Engineering for fire safety on a modern railway network
By Angela TAM
There is currently an unprecedented amount of new railway development in Hong Kong with five new lines - West Island Line (WIL), South Island Line (East) (SIL(E), Express Rail Link (XRL), Kwun Tong Line Extension (KTE) and Shatin to Central Link (SCL) - being built by the MTR Corporation.
Admiralty Station (ADM), already among the busiest interchange stations on the MTR network, is central to this plan: when fully completed, the station will become the interchange station for not only the Island Line and Tsuen Wan Line, but also SIL(E) (in 2015) and SCL (in 2020).
The station is undergoing a transformation to alleviate congestion with a more spacious environment, a major upgrade that entails the construction of four new platforms deep underground, to be connected by a new escalator network, as well as an expanded concourse and a new, improved Harcourt Garden above the new station structure. As technically challenging as it is to expand the station and align the new railway lines in a commercial area densely packed with office buildings and hotels, the expansion works would not have been feasible if conventional planning and design, including fire safety, approaches had been adopted.
The new portion of ADM will feature a large circulation atrium bringing natural light deep into the new basement levels and this has been made possible by an innovative solution developed in collaboration with the statutory authorities to address fire safety issues. Rather than forming fire compartments according to area or volume limitations to comply with code-based fire separation requirements for general buildings, a fire engineering approach was adopted to arrange suitable fire separation between the new and existing public circulation areas, together with smoke zoning and an innovative smoke control design for the atrium.
History of approach
The fire engineering approach, with proposed solutions submitted and vetted on a case-by-case basis, is not exactly new to railways in Hong Kong. It was first adopted for the Lantau and Airport Railway (LAR) project comprising the Airport Express Line (AEL) and Tung Chung Line (TCL) in the 1990s, and subsequently adopted for the Tseung Kwan O Line (TKL). According to Robert Seddon, MTR's architectural manager - strategies, fire safety strategy documents relating to means of escape, means of access, fire-resisting construction, smoke control, etc, covering stations, ancillary buildings and trackway areas have been in place since 1992. In addition to LAR and TKL, fire safety strategy documents have also been developed in the past for other major railway infrastructure projects including West Rail, Disneyland Resort Line, Ma On Shan Line, and the Kowloon Southern Link. The five new lines currently under construction are similarly based on tailor-made fire safety strategies.
ADM, which currently covers 9,000 sq m in three basement levels, will grow to 20,000 sq m spread over six basement levels. Its unique design is made possible through close partnership among the designer, MTR and the statutory authorities to address particular fire safety issues through fire engineering analysis. The new portion of the station will feature a 40 m high entrance atrium with ten escalators serving six levels, with the SIL(E) platform at the deepest level of -33 mPD. The escalators will be used for evacuation under fire scenarios not affecting the escalator atrium, while separate staircases will also be provided.
Given the volume of vertical space involved in the new atrium, a traditional dynamic smoke control strategy would possibly have required large ventilation fans of 2 m diameter like those serving the tunnels. This would have called for additional plant rooms as well as large, visually obtrusive ventilation shafts. To address the fire safety concern over the proposed use of static (natural) venting only for smoke control, an innovation was developed whereby natural smoke venting due to hot smoke buoyancy will be supported by dynamic smoke extraction using booster fans concealed within the atrium roof structure.
Evacuation within ADM is specially addressed by a "self-sufficiency approach" under which fire shutters at the level affected by a fire incident (and fire shutters at any other level subsequently affected) will be activated to separate the new and existing portions of the station. "While not compromising fire safety, the means of escape capacity at other, unaffected levels will then be maximised, allowing evacuees to move to ground level via all available non-incident zones, significantly reducing the time needed to clear the station," explained MTR's fire engineering manager Ir Dr Fiona S C Tsui.
Smoke curtains will separate the atrium void from the connected public circulation areas at various levels with just the localised openings remaining at the top and bottom of each escalator to facilitate onward evacuation to ground level. The strategy is designed to facilitate evacuation from an incident zone to a place of safe passage within 4.5 min. Flashing exit signage and the public address system will play important roles in directing evacuees to safety. Thus, in addition to an upgrading of the fire safety provisions in the existing ADM, such as fire hydrants and sprinklers, etc, the existing static signage will also be replaced with flashing exit signage.
There are altogether 29 signage modes and 50 evacuation scenarios that are defined according to the location of an incident. These modes of operation are pre-programmed, thus simplifying their implementation in response to a particular fire scenario and minimising human error.
Another large and unique railway station which has benefited from the fire engineering approach is the West Kowloon Terminus (WKT) of XRL. Containing 440,000 cu m of space, WKT is 20 times larger than a typical two-level station. Of the 15 platforms located at basement level 4, nine platforms are designed to accommodate long haul 16-car trains while the other six platforms serve eight-car regional shuttle trains.
