By Alejandra Velasco, Enrico Ronchi, Amitava Halder, Kalev Kuklane, Lund University, Sweden
Nowadays, underground metro systems offer a transportation option to millions of users around the world. The constantly increasing population over the years is linked to a higher capacity of and demand for subway systems. To supply such demand, designers are opting to build deeper metro stations; the Arsenalna metro station in Kiev is a clear example, since it has a vertical drop of 105 m; in Stockholm, the T-Sofia metro station is under construction and planned to have a 100 m depth. These facilities are considered “deep metro stations” and present several challenges from a fire safety perspective, mostly because stair-ascent evacuation would have to take place at any given point of the evacuation route.
Picture this hypothetical scenario: You are at one of these stations when you hear an emergency alarm. You need to evacuate as soon as possible, but what if you are carrying a backpack — would you be able to carry it through the evacuation route? You probably would answer “yes, of course,” since you know the backpack could be left behind if it represents a challenge to reaching a safe area. However, what if you are carrying your baby instead? You will have a completely different reply, since it is a different situation when the safety of your child is at stake.
Daily, it is usual to observe subway users with luggage since the subway serves to connect airports and train stations. It is also common to observe parents carrying their children in their arms or in the so-called “baby carrier backpacks” position; this is when the hypothetical scenario comes to mind as possible, and as fire engineers must consider them as a variable when involved in projects relating to stair-ascent evacuations.
Recent experimental studies1-4 have investigated the implications of fatigue on ascending evacuation performance. Nevertheless, there is one factor that previous fire safety research has considered to a lower extent: how the subway user’s evacuation performance is affected by carrying additional mass, also in relation to the geometrical configuration of the ascent.
Keeping in mind the knowledge gap on the effects of fatigue on people’s performance when applied to a stair-ascent evacuation, and by applying methods of exercise physiology in fire safety engineering, a study investigated how evacuation performance is affected by carrying mass in addition to the subject’s own body mass by performing a stair-climbing exercise. The experimental procedure was based on previous studies performed in Sweden1, 3, 4, 13 with simplifications in technical measurements, focusing on heart rate (HR), oxygen uptake (VO2) and perceived exertion.
What made the study novel was that it measured relevant parameters while including the carrying of additional mass in the same sample population. The additional mass chosen for the experimental set-up was 8 kg, which is the typical weight of hand luggage or the average weight of an eight-month-old infant.
The laboratory experiment was conducted with 21 participants in three different sessions. The methodology followed during Session 1 was based on the maximum oxygen uptake estimation of the participants while completing a sub-maximal test and using a braked ergometer bicycle;7 this test served to measure the participant’s HR while maintaining a cardiorespiratory activity for approximately 6 minutes.
The results supported the calculations for the ideal step rate prediction at which someone would theoretically be able to complete a 5-minute stair-climbing exercise without reaching exhaustion. Step rate predictions were based on the work by Kuklane and Halder4 and Halder et al.8 Meanwhile, the metabolic rate (M), which is a function of the external load L, walking speed V, grade of slope G and terrain coefficient η, was calculated using Pandolf’s equation.5
During Sessions 2 and 3, participants performed a 5-minute stair-climbing exercise on a stair machine. This machine was deemed to represent the escalators in a subway station evacuation route. The stair machine had fixed-step dimensions (20.5 cm height, 25 cm in depth) and 20 step rate levels to choose from (24–162 steps/min). Session 1 results determined which step rate was ideal for each participant to avoid exhaustion, making it important to follow the experimental guidelines as strictly as possible.
The Borg’s Scale (with a maximum of 20)9 was also used to figure the participant’s perceived exertion by using numbers and verbal anchors. The difference between these sessions was the inclusion of the additional mass into the stair-climbing exercise during one of them; for both sessions, participants were asked to slow down their step rate when they considered it as beneficial to complete the 5-minute exercise.
