|Assessment of Total Evacuation Systems for Tall Buildings|
Issue 72: Assessment of Total Evacuation Systems for Tall Buildings
By Enrico Ronchi, Ph.D., and Daniel Nilsson, Ph.D.
Building codes establish the minimum requirements for the safe design of high-rise buildings. Nevertheless, additional life safety measures are often necessary to mitigate the risks that arise from the complexity of these types of buildings and the possible difficulties in fire-fighting and rescue operations.
Recent events such as the World Trade Center evacuation1
have raised awareness on this topic. Several questions have been
prompted about the adequacy of the current emergency procedures for
high-rise buildings. What egress components (e.g., stairs, elevators,
refuge floors, sky-bridges, etc.) are suitable for evacuation of
high-rise buildings? What emergency procedures should be employed to
improve evacuation efficiency? These questions do not have simple
answers, and they often depend on the specifics of the building under
To date, research has mainly focused on the study of the
effectiveness of two egress components, namely (1) stairs and (2)
occupant evacuation elevators (OEEs). Little research has examined other
evacuation systems, which include combinations of these two exit
strategies as well as new egress components, such as sky bridges, for
The assessments of evacuation strategies in tall buildings often rely
on the use of egress models. If a model user is aware of the intrinsic
limitations of the models and the subsequent variability of the results,
egress models are efficient tools to analyze and compare different
egress strategies. They can be used to provide qualitative and
quantitative information on occupants’ use of different egress
components and strategies. In fact, they can allow the representation of
the occupants’ decision making process in the case of complex
Therefore, the Fire Protection Research Foundation at the National
Fire Protection Association has sponsored a project to investigate the
effectiveness of different total evacuation strategies in high-rise
buildings by means of egress modeling. The project consisted of a review
and compilation of existing information on this topic2 as well as a model case study.3
The review showed that evacuation models can be effectively employed
to study relocation strategies and safety issues associated with
high-rise buildings. The suitability of egress models for high-rise
building evacuations is associated with their flexibility in
representing different egress components and the complex behavioral
processes that may take place. The capabilities of egress models can be
enhanced by conducting further research on the understanding and
modeling of the impact of staff actions, group dynamics and people with
disabilities. Given the increasing height of buildings and the gradual
reduction in the physical skills of the population, the effects of
fatigue on evacuation also need further studies.
The model case study consists of two identical twin towers linked
with two sky-bridges at different heights. The towers are 50 floor
high-rise buildings, which use both vertical and horizontal egress
components, namely stairs, OEEs, service elevators, transfer floors and
sky-bridges. The total evacuation of a single tower has been simulated
by employing seven possible strategies, i.e., different combinations of
egress components. The strategies include either the use of only one
type of vertical egress components (stairs or elevators), or a
combination of vertical components (stairs and elevators) or a
combination of vertical and horizontal components (stairs, elevators,
transfer floors, and sky-bridges).
The evacuation strategies have been simulated employing a model based
on a continuous representation of the space. In order to provide a
cross validation of the results produced by Pathfinder, a model based on
a discrete representation of the space (a fine network model) has been
employed to simulate the base case (only stairs available for the
evacuation) and one scenario including the use of OEEs.
The comparison between the models has been made employing specified
calculations, i.e. the configuration of the inputs of the models is
based on complete information about the model geometry, occupant
characteristics, etc. Results showed that the range of variability of
the results between the two sub-models for stair and elevator modeling
allows a relative comparison between the evacuation strategies.
Differences in model results result from the modeling approaches and the
embedded sub-models for stairs and elevators.
The results of the case study reveal that egress strategies involving
the use of OEEs are not effective if not linked to appropriate
information to occupants about elevator usage. The strategy employing
only OEEs for the evacuation resulted as the most efficient strategy. If
occupants use sky-bridges to evacuate the building, evacuation times
would be significantly lower than the strategies involving the use of
stairs only or a combination of elevators and stairs without appropriate
information to the evacuees.
Therefore, the simulation work showed that the most effective
strategies for the 50 storey case study building under consideration
(the sole use of OEEs and the use of sky-bridges and transfer floors)
are hypothetical strategies that are generally not implemented in
today’s high-rise buildings. This may be due to a lack of understanding
regarding the behaviors of building occupants in the case of
The calibration of the modeling input for the case study has been made using experimental data4,5,6 and data from actual events.1
Nevertheless, the lack of data about the behaviors of the occupants in
the case of a combined use of different egress components is still
evident. Further behavioral research on the occupant’s decision making
process about the choice between multiple egress components would
increase the accuracy of model predictions.
Enrico Ronchi and Daniel Nilsson are with Lund University
2nd Quarter 2013 - First Responder Challenges in Very Tall Buildings -- Joe McElvaney, P.E., Phoenix (AZ) Fire Department
2nd Quarter 2011 - On Elevator Shaft Pressurization System Standards and Codes for Smoke Control in Tall Buildings -- Richard S. Miller, Ph.D. and Donald E. Beasley, Ph.D., Clemson University
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