In addition to more than 200 smoke control zones, the design of WKT must also take into account the requirements of boundary control as people who have entered the boundary control zone, like those in an airport's transit zone, cannot be evacuated directly to the streets of Hong Kong. Thus, special "place of ultimate safety" compounds have been designated for their identity reprocessing after an incident.
Arriving and department passengers within WKT must remain segregated for similar boundary control reasons, resulting in two distinct flows. The overly large internal volume also needs to be segregated. "We have utilised fire shutters and fire curtains to create individual compartments, with means of escape provided accordingly. Unlike a typical metro station, at WKT the 'way in' is not necessarily the 'way out', so a separate evacuation provision is necessary," Mr Seddon explained.
Emergency access is carefully considered taking into account the need to connect all levels as well as the station control room by a centrally-located, designated emergency entrance, supported by as many as 20 supplementary emergency entrances. "The most difficult areas to reach are the individual platforms on basement level 4. We have therefore provided a corridor, like an underpass, at basement level 5, near the middle of the station and connected to the designated emergency entrance, allowing enhanced access for emergency services," he said.
Supported by extensive studies, the fire-engineered design solutions proposed for ADM and WKT were ultimately accepted by the statutory authorities, paving the way for the possible adoption of such solutions on similar projects in Hong Kong in the future.
New fire engineering guide
In early 2012, the Fire Services Department (FSD) initiated the development of a "Guidelines on Formulation of Fire Safety Requirements for New Railway Infrastructures" document, based on the design agreements reached with the MTR for WIL, SIL(E), KTE and XRL and ongoing discussions on SCL. A draft version of the document was reviewed by both parties during the second half of 2012 and the first edition was officially issued in January 2013.
Railway designers can now apply a "railway prescriptive" approach with a higher degree of certainty that their designs will be acceptable to the statutory authorities. The new guidelines document is also expected to expedite statutory approval processes. While the design proposals for WIL, SIL(E), KTE and XRL were assessed on a case-by-case basis, a process that took more than three years, the process could potentially be reduced to one year in the future now that the details of what is acceptable, and what is not, are spelt out.
"Some requirements are more stringent now, but the key thing is having expectations defined," Mr Seddon said. "Case-by-case discussion is a time-consuming process and not the most efficient. To use an analogy, it's like ignoring past precedent in court cases and re-interpreting the law each time. The new railway guidelines are the result of discussions we've had with the FSD over the past five or six years, so the majority of the guidelines are already present in the current five new lines."
A second edition of the Railway Guidelines was officially issued in September 2013 and a notable feature of this latest edition is the inclusion of lift-assisted evacuation, a strategy which has been adopted on WIL and SIL(E) to address some challenging topographical constrains. The rationale is simple: if emergency services can safely use a fireman's lift in the event of an emergency, then why not adopt the same technology to evacuate some passengers in a controlled manner from deep stations?
"The innovative solution using high-capacity lifts to assist in the evacuation of deep railway stations was first developed during the detailed design of WIL in collaboration with the statutory authorities and emergency services personnel and subsequently applied in SIL(E), where the lifts typically serve only two levels - ground and station - with no intermediate floors," Ir Dr Tsui explained. "The lifts are served by separate lift machinery and powered by dual power feeds from separate sources that are protected by fire-resistant cables. Continuity of power supply for the lifts is assured, ensuring that the lifts will be available for evacuation when required."
A pressurised refuge lift lobby located one level above the regular lift lobby will allow passengers to wait safely for the lifts to arrive, in a safe and psychologically reassuring environment, before being evacuated to ground level. Fire curtains at the base of every lift shaft will further isolate the lifts from the rest of the station in an emergency. The approach may benefit not just future railway projects, but other building projects as well.
"The approach is a good first step for considering lift-assisted evacuation for super-high-rise buildings using refuge floors, as is increasingly the case overseas," Mr Seddon said.
The document issued by FSD is still a work in progress. The second edition identified an Appendix 7 concerning smoke control as "under preparation". It is a complex subject that will benefit from further discussions before the details can be included in an updated version. Even so, the current document is already making its contribution to the development of future new railway lines as studies commence in support of the Government's updated railway development strategy. The innovative fire strategies documented may prove useful not only for railway designers, but also for creators of large spaces, super-high-rises and other non-typical structures.
The large volume of space inside the circulation atrium at Admiralty Station calls for a fire engineering approach. Images: MTR
Obtrusive ventilation shafts would likely get in the way of a streamlined new entrance at Admiralty Station if a fire engineering approach had not been adopted
Evacuation arrangement at WKT
Lift-assisted evacuation for WIL stations
Designing a fire safety strategy for WKT is a challenge as it is 20 times the size of a typical MTR station