The Northwestern Pacific Basin Region in which this bridge is located is the most active cyclone basin in the world and almost one-third of the world’s tropical cyclones occur here each year.
Introduction
The lower deck of a bridge in China is being made ready for future light rail transit (LRT) operation. The 2 kilometre (1.2 mile) cable-stayed bridge with double-deck design has an upper deck with three traffic lanes in each direction, and an enclosed lower deck which will accommodate light rail transit operation and a vehicle traffic lane only to be used in the event of a strong typhoon, i.e., when for safety reasons the upper deck is closed to traffic.
This Northwestern Pacific Basin region, in which this bridge is located, is the most active cyclone basin in the world and almost one-third of the world’s tropical cyclones occur here each year. Tropical cyclone warning signals in Hong Kong/Macau are a set of signals used to indicate the threat or effects of a tropical cyclone. Signal 8 indicates that gale or storm force winds are expected, with a sustained wind speed of 63-117 kilometres per hour (39-73 miles per hour) from the quarter indicated and gusts that may exceed 180 kilometres per hour (112 miles per hour). In the past, the signals were physically hoisted, and the term “hoisted” is still in common usage. When typhoon signal No.8 is hoisted, LRT operations will be suspended and the lower deck of the bridge will be used for vehicle traffic only.
Parsons Brinckerhoff (now part of WSP | Parsons Brinckerhoff) is responsible for the design and upgrade of the mechanical and electrical (M&E) system of the lower deck of the bridge to cope with the future light rail transit (LRT) operation under normal circumstances and the vehicle traffic when typhoon signal No. 8 is hoisted.
The enclosed lower deck level of the bridge consists of two tubes of 2-lanes each approximately 2 kilometres (1.2 miles) in length. One side of the lower deck is exposed to ambient and the other side is connected to an underground station and a slip road to ambient environment. For this project, the M&E systems include:
- Tunnel Ventilation System (TVS);
- Fire Services (FS);
- Drainage System;
- Traffic Control and Surveillance System (TCSS);
- Electrical System (power supply for TVS, TCSS, FS, lighting system); and
- Monitoring and Control System.
Key Challenges and Solutions
Tunnel Ventilation System to be Operated Under a Typhoon Condition
During this time, in the event of a fire inside the lower deck (e.g., 10 MW vehicle fire), the tunnel ventilation system shall be able to operate at an external wind condition of 120 kilometres per hour (75 miles per hour). The additional airflow resistance induced by a typhoon scenario was taken into consideration in the tunnel ventilation fan selection. The special fan and ductwork configuration has been designed to meet the operational requirement and site constraint.
In general, semi-transverse ventilation is adopted for smoke control inside the typical section of the lower deck of the bridge. Smoke extraction fans will be installed along the wall of the outer lane grouping into different ventilation zones. During fire emergency ventilation, fans in a number of continuous ventilation zones will be operated together for extracting the smoke from the tunnel. Makeup air will come from both ends of the tunnel and the 800 millimeter (31-inch) diameter opening on the wall of the inner lane. For localized areas where semi-transverse ventilation cannot be provided, a push-pull ventilation system is adopted.
To facilitate passenger evacuation from the vehicles and train, an evacuation walkway at 1.4 meters (4.6 feet) above the floor level is provided along the tunnel.
The smoke spread behavior and the time duration required for a smoke-clear environment to be maintained for safe evacuation under the design ventilation capacity has been verified by computational fluid dynamic (CFD) simulation analysis. In particular, it is very challenging to determine the wind effect on the smoke behavior with the consideration of the bridge configuration, the arrangement of the portals, the magnitude of the wind speed, and the wind direction. Moreover, a computerized evacuation model is applied to determine the time duration required for evacuating all the passengers inside the tunnel.
Cost Effective Power Supply system
The lower deck of the bridge is divided into different ventilation operation zones. During operation of the ventilation system, only the fans serving the corresponding zone will be operated.
Also, not more than two zones will operate at the same time. The cable arrangement for the fan is specially designed to reduce the total amount of cable (which also reduces the overall weight of the cable so that reinforcement for the bridge is not required) to achieve a cost effective design.
Fire and Life Safety
The fire and life safety concept has been developed with reference to the requirements established in National Fire Protection Association (NPFA) 130 and NFPA 502 and in association with other related NFPA standards as quoted in NFPA 130 and NPFA 502. The Local Macau Fire Code has also been referenced in the fire and life safety design.
With the use of a fire engineering design approach, WSP | Parsons Brinckerhoff assisted the contractor in reducing the amount of modification work required and demonstrated that the proposed configuration of the tunnel evacuation walkway, the cross passage, and the fence opening achieves the evacuation time requirement.
Conclusion
The design phase is completed and the project is under construction. WSP | Parsons Brinckerhoff has been making significant contributions to assist the contractor to: develop a cost effective scheme for the M&E system; reduce the weight/ load input on the existing bridge structure; and enhance the safety provisions for the bridge, including resilience measures to ensure the safe operation of the enclosed lower deck under typhoon conditions.
