Retail buildings are highly visible to the public and as such, architects are tasked with creating aesthetically appealing spaces to entice occupants to enter and shop. Fire engineers have a tough time working on these sorts of projects given the objective to balance life safety levels without inhibiting the above innovative architectural design goal.
A typical design approach by complying with the Deemed to Satisfy (DtS) provisions will often impose restrictions to architectural design which is not palatable. It then falls upon the fire engineer to develop Performance Solutions and strike that desired balance.
This post lists out common Performance Solutions which we've encountered on retail developments and could serve as a starting point for your next retail project. Reach out to us to discuss how we can add value to your retail project.
For common Performance Solutions implemented on warehouse developments, see our previous post.
Fire Resistance Levels (FRLs) on Structure
Background
Buildings under the NCC are broadly classified under Type A, B or C (most onerous to least) which dictates the minimum levels of fire resistance required on structural elements.
In developments using concrete for structure, this is generally not a major issue as the thickness of the concrete can be controlled to achieve the desired FRL (at financial cost). Steel structure buildings on the other hand are not as easy to design for FRLs and in most cases, some level of fire resistant coating is provided on the structure to achieve the required FRL (which can be unsightly).
In shopping centres with exposed structure as part of architectural design, the application of intumescent paint or Vermiculite may not be an option. This is where our fire engineering team come in.
Performance Solution
To permit reduction in FRLs
The basics behind the reduction of FRLs lies in demonstrating that a fire would not likely create a scenario where the temperature or exposure heat can degrade the structure to the point of failure.
There are a number of ways to approach this. All of which are beyond the technical intent of this post and won't be discussed here. If it is something that is of interest to you and your project, please contact us directly.
Egress Travel Distances
Background
Class 6 retail buildings follow similar rules for egress travel distances as warehouses with an additional detail as shown below based on NCC 2019 Amendment 1.
Up to 20 metres travel to an exit or a point where travel to a choice of two different exits is possible within a total of 40 metres travel.
Distance to a single exit serving the a storey at the level of access to a road or open space can be up to 30 metres.
Some shopping centres make this very hard to achieve. Furthermore, introducing horizontal exits via fire walls and compartments is often not viable as it would have an adverse impact on internal visibility and also flow of occupants.
The positive part is that justification of this Performance Solution is all too common for fire engineers.
Performance Solution
To permit extended egress travel distances
Computational Fluid Dynamics (CFD) simulations are critical in the justification of this Performance Solution. The passage of fire and smoke within the building is compared to simulations of occupant evacuation. The Available Safe Egress Time (ASET) i.e. the time until untenable conditions are present for occupants is compared to the Required Safe Egress Time (RSET) i.e. the evacuation time. The below forms the basis of the comparison:
ASET > RSET x Safety Factor
Shopping centres are generally quite vast in size and as such, the CFD simulations are resource intensive. It therefore pays to get things right the first time! Tweaking and playing with models may be necessary to determine the correct combination of fire safety provisions to meet the above criteria. Good fire engineers will have the intuition to get within the ballpark on the first run.
Exit Separation Distances
Background
Distances between alternative exits within Class 6 buildings must be between 9-60 metres with no part of their respective exit paths converging to less than 6 metres apart. This is governed by NCC 2019 Amendment 1 clause D1.5.
Shopping centres are generally guilty of breaching the upper limit of the distance however, with some clever fire engineering, this solution can be justified in the same manner as extended travel distances.
Performance Solution
To permit extended exit separation distances
Exit Widths
Background
Exits and paths to them must meet minimum width requirements to reduce the risk of occupant queuing/slowing of evacuation. It's not surprising though that in retail settings there would be sections which are narrower such as back of house/stock areas. The Performance Solution that fire engineers must therefore justify is:
Performance Solution
To permit reduction in minimum exit width dimensions
These areas are typically restricted to staff only access and as such, the density of persons is lower in those areas in addition to the occupants present in those areas being familiar with exit locations and exit paths. These are generally the crucial elements to this argument.
Interconnection of Multiple Storeys
Background
Shopping centres are often big structures comprising multiple storeys. Designers take advantage of this grand-ness to the general public via atriums and voids connecting multiple storeys. Unfortunately, the NCC 2019 Amendment 1 is not kind to buildings with these features as it is effectively a hole in the floor for fire and smoke to migrate to multiple levels within a building.