Measurements for VO2, HR and perceived exertion were obtained during Sessions 2 and 3, and the predicted values for M were compared with the calculated ones obtained after the experimental sessions. The data analysis was made according to the following parameters: 1) velocities, 2) displacements with focus on vertical distances, 3) heart rate differences, 4) metabolic rate (as derivate of VO2), 5) variables correlations and 6) Borg’s scale.1, 4, 8, 10-12
All participants were able to complete the 5-minute stair-climbing exercise while carrying the additional mass (8 kg backpack), except one man, who asked to stop the machine after 3 minutes of exercise due to breathlessness complaints. All participants completed the 5-minute stair-climbing exercise without carrying the backpack.
When considering the results within the velocities, \4 out of 21 participants on average needed two step rate reductions while not carrying additional mass (1st at 3:00 min, 2nd at 3:35 min); meanwhile, while carrying an 8 kg backpack, 14 out of 21 participants reduced the step rate 3 times (1st, 3:03 min; 2nd, 3:37 min, 3rd, 4:21 min). In vertical displacements (Vdisp), participants could climb an average of 92 m when they were not carrying the backpack, compared with 84 m when they were influenced by the additional mass; this represents a performance reduction of 8.52%.
Participants reached a Vdisp of 57.93 m before asking for a step rate reduction when they were not carrying additional mass, in contrast with 42.75 m with the 8 kg backpack. These results show that people request a first step rate reduction at 15.18 m when carrying weight, representing a reduction of 26.20% in the participants’ performance for this specific sample and experimental conditions.
The Pearson’s correlation coefficient (r) was used to present the relations between the physiological variables found during the experimental phase and vertical displacements that influence the fire-safety designs. A stronger correlation was found between the metabolic rate and the vertical displacement, with r = 0.87; this indicates an increase of the energy expenditure when the subjects reached a higher position in the vertical direction.
An interesting result was in the correlation between the load/body mass and vertical displacement, where it could be seen that the impact of carrying weight was higher for the participants carrying the 8 kg backpack who had a body mass above the average, compared to those with a body mass lower than the average. Future studies can explore correlations related to gender, age, fitness conditions, etc.
Regarding the evaluation of participants’ perceived exertion while using the Borg’s Scale, it was found that under both experimental conditions, their perceived exertion did not change drastically during the first 2 minutes of the exercise but remained at 13 — “somewhat hard” — within the Borg’s scale. Changes appeared at the 3-min mark, when perceived exertion increased in both cases to a 15 — “hard-heavy” on the Borg’s scale.
Differences between the influences of the additional mass started to be more noticeable at the 4-min mark, when the perceived exertion consensus was 16 — “very hard” — for the experimental session while carrying the 8 kg backpack; it remained at 15 without the influence of the backpack. At minute 5, the general perceived exertion reached 18 — “extremely hard” — for the set-up with a backpack, while it remained steady at 15 of the Borg’s scale when a participant’s performance was not influenced by the additional mass.
Considering the aims of the experiment, it was concluded that: 1) The physiological limits of oxygen uptake, heart rate and metabolic rate are negatively affected by additional mass, proving that this factor should be considered in the fire-safety field when designing evacuation routes in deep subway stations; 2) the relation between the perceived exertion and the acceptable exertion levels during an evacuation should be considered as part of including ethical aspects (i.e., the need to assess the tolerable perceived exertion for an individual); and 3) the negative effect of the additional mass was confirmed, showing a need to include this in evacuation modelling tools.
Further information about the experimental assumptions, results and their implications can be found in the final report on this work.13
References
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3Arias, S., Ronchi, E., Norén, J., Delin, M., Kuklane, K., Halder, A., and Fridolf, K. An experiment on ascending evacuation on a long, stationary escalator. At 14th International Conference and Exhibition on Fire Science and Engineering (Interflam). Windsor, United Kingdom, 2016.
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13Velasco, A. The effects of fatigue during deep metro evacuations and its implications on evacuation modelling tools. Report 5572. Department of Fire Safety Engineering, Lund University, 2017.