A Review of Building Evacuation Models - NIST Virtual Library

A.5 STEPS
Developer: Mott MacDonald, UK
Purpose of the model: The purpose of this model is to simulate occupants in a normal or
emergency situation within different types of buildings, such as stadiums or office buildings 14-20 .
Availability to the public for use: The model is available for use by the end user from Mott
McDonald.
Modeling method: This is a movement/partial behavioral model. It contains pre-movement
abilities, occupant characteristics, patience factor, and family behavior.
Structure of model: This is a fine network system made up of a series of grid cells, in which
only one occupant can occupy each cell. The default grid cell size is 0.5m by 0.5 m. Another
“fine grid” option is available where more than one person can occupy a grid cell, but this option
is still in test mode.
Perspective of model: The model views the occupants individually and allows the user to give
individual traits to each person or groups of people in the simulation. The occupants also have
an individual view of the building, because the user can specify each occupant’s (or group’s)
“target” or checkpoint (exit), allowing for the user to aid in the mapping of a defined route for
certain groups of people. Also, for each target, each occupant group is assigned an awareness
factor between 0 and 1, specifying the fraction of that group which knows about the exit. If a 0
is specified for the occupant group and target, that denotes that no one in the group knows about
the target or exit, and the label of 1 would specify that everyone in the group knows about the
target or exit. The occupants choose the exit that they travel to according to the score assigned
to each exit. This score is based on the following four factors: 1) the shortest distance to the exit,
2) familiarity with the exit, 3) the number of occupants around the exit, and 4) the number of exit
lanes.
Occupant behavior: No behavior is simulated using this model. From the publications, it can
be argued that the behavior borders on implicit with the use of inputs such as patience and the
action of family groups moving together before exiting, however there was not enough evidence
of this capability to categorize this model as implicit.
Occupant movement: In high density situations or queuing, the movement speed is affected by
the availability of the next grid cell. In a grid cell, the individual has 8 possible decisions
surrounding the grid cell and the decision of where to go is based on which of the adjacent grid
cells has the lowest potential. When specifying an exit in STEPS, the program will calculate the
Potential Table which will provide the shortest distance from each grid cell to the target. A
recursive algorithm will be used by the program to find the distance from each grid cell to the
exit. The potential for exit cells is 0, and the program then jumps to each adjacent cell to
calculate its potential. If the program jumps to a cell using a diagonal move, STEPS will add
(Grid Size value*(Sqrt. 2)) to the cell’s current potential, and if the program jumps to a cell using
a horizontal or vertical move, STEPS will add the Grid Size value to the cell’s current potential.
A-12