Like airports, sea terminals also have to accommodate fluctuating numbers of people in large spaces. Kevin Wong examines the fire safety design challenges for a ferry terminal in Hong Kong.
Pier terminals for ferries need to accommodate large numbers of passengers in their daily operation. Similar to an airport buildings, they have a performance requirement that a smooth passenger flow is necessary for efficient operation. In some modern terminal designs, large floor plans and interconnected spaces to other levels – in terms of atrium or escalator voids – exist to create a clear, unobstructed view providing a pleasant environment.
But these functional and architectural requirements can impose additional challenges to the fire safety design of such buildings, so a prescriptive approach may not be the most appropriate method to provide a cost effective solution. In this article, the fire safety design of a modern pier terminal building is analysed to demonstrate the use of performance based fire engineering design solutions to satisfy its architectural and operational requirements.
The new terminal is being built near Hong Kong international airport and forms part of the airport island development. It will be an international terminal consisting of arrival and departure levels, check-in hall, security screening, baggage level and an underground Automatic People Mover (APM) station which connects to the existing airport terminal through a tunnel. Passengers from outside the immediate area can have their luggage checked-in at their home town and can then directly proceed to the air side of the airport terminal using APM ‘trains’, on arrival by ferry, without having to pass through Hong Kong immigration. This integrated design can facilitate passenger flow and keep administrative procedures at the terminal to a minimum.
The arrival hall is located at street level and consists of check-in facilities and a security screening area, which will allow passengers to proceed to the airside of the airport terminal. Escalators connect the arrival level with the underground APM station. The departure level will be located one floor above street level where, in addition to the sitting area, shops and catering outlets will be provided. These retail facilities will create mixed usages in the terminal building, as well as introducing additional fire loads. Escalators will also be provided to connect the departure level to the APM station underneath.
Along the ocean side of the building, escalators in the large atrium will be provided for passengers from the departure level descending to, but separated from, the arrival level for boarding the ferries. Since one of the main facades will be located along the seaside, evacuation will be limited to the back and sideways of building.
Open access
Due to the operational requirement for an unobstructed floor space to facilitate passenger movement, the separation of space into fire compartments under Hong Kong’s prescriptive code approach would have been impractical. In addition, different levels will be connected both horizontally and vertically by large atria to provide a wide-open space as an impressive first image to visitors. It was therefore proposed that the arrival and departure levels would be treated as one single fire compartment, something that is not allowed under the prescriptive code. The required maximum travel distance has also been extended from the prescriptive value, to allow for more flexibility in the planning of pier usage and eliminating the necessary escape path along the sea-facing side of the building. Based on the final architectural layout, the proposed maximum travel distances to protected corridors and staircases are 50m and 100m respectively, while the maximum dead end distance is not to exceed 55m.
The performance of the issues that have deviated from the prescriptive code were evaluated using a time line analysis, which compares the available safe egress time (ASET), i.e. the time of onset of untenable conditions, and the required safe egress time (RSET), i.e. the time required for occupants in the areas to complete their evacuation to the escape stairs or protected passages. When ASET is greater than RSET with a significant safety margin, the design will be considered to be acceptable.
In the case of smoke control, however, instead of a single fire compartment the arrival and departure levels will be divided into smoke zones using high level smoke barriers receded within the ceiling void. A dynamic smoke extraction system will maintain a smoke free zone in these spaces. The capacity of the smoke extraction system will be sized by using fundamental fire safety engineering principles to deal with the smoke production rate of the design fires, based on the credible fire load in the pier terminal, such as baggage in the circular area and waiting hall, as well as products in shops. A smoke detection system and sprinkler flow switch will be used to actuate the smoke extraction system, while an automatic fire sprinkler system will be provided to limit the heat release rate of the fire throughout the site, for life safety and property protection. Fast response sprinkler heads will be provided to give an early response to control the fire size in the retail facilities.
In the event of a fire, occupants on the arrival level will approach the final exits on one side of the building or, due to large floor plan, via protected corridors to the other side of building . Occupants on the departure level will escape through the staircases leading to street level, and finally to the open air. Unlike ordinary buildings, the occupant load at any one time is largely dependent on the ferry schedules. To this mix should be added: traffic flow rates; human behaviour; the time people spend in particular areas; and the number of staff available.
Using occupant load factors under the prescriptive code to estimate the actual occupant number for means of escape design was considered over- conservative. To prevent over-designing the means of escape provisions, the number of occupants for evacuation calculations is based on factors such as the passenger flow pattern, peak hourly departure and arrival flow rates, and the dwell time of passengers in each area. This methodology can result in a cost effective design in the means of escape provisions and realistic RSET assessment.
Large numbers of disembarking passengers and those waiting for the ferries may create an additional hazard in case of an emergency, due to their unfamiliarity with the pier building and local emergency procedures. So instead of a conventional alarm bell, a public address system will be provided throughout the pier to broadcast emergency warning messages and announcements to occupants. For example, announcements backed up by staff on the ground will guide passengers along the appropriate routes for evacuation. The PA system will also be used for delivering background music and will be provided with an emergency backup power supply. In addition, adequate means of egress in conjunction with exit signage will be provided to help people complete their evacuation, prior to the onset of untenable conditions.
Pontoons
Two pontoons – independent floating vessels separated from the pier building – will be located near the sea front next to the pier building. The only link between these and the pier building will be via pedestrian bridges with marine grade power coupling. Due to the nature of pontoons, it is impractical to provide an automatic fire suppression system, so their fire safety designs and those of the link bridges are based on a performance approach by considering a number of fire scenarios and emergency procedures:
– Fire on a ferry: Passengers on the ferry will escape towards the pier terminal using the pontoons and link bridges. If there is another ferry at the same pontoon, passengers from that ferry will be stopped disembarking immediately and the ferry will leave the pontoon for safety.
– Fire on a pontoon: Passengers already on the pontoon of fire origin will escape towards the pier terminal using the link bridges. Passengers on ferries that are buoyed at this pontoon will immediately stop alighting, and the ferries will leave the affected pontoon for safety. Ferries that are approaching the pontoon of fire origin will be informed to stay away from the pontoon or the whole pier terminal berths, depending on the severity of the fire.
– Fire on a link bridge: Passengers will escape either towards the pier terminal or the ferry from which they previously disembarked using the pontoons, depending on the location of the passengers relative to the fire.
With the proposed fire safety strategy, ASET is larger than RSET with a significant margin. The issues of oversized fire compartments and extended travel distances should not adversely affect life safety. It has therefore been shown that the operational and architectural requirements of a modern pier terminal building can be satisfied by using a performance based fire engineering approach, while providing a cost effective solution for a safe design.
Kelvin Wong is fire engineer with Arup in Hong Kong.
