By Claudius Hammann. A large telescope like the E-ELT (see Figure 1) planned by the ESO pushes technological boundaries with its building height of around 120 m and can be considered a revolutionary construction project due to its building costs of around € 1.2 billion .
The project focuses on control and measurement instruments: In order to capture images of distant galaxies, it required a very exposed location in the Atacama Desert at a foundation height of 3,060 m.
This places special challenges on the fire and personnel protection: Fire defences only exist in a very limited form, namely the voluntary Chilean fire service who face a journey of many hours to reach the building. Therefore, it was necessary to develop alternatives for a comprehensive safety concept without external rescue personnel.
This article is based on a Master‘s thesis and represents joint work between the University of Applied Sciences in Kaiserslautern, the Technical University of Munich and the ESO.
About the building
- Location: Atacama Desert (Chile)
- Start of construction 2014
- Construction period 10 years
- Planning since 2005
- Diameter of the main mirror: 39 m
- Surface area of the main mirror: 978 m2
- Height 100 m
- Width approx. 140 m
- Link to the live webcam for the construction project: www.eso.org/public/teles-instr/e-elt/
Evacuation calculations enable predictions to be made about the length of time it takes people to reach an exit or a safe area from their relevant position in the building. The required evacuation time is calculated from :
tEvacuation = tDetection + tAlarm + tReaction + tEscape
A detection time of 120 seconds  and an alarm time of 60 seconds was assumed . The escape time can be determined by inputting output parameters sourced from either a simulation program such as FDS+Evac or Pathfinder or methods for calculating the parameters by hand . The escape time was 225 seconds in the case of the E-ELT. As the organisational fire prevention still needs to be defined for the E-ELT, it was necessary to determine every possible combination. The reaction time is thus the only time measurement that is dependent on the classification of the alarm system, the complexity of the building and the fire protection management.
The attributes of each influencing factor were grouped into three levels (I to III), whereby level I defines the most favourable attributes for fire protection and level III the worst. There are thus 27 possible combinations for ∆t1 (first person starting to escape) and ∆t99 (all people except for the first one starting to escape); (see table).
If we now present the times from the table in graphical form, it is easy to recognise that the decisive factor for the reaction time is not the complexity of the building or the alarm system but the fire protection management (see Figure 1). Depending on the organisational fire protection, the times for tReaction [s] are Є [60;900]: The reaction time in seconds is an element between 60 and 900.
tEvacuation = 120 [s] + 60 [s] +
- 60 [s] for M1
- 120 [s] for M2 + 255,240 [s]
- 900 [s] for M3
This resulted in the following times for evacuating the E-ELT building:
tEvacuation M1 = 495,24 [s] = 8,25 [min]
tEvacuation M2 = 555,24 [s] = 9,25 [min]
tEvacuation M3 = 1.335,24 [s] = 22,25 [min]
Fire protection management is thus the factor with the greatest positive influence on the evacuation time. Accordingly, this influencing factor is usually the easiest to improve because it deals with guidelines, evacuation exercises and similar measures. It does not require modifications to the structure of the building or any upgrading of the active fire protection.
Possible alternative emergency routes
Buildings run by the European Union (including the ESO) are not subject to any building regulations in accordance with the Headquarters Agreement between the Federal Republic of Germany and the ESO because it is considered to be an extraterritorial area .
This also includes the construction site for the E-ELT in Chile. Alternative emergency routes, which do not pass through normal stairways or corridors, were defined as possibilities for evacuating the building.
In general, a differentiation was made here between stationary (permanently available) and semi-stationary (preinstalled and only used in the event of danger) emergency routes.
Due to the hemisphere-shaped, self-supporting form of the telescope dome, various solutions for providing the second emergency route were analysed. Not all of the details have yet been clarified due to the construction period of around ten years.
The two systems described below are conceivable according to the planning office: A viable solution for discussion is a spiral-shaped rescue tube (see Figure 3), which would be rolled out in the event of an emergency .
The most important key data for this solution are:
- Sliding speed of the person: around 2.10 m/s
- No feeling of falling
- Multiple users possible
- Rescue height 100 m
Another possible system for rescuing people within the dome is an abseil system:
In an emergency, the people evacuating the building will put on a safety harness and are then lowered by an automatic rope pulley installed within the dome. Without having to take any action themselves, these people will be steadily lowered to the base of the telescope on a rope. The rope will continue to lower them without letting them fall even if they have lost consciousness. A prerequisite for this type of solution is that only trained personnel are allowed to work at the telescope.
In order to provide the people evacuating the building with sufficient air to breathe, the flow behaviour of smoke from a fire in the large telescope plays a vital role.
The following questions were relevant in this context:
- What position on the telescope is most favourable for the extraction of smoke in the event of a fire when taking into account the wind direction?
- Do the partitions (flaps) on the building envelope of the telescope influence the behaviour of the smoke extraction?
- Is it possible to transfer the findings for E-ELT to other telescopes with different shaped buildings?
Following the experiences of recent years where fires already broke out at telescopes during their construction phases and resulted in fatalities in some cases, there has been a change towards the exclusive use of non-flammable building materials and insulation . The only flammable materials in current telescope construction projects are the cables, hydraulic oil and measurement instruments. The expected temperatures in the event of a fire are thus low, aside from the potential deflagration of hydraulic oil, which can, however, be avoided by taking appropriate care in the construction phase and using internals safety systems.
The above-mentioned questions were examined using a physical (see Figure 4) and a mathematical model. The most favourable flow behaviour was verified using a cold smoke experiment on a 1:100 scale model of the E-ELT and a 1:50 scale model of the VLT (Very Large Telescope). The telescope models were flooded with cold smoke and various wind directions were simulated. The resulting smoke extraction times in seconds show that every different building shape for a telescope needs to be considered separately (Figure 2). Open partitions in the dome always have a positive effect on the behaviour of the smoke extraction. The most favourable smoke extraction in the tests was found in the following wind directions:
- 45° and 90° at the E-ELT (partitions open or closed),
- 90° at the VLT (partitions open or closed).
As electrical systems may also fail depending on the location of the fire, an emergency generator with sufficient power output should be installed so that the telescope can be rotated towards the most favourable wind direction even if the mains power supply is lost. The control system should be linked to the fire detection system and already trigger the required positioning of the telescope as soon as the fire is detected. These findings were also supported by the mathematical model.
In order to guarantee the safety of people in large telescopes despite the difficult conditions, it is important to take into account,
- which emergency route the people evacuating the building will take,
- how long they need to reach safety and
- how it is possible to keep the emergency routes free of smoke for as long as possible.
In general, safety concepts for large telescopes in suburban areas without fire defences provided by local fire services deliver relevant findings and represent an interesting field of research in fire protection.
Claudius Hammann: Masters in engineering, head of preventative fire protection as a fire service officer in the fire department at TU Munich; responsible for standard and special constructions on the research campus at Garching; received his doctorate in engineering from the Nuclear Technology Department at TU Munich in the field of research looking at the reliability of safety technology.
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The article was published in FeuerTrutz International, issue 1.2018 (January 2018).
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