Challenges in MODERN Fire Safety Design And Fire Fighting Operation

Introduction

As buildings evolve over times, there are new challenges faced in fire safety design. Some of the major changes in modern buildings are as follows:-

·??????The type of furniture.

·??????The size of modern rooms which are either smaller in foot print or are transformed into open area concept such as to an office and home.

·??????Effects of fuel load on the sprinkler design.

·??????Effect of sprinklers on the smoke control system.

·??????Buildings with “green building” treatment.

·??????Security issues versus prompt and safe fire evacuation, etc.

·??????Timber building construction.

Each of the above will be discussed in detailed with its respective challenges. In order for a fire engineer to prescribe a fire safety plan and design, the engineer must understand the challenges faced and be equipped with solutions that will ensure a safe building in terms of fire safety design and operation. Otherwise, the engineered solutions will fall short and may not even work properly in the actual fire scenario.

Furniture

Modern furniture are made up of modern composite and synthetic materials such as fibreglass, plastic, synthetic foams, engineered timber as compared to the good old timber construction of legacy furniture. The burning rate and rate of heat released is different for modern furniture, thus the nature of the fire growth is affected. The old legacy furniture has growth rate of slow to medium while the upholstered furniture now has an ultra fast growth rate.

Figure 1

The heat release rate is a very important parameter that the engineer needs to establish especially if he is carrying out performance based design where he is required to input the design fire or when he is determining the fire size for input to the design a of smoke control system. The rate of heat release rate together with room dimensions will affect system performance such as the time taken for activation of the sprinkler, heat detectors, etc. The fire size is considered constant once the sprinkler is activated.

Modern Room Dimensions And Adaptation to Fire Fighting Operations

Room dimensions for modern buildings are going in two opposite directions from the olden days:-

1.????Room size is smaller as space is now getting more expensive. And more furniture and materials are packed into a smaller room. The fire behaviour changes from fuel controlled fire to ventilation controlled fire. Thus, the oxygen to support combustion runs out before the fuel does. Once this happens, the flaming fire dies down and the entire space is filled with hot smoke particles which can be combustible once it receives fresh oxygen supply. This normally happens when window breaks due to heat or when the fire fighters introduce some ventilation to get rid of smoke which in turn also introduces fresh air into the space or just simple opening of doors by fire fighters at the wrong location. This phenomena is called backdraught and can be very dangerous for fire fighters as the fire burns back to the source of oxygen such as door opening.?

It is good to keep in mind of ventilation limit fire when carrying out fire fighting operations in order to be safe such that venting is only provided when the water is ready to be applied to the fire. The effects on wind on the ventilation effect is also very important. In fact, if there is no life at risk, it may be best to fight fire just from outside the building without carrying out any ventilation.

In addition, it will also affect the way fire investigations are carried out as the worst damaged spot may not be the fire origin anymore if the fire is ventilation limited.

Secondly, for small room with limited height, it is easier to reach flash over when everything inside the room reaches the auto ignition temperature at the same time. Thus the importance of passive fire fighting components such as fire doors and fire walls cannot be overly emphasised to contain the fire to the room of origin, pending the attendance of fire fighters. Thus when approaching small rooms, fire fighters should feel the heat on the door leaves and if hot, be prepared with fog spray before opening the door.

Figure 2

1.????At the same time, some spaces are larger and higher as the open area concept is adopted for offices, malls and even homes. Such big spaces will have more combustible fuels and the lack of passive fire fighting components such as doors and walls will mean greater chances of fire spreading in the largely available space. For some, the prescribed maximum space in terms of area and volume specified in building codes may be exceeded and thus performance based fire design need to be carried out which may involve introducing smoke control system and active fire fighting devices which are automatic to detect and contain the initial fire spread. Care must be taken also in terms of false alarm due to the selected fire detection devices which may be too sensitive or prescribed for the wrong room environment, even worse if the selected fire detection devices are unable to detect the fire.

2.????For high ceiling areas, challenges that are faced are related to automatic fire extinguishment, especially sprinklers. Sprinklers are seldom able to activate beyond 15m for prescribed design fire size unless the fire has grown to be very big as illustrated below.

High ceiling fire protection system.

It cannot be overly emphasized that sprinklers cannot be used in areas with high ceiling. To activate a sprinkler point, it would require a very high heat release rate which exceeds the normal design parameters. The following table is simulated using Certified Fire Investigator (CFI trainer) calculator for the time taken to activate a sprinkler point for different ceiling height and input heat release rate. The calculation is based on standard response sprinkler of 68 deg C activation temperature and K=80. The input heat release rate is based on the prescribed design fire from Guide to Fire Protection In Malaysia.

