Commercial

As the Smoke Rises, the Elevator Shaft Vents Must Go

Fri 10/28/2022 - 20:06

As the Smoke Rises, Elevator Shaft Vents Must Go

Given the fact that stack effect demonstrably exacerbates smoke spread through elevator shafts, it makes good sense to eliminate hoistway ventilation.  Venting the shafts never made much sense in the context of the energy code and frankly ran counter to the energy code’s intent, but thanks to the CTC Elevator Study Group and other researchers’ efforts we now know that shaft ventilation never really worked from a life safety standpoint either.  Eliminating the hoistway vent should be one simple code change the entire industry can celebrate.

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It is an undisputed fact among fire safety professionals that chances are good that the fire won’t kill you, but the smoke will[1].  Smoke inhalation is the leading cause of death and serious injuries in fires and the biggest potential hazard any building occupants may face.  Therefore, controlling, and mitigating smoke spread has long been a focus of building codes.

A key area where smoke spreads in buildings is in elevator shafts.  As heat from a fire rises, the natural convection movement draws smoke upward and can spread vertically to all the floors of a building.  Traditionally, we mitigate this smoke spread through compartmentalizing portions of the building away from each other.  Section 708 of the Building Code (BC) dictates than in all but a few limited situations, shafts must be enclosed by fire-resistant rated construction.  Any floor opening between three or more stories (two after November 7th, and already in most places outside NYC) is considered an “atrium” and is subject to special requirements, such as active smoke control systems in the form of jet fans.  Since the 1968 Building Code, the law has also required for elevator shafts to be ventilated[2].  In case smoke were to enter the shaft the thinking was providing a vent at the top would be the easiest way to prevent it from spreading further and allow easier smoke extraction post-fire.  Prior to the 2014 Code, a portion of the vent also had to remain permanently open.  However, this presents a problem as providing an open vent on the top of a shaft is also a great way to let conditioned air fly out of the building.  It turns out that not only is the vent a CO2 emitting waste of money, but it also makes smoke spread worse for the very same reason it lets heat out the building in the dead of winter – stack effect.

Stack Effect Graphic RepresentationStack effect is a product of the pressure differential buildings experience as they get taller.  Stack effect is most pronounced in very tall buildings, such as those over 420’ high, and is magnified during winter.  At grade the pressure is usually higher and so too is usually the air temperature.  The further up we go, the lower the pressure and temperature, which is why airplanes are pressurized and it’s the reason people may feel a “squeeze” in their ears going up or down the elevator in a tall building. 

Fluids, such as any gas or liquid, will always move from areas of high pressure and temperature to areas with low pressure and temperature as that fluid seeks to even out the pressure and temperature differential.  This naturally draws air up shafts if the pressure and temperature difference are great enough to warrant the effort and shaft vents make this easier as they connect low pressure exterior air with high pressure interior air.  As temperature goes hand in hand with pressure, the differential and subsequent stack effect is at its greatest in in winter.

Per the diagram provided in the CTC study, the building experiences a “squeezing” effect on the lower floors below the neutral plane and a “pushing” effect on the upper floors as warm air rises like a chimney.  Throw in a fire and the temperature and pressure differential become even greater, especially if the fire breaks out on a cold windy day in a high-rise building with an open elevator shaft vent at the top.  As the heat rises, so too will the smoke and it can ride that shaft straight up to the upper floors of the building – where besides the fire floor the most deaths are prone to occur as witnessed in the Twin Peaks high rise fire earlier this year where 14 of the 18 deaths were concentrated on the upper floors[3].

The issue with keeping smoke out of elevator shafts is elevator doors are prone to leak rather badly due to their required horizontal sliding motions.  A bevy of options are available to creative designers to address this problem[4], but the code prefers enclosed elevator lobbies to compartmentalize the shafts from the rest of the floor.  If a lobby isn’t provided, the code permits pressurized air be blown into the shaft to keep smoke out[5].  Worth noting is that prior to 2004 the 1968 Building Code relied heavily on the shaft vent for smoke mitigation and didn’t even require elevator lobbies in high-rise office buildings, which changed with the post 9/11 Local Law 26 of 2004.  Nationally, the IBC has required elevator lobbies in high rise buildings since its inception in 2000, and prior to that the Uniform Building Code has required them since the 1970’s[6]

