Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 56

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 55
56 Safety Equipment tected to resist fire damage. Safety critical equipment in road tunnels must be able to function in the event of fire. The The ventilation system is a crucial safety measure when a physical location, such as a lower level, may also reduce fire fire occurs, because it allows for smoke-free escape routes. damage. Different countries have different requirements The ventilation system is designed to control the smoke for the equipment. For example, a minimum temperature spread, and this can be achieved by blowing smoke in one requirement in the NFPA 502 for the tunnel ventilation fans, direction, supporting smoke stratification and extracting dampers, and sound attenuators is 250C (482F) for 1 h of smoke at the ceiling or near the ceiling, or blowing smoke exposure. A higher temperature is used depending on the in one direction and extracting it at a few places. results of the calculations. Many international standards have requirements of 250C (482F) for 90 min of exposure and Alarm systems including telephones, push buttons, pull a maximum of 400C (752F). boxes, detection, and surveillance are important to alert the operator and thereby activate the emergency procedures, The development of emergency response plans requires a ventilation systems, etc. Unfortunately, those systems are consideration of activities before, during, and after the incident, seldom utilized by tunnel users. In 46 tunnel fire incidents in Austria, the emergency telephone was used only 10 times and covers: (22%). Pull boxes/push buttons were used only four times (9%) (42). Prevention and training, Accident management, and Communications systems influence the evacuation of Fire emergency operations. the tunnel during a fire and thereby reduce the number of people at risk by exposure. The communications systems can be radio re-broadcast (audio warning) and message boards INTEGRATED APPROACH TO SAFETY (visual notification). Once the evacuation has been initiated, IN TUNNELS it is important that the tunnel users reach the safe area as quickly as possible. Exit signs, exit route guidance, lighting, Safety is a result of the integration of the infrastructural and markings are mitigation measures that can make the escape more efficient. All these elements are discussed in measures, the operation of the tunnel, and user behavior, as details in the NFPA 502. well as preparedness and incident management. The assess- ment of fire safety in tunnels is a complex issue, where broad Response to Fire (Fire Resistance of Safety Systems) multi-disciplinary knowledge and application of different physical models are necessary to explore the causes and devel- It is particularly important that the installations necessary opment of fires to evaluate measures to prevent and reduce during an emergency continue to function for a suitable dura- their consequences. In general, an overview of the entire sys- tion during a fire. tem is necessary to determine the best possible actions (43). Reduction of Consequences to Structure and Equipment The systems to take into account comprise: Safety Equipment The occurrence and physics of fire development. The tunnel systems; that is, Fire suppression systems can potentially prevent severe Infrastructure and damages to the structure and to the equipment once acti- vated at an early stage of fire development. It is possible to Operation. have fixed installations in the traffic space to suppress fires Human behavior of users, operators, and emergency in vehicles or other sources. Often, the purpose of the fixed services. suppression installations is not to extinguish the fires but to Other factors influencing safety. control and limit fire development. (See Annex of NFPA 502 2008 edition for additional information.) Prescriptive Approach Response to Fire: Structural Fire Resistance and Fire Protection Traditionally fire safety standards for tunnels and other struc- tures have been prescriptive; they have contained minimum One of the serious consequences of a fire is damage to the requirements that must be fulfilled. These requirements have tunnel structure and its ultimate collapse. By suitable design of the tunnel structure and by passive fire and/or fixed fire been established over the years based on experience, tradition, suppression protection, the tunnel can withstand the rele- and engineering/expert judgment. They apply in principle vant fire scenario and tunnel rehabilitation and repair costs by absolute evaluation of safety: if the design is in accor- can be reduced or eliminated. dance with the standard, the safety is acceptable; if not, it is unacceptable. The advantage of a prescriptive standard is Fire Resistance of Equipment, Power Supply, that it is not difficult to use and it ensures a minimum level and Cabling of requirements. On the other hand, prescriptive standards may not be applicable to unusual situations and in some cases The installations are often the first part of the tunnel system to suffer damage in a fire. By suitable design and passive or may not be able to take into account the interaction between active fire protection systems the damage to the installations different parts of the tunnel structure, installations, and the can be reduced. The cabling and other installations are pro- local conditions.

