By Rob States, Dan McKinney and Bruce Dandie
Standards and codes require that transit stations provide a safe environment for patrons to escape a fire event at the station. In the public areas of the station, a fire event can be caused by a train fire or some other type of fire on the platform or concourse area such as a baggage fire or a fire in a trash bin. The design approach to provide ventilation and smoke control for transit stations is well established for enclosed stations attached to underground tunnel networks, and for exposed above ground stations where heat or smoke is not trapped in the evacuation paths.
However, in an attempt to make transit stations more appealing to the traveling public, municipalities are making their stations more architecturally appealing and are partially enclosing the stations for passenger comfort so that patrons are not exposed to the elements. This means that the standard design for smoke management at transit stations is no longer appropriate. Conventional ventilation strategies that have been used in the past simply do not work when applied to a partially submerged station that is subjected to ambient wind effects that influence the smoke flow and can overpower a conventional tunnel ventilation fan system.
AECOM’s Tunnel Ventilation team worked with a transportation district and municipality to engineer a smoke management system to a partially submerged station through the use of computational fluid dynamic (CFD) techniques. The ventilation design provided a code-compliant system that provides the transit station with safe evacuation conditions without sacrificing the station’s open feel and weather protection.
The transit station is located in a suburban area where the track outside the station runs in a retained cut for some distance before it rises to grade level. This opens the track and station to the influence of wind and aerodynamically isolates the station from other elements. As the illustration shows, large pedestrian openings into the surface-level concourse are subject to influence by ambient wind, as are the roof skylights. To appropriately model this station configuration, the station, its connected wind domain and the exposed track ways all have to be modeled. This results in a CFD model representing an area nearly half a mile long and more than 800 ft wide.
The station platform accommodates 10 subway cars, any one of which can be involved in a fire. Wind direction and strength at the time of the incident are also unpredictable. The combination of these unknowns creates a multitude of operational permutations that must be adequately handled by the station ventilation system. From a design perspective, this creates a huge program issue, especially when each simulation consumes a week of computer time due to the size of the CFD model. This becomes further problematic as not all potential design solutions will be effective in all design cases. This necessitated that a number of ventilation configurations and solutions be explored.
In such a computationally intense project, it is uneconomic both from a time and resource perspective, to solve a highly detailed model for each condition. Instead, a simplified model to quantify the design’s sensitivities and find critical wind directions and fire locations was solved initially. In these initial simulations, it was determined that the station’s skylights generated significant internal recirculation eddies that rapidly distributed smoke throughout the station. Consequently, the control of the recirculation eddies became a priority. A second initial finding demonstrated that wind directed along the track was significantly coupled to the station’s platform area. It was also established that in wind speeds below 5 miles per hour (mph), the station behaved nearly identically to the zero wind case, while there was a transition above 10 mph to conditions dominated by wind.
Once the station’s initial design and configuration were analyzed, and its inherent design attributes were understood, various ventilation strategies were applied to provide an appropriate design. The value of configuring a ventilation system early in the station’s design cycle cannot be over-emphasized. Design features that have been previously established can become obstacles to an effective ventilation system and can add cost and complexity to the system.
The station design included roof mounted skylights with side louvers that provided sunlight to patrons below, and a buoyancy driven path for smoke and fumes from a fire event to escape. Though the skylight appears to be an asset to the ventilation strategy, CFD modeling showed that winds readily enter the skylight louvers on one side and exit on the other. This effectively creates a traction boundary condition above the concourse that drove a recirculation zone. In low wind, the buoyancy forces from the smoke dominate, but in modest wind, the recirculation flow intercepts rising smoke and distributes it chaotically throughout the station, obscuring the evacuation paths.
To control this effect, a “wind fence” was added to the station configuration. This fence serves to protect the upwind side from direct wind impingement and deflects the wind over the top of the skylight. This effectively turns the entire annular region between the skylight and fence into an entrainment interface that couples to the ambient wind and provides smoke extraction from the station area regardless of which direction the wind blows. The net effect of this wind fence changed flow inside the station from chaotic recirculation to a vertically extracting flow that assisted the buoyant removal of smoke and allowed the retention of the skylights.
In addition to the wind fence, exhaust fans were placed on the roof of the station and ducted to the platform level via chimneys separated from the concourse area to aid smoke extraction. The roof fans were configured to take advantage of the natural buoyancy of smoke and assist with smoke extraction. Importantly, from the client’s perspective, the roof fans were located away from the patron’s visibility and therefore do not degrade station aesthetics. The design requirement was determined to be 100 kCFM of roof-mounted flow extraction during a fire event. The final ventilation design provided six fans, each with 20 kCFM of nominal flow delivery, operating in exhaust. This configuration allows one fan to be out of service during a fire event.
However, if the out-of-service fan were allowed to back flow, it would short circuit the remaining fans and reduce the chimney exhaust capacity. As a consequence, gravity dampers were placed on the exhaust of each fan. During fan operation, the damper is opened by fan flow. Should a fan not be operating, the closed gravity damper prevents back flow.
The final station design integrated novel ventilation strategies into the station’s architecture to maintain aesthetics. As a result, the transit district was pleased with the design simplicity and low maintenance requirements. The municipality was also pleased with the open architectural feel of the station, which appealed to rider’s aesthetics and protected them from the elements.
Rob States and Dan McKinney are Tunnel Ventilation Engineers with AECOM Transportation. Bruce Dandie is the Manager of Tunnel Ventilation Services at AECOM Transportation. For additional information on this topic or on Tunnel Ventilation Services, you may contact Bruce at Bruce.Dandie@AECOM.com.