Historically, the codes’ primary focus was always on smoke and fire and maximizing the time available to egress the building – rate this column for so many hours, provide enough stair width to let occupants evacuate down and first responders up, keep flammable materials out of assembly spaces, and the like are all things most building safety professionals have in mind when they think of the codes.   However, with the threat presented by climate change and rising energy costs the codes have had standards for energy conservation since 1975[7].  Naturally, providing a permanently open vent at the top of an elevator shaft is going to let a lot of heat out during winter[8], and even providing a damper as is the current custom under the exception to BC 3004.5.1 and required under Energy Conservation Code section (ECC) 402.5.5 is an inelegant compromise.  Dampers, even good ones, tend to leak a bit of air and they provide a highly conductive thermal bridge right at the worst place in a building for one, the place where the interior temperature is naturally highest, and the exterior temperature is coldest in winter.  Because of this the more energy conscious side of the industry would rather get rid of the vent entirely.

Thankfully, the rest of the industry reached a similar conclusion during the 2012-2015 ICC revision cycle.  Prior to the cycle the ICC requested its Code Technology Committee (CTC) investigate the role elevator lobbies and vents play in preventing smoke from spreading throughout a building.  The findings of the study were controversial, as they actually proposed getting rid of elevator lobbies for all buildings less than 420’ high[9].  The study showed that 420’ was the point at which the stack effect became large enough to play a substantial role in smoke spread, predominantly because sprinklers did such a good job dampening spread.  Getting rid of the lobby below 420’ was hard to accept by many in the industry, as William Fitch noted in public comment to the rule change (FS66-12) that the study’s conclusions were “embarrassingly inadequate and incorrect”[10].   The rule’s detractors felt the study omitted key factors, such as use of lobbies by first responders to stage to fight fires on the floor above, and over reliance on sprinklers to control smoke.  Ultimately the ICC agreed and did not enact the proposed change from its own study group.

While the effort to remove the need for lobbies in high rise buildings below 420’ failed, another related provision to eliminate the hoistway venting requirement passed and made it in the 2015 IBC.  As part of the public commentary on the proposed rule change to eliminate hoistway vent (G166-12), Johnathan Siu and two other colleagues proposed to eliminate the vent requirement entirely, stating “the concern is that such venting may have the effect of drawing smoke through the building where it is not appropriate.  This is a specific concern after consideration of overall smoke movement by the CTC Elevator Lobby Study Group related to stack effect and preventing smoke movement throughout the building”[11].  In its summary of Committee Actions, the ICC approved the change with the reason “hoistway venting was no longer necessary and creates conflicts within the code.  In addition, hoistway venting openings are a huge source of conditioned air loss”[12].  Eliminating the vent was an easy enough change to make to the ICC Codes as the elevator code standard, the ASME A17.1, had already eliminated the vent provision as of the 2010 edition.  Since 2015 the ICC no longer requires hoistway venting in any way, and as the 2022 NYC Codes are based on the 2015 ICC Codes, our city will also be going ventless after November 7th[13].

While we are losing the vents, the elevator lobbies and their pressurization alternatives remain safely ensconced in the codes for the foreseeable future.  Elevator lobbies are now featured in the new 2022 Code under section 3006, which due to how our codes work could mean they will even be required in prior-code buildings undergoing substantial alterations say, where a new elevator shaft is added.  General shaft enclosure provisions remain in Chapter 7 but have been moved to section 713 from 708.

Worth noting is besides the CTC study there have been some other studies investigating the impact of stack effect in shafts on smoke spread with many coming from China.  A study, Simulation on spread of fire smoke in the elevator shaft for a high-rise building, by Yunchun Xia from Anhui Jianzhu University[14] used computation fluid dynamics (CFD) to simulate various fire scenarios using different ventilation rates of the shaft.  Higher ventilation rates lead to increased pressure, increased smoke volume, and infiltration in a linear fashion, as evidenced by the following graphics taken from the study:

Velocities of smoke outside the elevator cab

Pressure and flow rate of smoke at exit

 

Smoke distribution in the elevator shaft and pre chamber

Sources

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