The stack effect and considerations for smoke control

The stack effect is a natural phenomenon that occurs predominantly in tall buildings, where difference in air temperature results in pressure between the inside and outside of a building and causes air to flow through vertical spaces. In the event of a fire, the stack effect can promote smoke and hot gases to rise and accumulate in the upper floors of a building, while drawing fresh air from the lower floors. This can create a hazardous situation where smoke and heat are drawn into the stairwells, which are critical as escape routes for occupants of the building as well as for fire rescue service intervention.

As illustrated in Figure 1, the stack effect can interfere with the operation of certain types of smoke control systems by encouraging a flow of air that is opposite to the desired direction of airflow.

The internal and external temperature distributions and gradients before a fire are not always accurately represented by practitioners in design approaches that utilise Computational Fluid Dynamics (CFD) based fire modelling, such as the widely used Fire Dynamics Simulator (FDS). Moreover, when these pre-fire temperature conditions are included in the model, the prevalent use of the default inert wall thermal boundary conditions in FDS may significantly influence the preservation of temperature gradients. This happens because these conditions model an infinite heat transfer to keep the wall temperature at a steady 20°C.

Stack effect is often overlooked in smoke control design with some guidance, such as EN12101-6-2006 Annex B (informative), even suggesting to intentionally reduce or remove the impact stack effect during the commissioning of smoke control systems: "B.2 Where stack effect is likely to be a significant factor, this may be minimized by operating the pressure differential system for a period of one hour before testing so that the external air and shaft temperatures can equalize."

This report highlights alarming observations where the impact of stack effect in tall buildings undermined the active smoke control systems and illustrates how, if not suitably designed for, means of escape and fire rescue operations may be compromised.

Responding to reports of various fire system faults, recent investigations were carried out during winter and spring months at several tall buildings.

For Observation Test 1 the building was a 40-storey residential building with a naturally ventilating Automatic Opening Vent (AOV) provided at the top of the escape stairwell intended to act as a make-up air source for a smoke extract shaft within the lobby. The interior temperature was 18°C-26°C, and the exterior was 8°C -10°C.

Figure 2(A) illustrates the typical idealised flow to remove air via a smoke shaft in the lift lobby (driven by a smoke extract fan) with airflow being drawn from the stairwell’s open AOV. During a test of the smoke control systems, the main entrance and exterior stair doors at ground level were open.

Figure 2(B) shows what was observed during the test, namely how the smoke exhaust systems were unable to counter stack effect driven flow up the stair shaft. This required retroactive amendments to the system which may not have been undertaken in other buildings, particularly those commissioned on warmer days.

Finally, Figure 2(C) demonstrates that if a fire occurred while the building was experiencing a winter stack effect condition, smoke could be actively pulled into the stair as the exhaust shaft is unable to overcome the draw of air into the stair. Furthermore, it shows it is likely to be exacerbated by fire driven mechanisms such as a buoyancy driven upwards air flow, increasing pressure in the lobby corridor.

For Observation Test 2 the building was a >30 storey office building with protected firefighting lobbies containing a dedicated firefighting lift and smoke extract shaft. The escape stair which opens into the firefighting lobbies at each level was provided with an AOV for natural make-up air. The interior temperature was 22°C-26°C, and the exterior was 5°C -8°C. Fire curtains were included in the smoke control system to provide compartmentation to reduce smoke movement through the lift shafts in lieu of fixed lobbies. These fire curtains were designed to activate on the ground (escape) level and the level where the fire is detected.

Figure 3(A) illustrates the idealised design flow, which was observed to not sufficiently account for flow through the firefighting lift. The exhaust function was intended to generate a pressure differential across the escape stair door to prevent airflow into the escape stairwell. During the test, the lift motor room vent was open as well as the main entrance and exterior stair doors at ground level.

Similar to Observation Test 1, Figure 3(B) shows how, during the test, the air flows within the escape stair were reversed compared to the design flow. Additionally, the combined force of the smoke extract system and stack effect caused fire curtains in front of the lift doors to fail to fully close on the fire floor and the ground floor.  However, it should be noted that in a similar case with a taller building, the ground level fire curtains were pulled out of the runners when the ground level doors from the lobby to the exterior were opened. In some instances, particularly when the firefighting lobby door to the main office floorspace was opened, the firefighting lobby exhaust was also unable to prevent airflow into the adjacent escape stairwell.

Figure 3(C) demonstrates that if a fire occurred while the building was experiencing a winter stack effect condition, vents at the top of the building will likely exhaust warm air (even on milder days), compromising the design intent of the smoke control system.

Observations have also shown that the winter stack effect can be further exaggerated when escape stairs are glazed. A glazed stair with high solar gain can allow sunlight to enter the building and heat up the surfaces and air inside, creating a larger temperature difference between the interior and exterior of the building. If the glazed stair is in an area of the building where the stack effect is already strong, such as a tall building, the high solar gain can enhance this effect significantly.

It was observed that the failure of the smoke control systems to prevent airflow into the escape stairwell was due to the lack of consideration for the stack effect in both the design and guidance assumptions. Specifically, standard testing methods allow for stack effect to be ignored and considers stairwells in isolation to all other vertical shafts.

This is not representative of modern building behaviour. Recent publications, amendments and additions to codes have begun to identify stack effect, and wind, as key design conditions. However, these are often limited to 60m+ tall buildings (EN12101-13). While being a predominant cause of issues in tall buildings stack effect, wind-driven flow will be present in all buildings and can affect air movement in even low-medium rise buildings.

If a fire occurred in a tall building that was experiencing a stack effect condition due to cold external temperatures, and the building was not suitably designed to accommodate the stack effect, then smoke could be drawn into the escape stair. While this report focuses on active depressurising systems, similar stack effect air flows in fully naturally ventilated stairwells may result in similar issues in taller buildings.

Stack effect should be reflected in the modelling, design and testing of smoke control systems. Specific attention should be put on medium-high rise and taller buildings, and layouts which provide open connections at the top of interior vertical shafts and the exterior.

Within the industry, a greater emphasis needs to be placed on ensuring that testing and commissioning is carried out in realistic and suitably representative conditions, e.g. occupied levels heated. Testing which ignores the stack effect and considers stairwells in isolation should not be considered suitably representative.