OCR for page 55
57 Performance-based Approach for more flexibility. The performance-based approach gen- erally requires more data and procedures resulting in a more In recent years, the national and international standards have complex and time-consuming design. For the designer, a tended to favor the performance-based approach. NFPA 502 is prescriptive approach is an advantage with respect to liability. not an exception. By application of fire performance concepts, fire safety is achieved based on a scientific understanding of A design fire is an idealization of a real fire that might the fire phenomena, of the effects of fire, and of the reaction occur. A design fire scenario is the interaction of the design and behavior of people. Emphasis is given to the safety of life, fire with its environment, which includes the impact of the whereas fire safety engineering can also be used to assess fire on the geometrical features of the tunnel, the ventilation property loss, interruption of service, contamination of the and other fire safety systems in the tunnel, occupants, and environment, etc. Furthermore, risk of fire and its effects are other factors. quantified and the optimum safety measures are evaluated. Nobody can precisely predict every fire scenario given the By a performance-based approach, the regulatory require- range of variables and human behavior. It is not known ments are given on a more general level specifying the safety of the users, economic values, and so forth. This may result What will cause the fire (collision, electrical problem, in different interpretations leading to undersized fire and terrorism)? underdesigned safety systems. What exactly will be burning (goods, furniture, car body, etc.), including the number of vehicles involved Fire safety engineering will normally involve the following in the fire? steps: Where will (in which part of the tunnel) the fire occur? When will the fire start (month, time of the day, etc.)? Qualitative design review: What the outside environmental conditions will be at Definition of objectives and safety criteria, with the time of incidence (winds, hurricane, earthquake)? reference to performance-based standard require- What will be the traffic conditions? ments and coordination with the authorities having How will the tunnel users' behave in an emergency? jurisdictions; How will the operators behave during an emergency? Definition of the tunnel system; Identification of fire hazards; Therefore, the designer makes a number of assumptions to Selection and definition of fire scenarios; ensure that the design will save lives and retain structural Identification of methods of analysis; and integrity under most of the foreseeable fire scenarios. Identification of design options. Quantitative analysis of design using the appropriate International standards on preventive fire protection are subsystems: based on a risk approach. In a European study, it was found Fire ignition, development of heat and smoke; that risk estimates produced by different users differed by Spread of fire, heat, and smoke; "several orders of magnitudes." The estimates varied signi- Structural response to fire; ficantly from case to case. Serious concerns on risk analysis Detection, activation, and suppression; and have recently been found in the Channel Tunnel design that Behavior of tunnel users and influence of fire on life has already experienced several large fire events since its safety. opening. Assessment of the outcome of the analysis and evalua- tion against criteria. The PIARC report reiterates the need for a greater focus on the definition of appropriate fire scenarios dealing with The objectives and the associated acceptance criteria specific aspects of tunnel fire safety. This can be achieved by used in a performance-based approach is clearly defined and accurate specification of the input and output characteristics established for the particular design. The acceptance criteria, of design fires. which establish the adequacy of the design, can be according to the following approaches: The main cause of death in a fire is related to inhalation of smoke and hot gases and not from the fire itself. Therefore, Deterministic (including, when appropriate, safety with respect to life safety, attention is given to the determi- factors). nation and mitigation spread of (possibly toxic) hot gases and Probabilistic (risk-based used in European countries). smoke. Some key design fire scenarios relevant to the fire Comparative (comparison of performance with accepted safety in tunnels are listed here: codes of practice). 1. Design fire scenario for ventilation and other systems The deterministic and the comparative approaches are to (e.g., fixed fire suppressions) design and assessment-- some extent similar to the prescriptive approach, but allow Smoke ventilation in tunnels needs to be designed on