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 consideration.

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 tall buildings.

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 evacuation scenarios.

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 non-conventional strategies.

The calibration of the modeling input for the case study has been made using experimental data^4,5,6 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

References

1. Averill, J., Mileti, D., Peacock, R., Kuligowski, E., Groner, N., Proulx, G., Reneke, A. & Nelson, H. "Final Report on the Collapse of the World Trade Center Towers, Federal Building and Fire Safety Investigation of the World Trade Center Disaster, Occupant Behavior, Egress and Emergency Communications," NCSTAR 1-7, National Institute of Standards and Technology, Gaithersburg, MD, 2005.
2. Ronchi, E. & Nilsson D. "Assessment of Total Evacuation Systems for Tall Buildings," Fire Protection Research Foundation, Quincy, MA, 2013
3. Ronchi, E. & Nilsson, D. "Assessment of Total Evacuation Systems for Tall Buildings: Literature Review," Fire Protection Research Foundation, Quincy, MA, 2013.
4. Heyes, E. "Human Behaviour Considerations in the Use of Lifts for Evacuation from High Rise Commercial Buildings." Ph.D. Dissertation. Department of Civil Engineering. University of Canterbury, Christchurch, New Zealand, 2009.
5. Kinsey, M. "Vertical Transport Evacuation Modelling." Ph.D. Dissertation. Fire Safety Engineering Group, University of Greenwich, UK, 2011.
6. Jönsson, A., Andersson, J. & Nilsson, D. "A Risk Perception Analysis of Elevator Evacuation in High-Rise Buildings." Proceedings of the 5thHuman Behaviour in Fire Symposium, Interscience Communications, London, 2012.

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