As can be seen for high ceiling area where ceiling jet temperature does actually exceed the sprinkler activation temperature, the response time can be shortened by using a quick response sprinkler with half the Response Time Index (RTI) value. This is also commonly adopted in storage rack design.

When sprinkler system could not be used, the option would be to use a fire detection system that can be activated for high ceiling installation such as beam detector installed at wall and install a deluge system which is activated by the fire detection system. Another method would be to install a water monitor activated by a built in flame detector, which starts to see its usages in high ceiling auditoriums and stage in many other countries. In Malaysia, we have started to implement water monitor systems in a few large auditoriums but the technology is still not widely practised yet.

Figure 4

Effect of Fuel Load on Sprinker Design

In accordance to BS EN12845, the designed sprinkler flow density with standard spacing follows the prescribed hazard whether the space is classified as light hazard or ordinary hazard or special hazard with reference to the type of usage and space dimension. However, in high ceiling areas, the spacing of sprinkler may need to be adjusted from the prescribed design spacing to ensure activation as per Figure 3.

As a point of interest, the potential fire size of different type of buildings may have increased over the time due to additional fuel load from modern furniture and thus the occupancy hazard prescribed in standards may no longer be applicable as a higher water flow rate and closer sprinkler spacing is required to control a bigger size fire. Therefore, as a whole, it affects the performance of the sprinkler system in extinguishing fire.

Effects of Sprinkler on Smoke Control System

With the ventilation limited fire in mind, it should be noted that the implementation of sprinkler which discharges at the initial stage of a fire to douse out the fire and the subsequent activation of a smoke fan after some time delay would be complimentary. ?In this case, the smoke fan helps to disperse out the smoke products without actually increasing the fire size.

In terms of design, the question we need to ask is whether smoke extraction or fresh air pressurisation should be introduced when there is no sprinkler in the building. If we study the design standard carefully, smoke control system can only be implemented if the design fire is limited in sprinklered buildings. But nothing is being mentioned on the sprinkler system when implementing smoke control via dilution and purging with fresh air. It is noted that fire size is not used to calculate the capacity of the smoke purging fans. However, we should note that smoke purging fans purpose are not for evacuation safety, rather it is for fire fighters to enter the building for mop up operation once the fire is controlled by sprinkler or other means.

Figure 5

·??????Pressurisation as smoke control

We may also need to consider the location of the fire. When a fire originates in room connected to protected corridor, we need to consider whether it would be advisable to install pressurisation fan in the corridor area. The pressurisation fan will provide positive pressure to prevent smoke from entering the corridor from the room of the fire origin which is good. However, some fresh air will eventually flow underneath the door to feed the fire (until the room pressure equalises with the corridor pressure due to pressure developed by the fire products), which means that the fire will grow initially inside the room or origin. ?Another scenario would be that it provides free flow of oxygen when someone leaves the room door opened during fire and the door fails to close automatically. Thus, it is important for the designer that the gap around the doors remain small and the fire doors are closed all the time for the pressurisation design to work.

·??????Smoke extraction system

On the other hand, with a smoke extraction system in the corridor, the system will first extract air from corridor and when the pressure is too low in the corridor, some air/smoke will travel from connecting rooms to the corridor, making the room positive pressure relative to the corridor. The system is intended to keep the smoke layer above the evacuation routes. This will be like the smoke extract in the mall corridor.

Finally, we need to consider what happens if the fire originates from the protected corridors which seldom happens unless the tenants keep combustible materials in the corridor space. This will be the worst scenario if fresh air pressurisation is applied while if smoke exhaust is applied, it will take some time for fresh air from the room to infiltrate the corridor.

All in all, it is recommended that the sprinkler system to be installed where there is intention to apply mechanical ventilation during a fire.?

Green Building Treatment And Facade Fire.

In the past, it is seldom required for consideration of external building fire as there is lack of combustible materials on the external of a building that can sustain a fire. Most walls are just double masonary walls with good fire rating and a cavity in between. In addition, there was a policy to separate openings (windows, louvres, etc) of different height by a certain vertical or horizontal wall projection. This is not so in modern buildings with heavy claddings and balconies projection which can be occupied with combustible furnitures. The claddings are normally insulated to improve the energy efficiency of the building in terms of heating and cooling requirements.

