoke free clear layer, which, on the surface of the presentation, does seem to comply with a portion of the building code. However, the code must be complied with completely, not just a haphazard approach.
The designer needs to be extremely cautious in selecting the ventilation method required when trying to ventilate hot smoky gasses from the building. This has been a simplistic view on the utilisation of windows and elements. Not discussed in this article are the make-up air requirements and electrical requirements. Actuation systems and the associated challenges of offsetting the centroid moments when pivoting the sash in the fixed frame, bearing in mind the negative effects of the friction stays. The crucial item is the selection of the actuator to overcome the static forces of the window, coupled with the wind load that should be obtained from the facade engineer.
I have stayed away from condemning the selection of the windows based on non-compliance with the EN 12101 code. However, before we toss out the code, we need to think of the following. How will these windows perform at elevated temperatures? As a designer, are you satisfied that the occupants are safe? Are we achieving the underlying aim of a “Life Safety System”? When does the requirement of life safety end? When the occupant is off the fire floor, or when the occupant is outside the building? If the untested window loses it glazing component, and the glazing drops down the external facade of the building and injures a person external to the perimeter of the building, has the designer complied with supplying a life safety system?
There is a lot to consider when selecting the correct equipment for a smoke ventilation system. It is the designer’s responsibility to understand the reaction of the selected equipment during the entire ventilation process.