External fire-fighting in Midtbykvartalet – A feasibility study

The property developer E. C. Dahls Eiendom (ECDE) plans a building complex in a quarter in the city centre in Trondheim, the "Midtbykvartalet". The building will be enclosed by existing building blocks which to varying degrees hinder the fire service's access and efforts. Also, since the new building is intended for residential purposes, it will be necessary to install windows in fire rated walls against adjacent building. These factors result in deviations from a number of performance requirements in the guidelines to the regulations on technical requirements for construction works and there is therefore a need to find alternative solutions. It must be documented that these alternative solutions have at least as good an effect on fire safety as pre-accepted solutions would have. A potential side-effect of new, alternative solutions is that these can also, to some extent, protect the existing wooden buildings in the quarter.

The aim of this report has been to identify the state-of-the-art within active fire protection measures for external fire-fighting and to obtain an overview of existing solutions and manufacturers and to carry out an assessment of the potential of these solutions.

Risk scenarios

An overview of existing buildings in the Midtbykvartalet is presented as well as an overall description of the plans for development. Based on this, several scenarios have been identified to reveal the potential fire-spread hazard between the existing buildings and the planned building. Furthermore, a qualitative risk assessment has been carried out.

A literature study describes the state-of-the-art in water-based extinguishing systems for outdoor use. It deals with fixed extinguishers (facade sprinklers, water curtains), dynamic extinguishers, foam extinguishers, fire gels, as well as with sprinkler systems' effect and reliability. Furthermore, existing solutions (e.g. facade sprinklers, water curtains, water cannons and water mist turbines) have been surveyed, existing documentation described and assessed regarding suitability for use in the Midtbykvartalet.

From the identified scenarios, it appears that fires in existing buildings are more likely to spread to the new building than a fire from the new building to existing buildings. The greatest danger to the new building will be if a fire spreads in existing buildings, up through the roof, through windows or along the facade to the roof. In many cases, the fire service will have good access with their ladder trucks etc. to perform extinguishing efforts, at least in the early phase of the fire. But the risk of rapid internal fire spread, which may include several of the older buildings, can create a challenging situation for the fire service and a risk for the new building. In case of fire spread to the new building, the fire department will, due to the position and height of the building, have difficulty with aerial rescue and evacuation from the new building's higher floors.

Concept for the Midtbykvartalet

A combination of a static and a dynamic extinguishing system will provide the best balance between system robustness, extinguishing effect and flexibility for the Midtbykvartalet. Facade sprinklers are considered the most suitable static system solution. Facade sprinklers will primarily cool the facade of the new building and absorb heat radiation from a potential fire in the existing buildings, but will not be suitable for extinguishing or actively fighting a fire within the existing buildings. The design and planning of facade sprinklers shall take the design of windows, balconies and roof terraces into account, which have been identified as vulnerable points in the firewalls.

Dynamic systems such as water cannons and water mist turbines can be used to cool facades and to actively fight a fire over relatively long distances. In addition, such systems can be established so that the fire service can take over control of the extinguishing system as needed. This property is important, because of the height of the new building and because of how it is surrounded by existing buildings.

Both water cannons and water mist turbines can be combined with an automatic control that allows you to fight a fire at an early stage, even before the arrival of the fire service, as long as early detection is achieved. A fire that spreads within the existing buildings will not be possible to extinguish with permanently installed dynamic systems. Therefore, the cooling and extinguishing effect of such systems must be evaluated based on the scenario of a large fire in the neighbouring building, which has not been done before.

The cooling and extinguishing effects for both systems are largely dependent on their control system. The control system must be capable of aiming the water cannon or the water mist turbine at the fire, if required compensating for wind effects and selecting an appropriate water jet mode. Therefore, in order to adapt the control system to the Midtbykvartalet, it is necessary to quantify the cooling and extinguishing effect of such a system in advance and with regard to a potentially large fire in the adjacent existing buildings.

Selected water mist turbines have the option of operating in a full jet mode, like a water cannon. Therefore, such systems are considered more flexible than water cannons. However, water mist turbines set large amounts of air in motion and generate turbulence that can affect the fire. It is therefore important to investigate if and in which cases this can aggravate the fire and have a negative effect on other areas in the quarter.