Figure 6

Figure 7

As from figure 7 above, both scenarios for spread of fire can occur concurrently:-

1.????Fire spread externally on the combustible external cladding. The origin of the fire could be inside the building and the fire breaks through the window and ignites the cladding.

2.????Fire spreading from claddings back into the building via windows surrounded by the combustible external cladding. This happens when the cladding is placed adjacent to all the windows in vertically aligned arrangement.

The second scenario is especially dangerous in terms of fire safety of the building and it could be prevented by using non combustible external cladding/ green materials or by the installation of sprinkler system inside the building. Where there is budget constraint of installing non combustible green materials, a fire stop or break could be installed say every 2 floors comprising of non combustible green materials such as rockwool. On the other hand, certain parties may recommend the installation of horizontal fire stop projection to prevent the fire from spreading up along the facade structure. However this is subjected to correct design and installation at site in terms of construction and distance apart of horizontal stop.?

However, if the fire were to start from the outside where there are balconies and furnitures, the installation of sprinkler would not assist as high wind speed at top floor will prevent the sprinkler water droplets from reaching the fire fuel.

As such, the best solution is to refrain from the installation of external wall claddings (especially on high rise building) that do not pass the flame spread, drop test, smoke test, combustibility test, etc or full scale as prescribed by BS8414 or NFPA 285 or equivalent ISO standard. Care must be taken to which standard the test has been carried out. Most commonly adopted tests standards in Europe are BS EN ISO 1182 for non combustibility test, EN ISO 1716 for gross calorific potential, EN 13823 for single burning item test, EN ISO 11925-2 for ignitability test. The materials are then classified in accordance to BS EN 13501 in accordance to performance criterias to the test standards.

Security issues versus prompt and safe fire evacuation

Modern buildings are equipped with the tightest level of security control. CCTVs are installed at common areas and high security areas without encroaching into the privacy of building tenants. At the same time, access to the building is controlled and limited to authorized personnel and may vary in accordance to the operating hours of different sections of the building. At the same time, when there is fire, it is a requirement that everyone in the building can escape in an efficient and timely manner (except for high security areas such as prisons).?This is now increasingly challenging in terms of the design of the building access control system whether the fire signal should trigger all electronic locks to be opened simultaneously or allow the tenants inside building to gain access by manually activating break glasses. It is not uncommon to watch in television that criminals create fire alarms to gain access into the building.

Timber highrise construction

Timber highrise construction is the latest challenge in the fire safety industry. Buildings as high as 25 storeys are being built with timber as the main structures of the building. The present approved document B code in UK cannot be used to address the timber structures which need to be assessed through other means. Timber structures can add to fire load of the building unlike steel structures and concrete/ masonary walls and floors. The time temperature curve applied to furnace to test the fire performance of the load bearing materials need to be considered carefully as the furnace heat input needs to be reduced to maintain the time temperature fire curve of the test standards due to heat added by timber constructions. Whether how valid this method of test is, will need to be evaluated. In another challenge, would be the fire safety of the building during construction as timber poses a higher hazard in?the building unfinished state as compared to traditional structure materials.

Figure 8

Conclusion

As modern buildings evolve in terms of size, dimension, building materials, security implementation, there would be challenges in fire safety design. The fire safety design of modern buildings should take into account the fire dynamics, the limitation of prescriptive fire design solutions, the implementation of performance base approach where prescriptive solution could not be carried out, etc. When facing challenges in fire safety design, it is always best to fall back on fire chemistry and fire dynamics as the basis and check point to ensure that the fire solutions implemented are practical and cost effective.

References:-

1.????“Guide to Fire Protection In Malaysia” , 2000 edition Page 225

2.????BS 5306 Part 2, 1999 edition, Table 3 page 15, Table 4 page 16

3.????NFPA 92 Standard For Smoke Control

4.????Grenfell Fire- Wikipedia

5.????Lawrence J. Machetti, Fire Dynamic Series- Fire Protection Fundamentals, Page B5, Figure B.1.3

The writer

Ir. Tan Yoke Lee graduated from University of Melbourne, Australia with a degree in Mechanical Engineering in 1995. She had 10 years of working experience in construction companies before she joined the consulting engineer Jurutera Perunding Intelek Sdn Bhd from 2007 until December 2021. She was the head of engineering department in the past company and is a member of both IEM and IFE UK. She has given numerous talks in fire and ACMV to IEM members for CPD points and also to schools such as?polyteknik KK, UMS and UCSI.