Evacuation Modelling in Design Jeremy Gardner Jeremy Gardner Associates
Evacuation Modelling in Design Fire Engineers view Conventional Codes Fire engineering Key Issues for Fire Engineer Which model Cost CAD Occupant numbers Pre – movement time Exit choice Management Disabled Simple Case Study How useful is it for building design?
Code Approach to Means of Escape Turn your back on the fire and walk to a place of safety Exits sized on a notional 2.5 mins Occupancy type, fire load etc not considered for exit sizing No target for total evac time Works well for most simple buildings Not the most efficient approach for large or complex buildings.
Post War Building Studies Empire Palace Theatre Fire 1911 Audience escaped safely 2.5 min notional escape time
Fire Engineering Fire engineers role Performance based regulation in UK and Ireland ADB, Technical Handbook, TGD B etc are guidance only Alternative is a fire engineering solution A fully fire engineered approach to the whole building is rare Most fire engineering studies are concerned with a limited number of departures from the code The fire engineer prepares a fire strategy - the equivalent of the Code i.e. requirements for Means of escape Structural fire resistance and Compartmentation Facilities for fire fighting Sprinklers, fire alarms, smoke control etc Solutions are specific to each building and should result in more design flexibility, a more efficient and safer building.
Time to Untenable Conditions Compared With Time Needed For Escape
Evacuation Modelling in Design Which Model Movement only models Behavioural models Risk models NIST report 2005 reviewed 30 models Technical support Cost Biggest cost is the time needed to become reasonably competent in using them Importing CAD drawings Should be much quicker and potentially fewer mistakes Difficult to import the architects drawing directly Trace over the architects CAD plan and then import the traced plan as a DXF file.
Occupant Numbers Floor space factors can overestimate Offices Code - 6 m 2 per person SFPE Handbook 25 m 2 per person – surveys in 1970s Workstation count is 15% overestimate RICS Survey – average 16.3 m 2 per person.
Occupant Numbers Shopping Centre Occupancy Study Shops 4 m 2 /person, Malls 0.7 m 2 / person
The time to begin escaping can be longer than the time taken to escape Complex, many variables and not well understood Data is very limited Key factors: Awake or asleep, familiar or unfamiliar, building type, type of alarm, building complexity and management actions BS 7974 PD6 contains some useful information e.g. Office – first few people in 1min Retail – first few in 15min if not well managed.
Exit Choice Do the public use emergency only exit corridors?
Exit Choice Behaviour Security considerations often mean that fire exit routes are not permitted for public use unless there is an emergency In many public buildings emergency only routes may not be used at all, or only as a last resort In a very well managed building the staff will encourage use of emergency exits Exit choice assumptions will have a major impact on the modelling results.
Exit Choice Behaviour Evacuation models mostly assume the nearest exit is used or, alternatively a user defined exit Additional factors in some models include Congestion at exits Preference to use familiar routes Signage visibility.
Management Motivation to Escape Sounder Voice alarm Smoke visible and flames visible Management staff action Smoke and / or fire seriously affects occupants.
Management Motivation to Escape What happens if occupants become highly motivated to escape? Personal space invaded as density increases Flowrate through exits increase If exits can’t cope with peak flow rates then density increases, speed slows, flow rate through exits reduce Risk of crush and blockage of escape routes Panic is not a helpful term, but competitive rushing does occur even in non fire or emergency situations e.g. Ikea and Primark store openings.
Management Management is possibly the most important factor of all In the past concerns about causing panic may have caused management to delay the evacuation Can have a major impact on pre-movement time and the use of emergency exits rather than just familiar routes The management approach to evacuation of the disabled will also have an impact on evacuation time.
Disabled Changes to legislation to improve disabled access to buildings means disabled evacuation needs more careful consideration than it received in the past Models can cater for this, but the user may need to consider this in detail University of Ulster research.
How Useful For Building Design? Some Examples Impact of Fire Fighting operations on Phased Evacuation Comparison of 3 large stairs with 4 smaller stairs Alternative shopping centre mall escape corridors Office density at 10 m 2 / person or 16.3 m 2 / person Phased Evacuation - evacuate the fire floor and floor above or just the fire floor?
Example Phased Evacuation Office Building ADB Guidance changed in April 07 Previously - initially evacuate the fire floor and floor above New guidance – initially evacuate the fire floor only Computer simulations to compare both methods.
Example Phased Evacuation Office Building Typical office building Net floor area per storey 2500 m 2 3 stairs 1200mm wide Modelling using Simulex No pre-movement time 10 m 2 / person Simple model but ok for comparison.
Example Phased Evacuation Office Building Evacuation time from fire floor Fire floor and floor above: 2 min 55 secs Fire floor only: 2 min 12 secs Supports change in ADB guidance.
How Useful For Building Design? What is the alternative? The only practical way to estimate travel, queuing and flow times through exits except for very simple buildings More confidence in results for buildings where staff actions, exit choice, pre-movement time are more straightforward e.g. office buildings In more complex situations it is essential to model a range of different assumptions and this will give a wide range of evacuation times Assumptions are less important for a comparison of alternative approaches.
How Useful For Building Design? Why Don’t We Use Evacuation Modelling More Often? Assumptions that users are uncertain about can determine the result Occupant numbers Pre-movement time Use of unfamiliar exits Staff actions Escape speed of disabled occupants Fire engineers are more comfortable with fire physics e.g. smoke modelling than human behaviour No target time for building evacuation Can pose more questions than it answers Building control and the Fire Authority acceptance Using worst case assumptions, even the safest code compliant buildings can be shown to be dangerous Can take a lot of time.
Evacuation Modelling in Design Current code approach to escape route design is very simplistic Codes should be changed to encourage Designers and Management to find ways to use emergency exits as normal circulation routes More research is needed e.g. Pre-movement time Exit choice Modelling allows more detailed consideration of ASET vs RSET Evacuation modelling is a useful tool for fire engineers with great potential for the future.