Computational Egress Modeling in the Covid-19 World

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David Stacy, P.E.


July 14, 2021

Fire protection engineers often utilize computational egress modeling tools in order to assess occupant movement and expected locations throughout a space, specifically when performing an available safe egress time (ASET)/required safe egress time (RSET) analysis.  In this type of approach, smoke control systems and general tenability in a building can be assessed by comparing the ASET (an output from a computational fire model) to the RSET (an output from the egress modeling).  Under these analyses, we can demonstrate that occupants can safely egress from a building, or portion thereof, prior to untenable conditions developing; this is typically assessed on the pass/fail performance metrics of visibility, temperature, and toxic gas concentrations.

Model Rendering of an ASET/RSET Analysis Utilizing FDS and Pathfinder

We are now faced with adapting to and battling the global spread of the SARS-CoV-2 virus and the associated Covid-19.  With waves of these cases remaining unpredictable, building owners, facility managers, and event planning personnel are tasked with the significant operational goal of “how can we safely begin to re-occupy our spaces”.  The egress modeling software we utilize at Performance Based Fire Protection Engineering, PLLC, Pathfinder created by Thunderhead Engineering, has historically been successfully utilized in understanding occupant movement from simple office buildings to large assembly spaces such are arenas, under both emergency and non-emergency conditions.  Working with design professionals, they have identified that this software also needs to be tailored to include tools, quickly vetted, in order to help understand social distancing and proposed movements throughout a building.

Pathfinder now includes specific commands and data outputs that will help us understand how occupants would be expected to behave with one another in a building, following proposed social distancing standards.  By making small changes to the occupiable area, such as one-way flows, dedicated ingress and egress points, and spatial reconfiguration, we can evaluate how each of these design changes impacts the expected exposure to occupants and their general movement throughout the building.  Some of the tools now included incorporate the following diagnostics:

Output Data for Social Distancing

By utilizing a parameter indicating a proposed social distancing constant (i.e. 6 feet), we can be provided with numerical data that will indicate which occupants experience conditions in excess of this threshold, how many times they fail to maintain the social distancing, and their total exposure time (accumulated exposure) throughout the simulation time.

Graphical Depiction of Social/Physical Radius

In addition to numerical output data (graphs), Pathfinder provides a visual representation of the occupant movements throughout a space.  With the new social distancing features, a distancing “ring” will be shown around each occupant.  This ring is effective in yielding the next types of results, described below under Occupant Contours.

New Occupant Contours to Quantify Exposure

Two new additional visual outputs are provided in our assessment, utilized to help evaluate physical distance concerns between occupants; these include “social linkage” and “social usage”.  Through utilizing the social linkage output, the defined desired radius around an occupant is included in the model, and the number of other occupants/agents who are within this radius are accounted for.  Social usage is similar to the linkage, however in this case for every location on the walkable mesh itself (not tied to a specific occupant), the number of agents within a defined radius are calculated and assigned to that mesh point.  This can be utilized to understand “hot-spots” in a building that should be re-evaluated further.

Social Usage Contours Displayed in a Proposed Occupant Movement Plan.

Our egress modeling tools have been updated to account for the new need in the A/E field, addressing social distancing and general occupant movement in order for tenants, building owners, and facility operators, to determine how they can more safely and effectively re-open.  Thunderhead Engineering’s Pathfinder software can be utilized by fire protection engineers to evaluate specific buildings under varied occupant loading, movement arrangements, and guided directions, as well as the effect of staggered entry and temperature check-points, and other egress arrangements associated with the complicated development of a reopening plan for a facility.

Although an egress model may not necessarily be required in order for you or your client to get their building back open, it surely can be used to validate your proposed approach and have an understanding of the expected impact that your plan may have. Furthermore, providing a visualization to a landlord, AHJ, or tenant can assist you in getting back up and running as quickly as possible.

By engaging a fire protection engineer to perform this analysis, we also are able to bring in our expertise in life safety code consulting.  It is not uncommon during these times to notice in a mercantile or assembly occupancy exits blocked by merchandise or obstructions, in an effort to direct occupants to perform certain movements.  The intentions are good, but scenarios such as these as well as elongated one-way travel induced by temporary partition walls or similar effects can have a significant impact to life safety during a fire event or other emergency.  It is important for your re-opening plans to consider life safety and building code requirements in order to provide a holistic approach to this plan.

If you would like more information on how this modeling approach can assist you or one of your clients, please reach out to Performance Based Fire Protection Engineering, PLLC by contacting Principal & Founder David Stacy at  We would be glad to see if this type of approach would prove beneficial to you as well as walk-you-through various case studies. Below is a representation of one such representative model, provided on our YouTube page.