Part G3 of the NCC covers atrium construction and can be restrictive and similarly, NCC clause D1.12 allows only 3 storeys to be interconnected and that is in a building with automatic fire sprinklers.
Performance Solution
To permit interconnection of more than 3 storeys
Fire engineers take advantage of CFD modelling techniques and mechanical system operation in fire mode to prevent fire and smoke migration between multiple storeys. By maintaining a pressure differential across two storeys, the flow of smoke and heat can be controlled. This is the basic principle behind zone pressurisation.
Extended Hydrant Hose Length Coverage
Background
Hydrant hose length coverage is almost always a problem in retail settings as hydrants are tucked away in corners outside and reach with a 60-metre hose will unlikely reach all points in common malls and not to mention the larger tenancies (supermarkets).
Although it's a tough sell to the fire brigade for brand new sites, it's not to say it's impossible for a good fire engineer.
Performance Solution
To permit use of a third hydrant hose length for full coverage
Use of a third hydrant hose length requires additional weight to be borne by the fire brigade for set up and firefighting, the Performance Solution must demonstrate, normally by a Fire Brigade Intervention Model (FBIM), that set up and fire attack times are reasonable. This is ultimately at the discretion of the fire brigade.
Omission of Hydrant Radiant Heat Barriers
Background
Risk of firefighting personnel to heat exposure from a burning building is normally addressed within Australian Standard 2419.1-2005 by requiring radiant heat barriers of FRL 90/90/90 to hydrants located within 10 metres of a building they serve.
Although with concrete/masonry construction this can be achieved, there are feature walls of glass or other non fire-rated materials which cannot achieve the right FRL.
Performance Solution
To permit omission of required radiant heat barriers protecting external hydrants
This is a textbook justification most of the time for fire engineers when sprinklers are installed within the building especially since new versions of Australian Standard 2419.1 are on the horizon which build in these concessions as a Deemed to Satisfy clause.
In the near future, this Performance Solution may be redundant.
Compromised Fire Hose Reel Coverage
Background
Let's face it: Fire hose reels will never be a visually appealing piece of equipment. On top of that, placing fire hose reels within the building can cause obstruction and tripping hazards. It is therefore up to the fire engineer to work through some create solutions to rationalise the location and quantity of these eyesores.
Performance Solution
To permit the provision of supplementary portable fire extinguishers in lieu of full fire hose reel coverage
The fire brigade often don't care for fire hose reels since they don't use them in a fire scenario. They're exclusively for occupant use in first aid firefighting. As such, the successful justification of this Performance Solution hinges on the fire engineer demonstrating that without hose reels, an equivalent form of firefighting is available for occupants. This is often via supplementary water fire extinguishers.
Rationalisation of Smoke Exhaust Provisions
Background
Class 6 buildings are unfortunately hit with a requirement under the NCC 2019 Amendment 1 to segregate open spaces in excess of 2000 m2 floor area or 60-metre long malls at the ceiling level with unsightly baffles. This aims to limit the migration of smoke away from the fire source in vast retail spaces.
The NCC also specifies minimum smoke exhaust quantities to be used. This may not always result in an optimised system as the inputs are building class, sprinkler protection status and smoke layer height. No mention of architectural features or other fire safety mechanisms.
Your trusty fire engineering consultant can help with this Performance Solution.
Performance Solution
To permit reduction in smoke exhaust quantities and omit a required smoke baffle
You guessed it. CFD modelling is used here again to demonstrate that the use of sequentially operating smoke exhaust fans in lieu of baffles results in effective smoke migration mitigation. Furthermore, rationalisation of smoke exhaust quantities hinges on the ASET/RSET analysis described above for egress related Performance Solutions.
Conclusion
Retail spaces are not like warehouses in the sense that fire safety measures can be more readily implemented on the latter without concern for aesthetic impact. Fire engineering Performance Solutions on retail projects is almost as much of an art form as the architectural design itself (don't tell architects we said that!).
Get in touch with us on your next retail project and experience the outcomes we can achieve for you.