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Author Topic: SFPE Mode V Steering Mode  (Read 4933 times)


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SFPE Mode V Steering Mode
« on: September 27, 2016, 02:47:26 pm »

This is probably a frequently discussed issue so apologies if people are tired reading about it. I have done a bit of research and I am aware of the basics of it but I am still unsure about the whole matter. I am relatively new to Pathfinder, I love using it and want to get things right so I appreciate any comments that may have been addressed elsewhere. Links to other discussions are welcome as well as most I have found are quite old and possibly dated by now.

Is there a time to use SFPE and a time to use Steering mode?

How do you decide?

I am trying to find the RSET from 2 buildings at the minute and have almost got my models set up. One is a stand in a football stadium, relatively simple, 4500 occupants, 8 exits all at the top of the stand. I ran it on steering mode and was getting a time of 18 minutes. This is not fully accurate as I introduced 2 mid height exits to see the effect and it slowed things down but this is the model I hit run on the SFPE mode so for the purpose of this discussion I will use it. The SFPE mode result finished at 3600 seconds with 62 people still to evacuate. All I did was change the mode, all other parameters were exactly the same. I had run a small sample simulation with 560 occupants exiting from 2 rooms out a single exit and I got a 140 second difference in the results. I was surprised at this difference but the 42 minute difference in the stadium evac was quite alarming. The actual evacuation after a game was recorded at 9 or 10 minutes.

My 2nd building is a shopping mall with 13,000+ occupants so as you can imagine I am not too keen to use SFPE mode on this as I think it will completely blow our fire design based on the stadium time.

Am I wrong in thinking I can just switch modes? Do you have to make a number of adjustments to use SFPE mode and get a realistic answer?

If I use steering mode what are my justifications to the AHJ for this choice and not using SFPE mode?

Is there a wrong choice for a certain situation/simulation?




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Re: SFPE Mode V Steering Mode
« Reply #1 on: October 02, 2016, 10:27:29 am »

So SFPE mode and Steering mode are very different ways to calculate the time to egress from a building.  If you look into the SFPE Handbook, they developed a hydraulic model for egress.  Basically, this hydraulic model says it takes X time to move from your location to a door, Y time per person to move through that door, etc.  In this model it will move all occupants to an exit and allow multiple persons to occupy the same space while waiting to egress through the egress component.  From that point it will utilize a rate to determine how many people can move through that egress component per time.

The steering model is (I believe...) a Thunderhead developed algorithm that accounts for space of persons.  If you run both models and look at the results, you will see that the Steering model results in bunching and more realistic movement while the SFPE model allows the people to move in a straight line and occupy the same space.

It is your decision to determine which model is best.  Obviously you need to be conservative and defend this position either way.

Charlie Thornton

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Re: SFPE Mode V Steering Mode
« Reply #2 on: October 17, 2016, 11:40:14 am »

The hope is that SFPE mode and Steering mode will give similar times. Running both on the same model offers an opportunity to understand what happens under the two completely different models and there is usually something to learn when the answers are drasticly different. When SFPE mode runs slow, it's usually because a room has become extremely crowded and the resulting hydraulic flow equations threaten to go to zero for occupant velocity. There are two settings in Pathfinder that impact this:

Simulation Parameters > Behavior > "Behavior Mode" = SFPE > Max Room Density

This parameter will stop SFPE occupants from entering a room if doing so would cause the density in the room to rise above the given value (default = 1.88 pers/m2). This default is basically the point of diminishing returns for the density-velocity function and sticking to this default will prevent room filling from bogging down a simulation. It does not protect you from rooms that are initially very crowded.

Simulation Parameters > Behavior > "Behavior Mode" = SFPE > Minimum Speed Fraction

This parameter is the floor that prevents the density-velocity function from going to zero if you've used a non-default value for Max Room Density or overfilled a room in the initial condition.

To understand why SFPE mode is running slow, the best quantity to understand is room density. You may be able to see this in the 3D results or it may be helpful to run the debug viewer (Debug Simulation rather than Run Simulation) which automatically changes room color based on room density. This video shows the color changes in debug view as the room density changes:


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Re: SFPE Mode V Steering Mode
« Reply #3 on: October 18, 2016, 08:14:37 am »

Would it be possible to send us the two models so that we could understand what is happening? If you do not want to share publicly, please send directly to

Dan Swenson
Thunderhead Engineering