Egress Parameters Influencing Emergency Evacuation in High-Rise Buildings

Abstract

Fire in buildings pose a significant threat to occupants, first responders as well as the structural system. Rapid spread of fire and smoke in buildings can hinder the process of evacuation resulting in loss of human life. Such situations call for a reliable egress system that provides safe evacuation of occupants in minimal time. Updating the occupants and first responders with much-needed situational awareness such as accessible stairwells and exits during the disaster can not only lead to efficient evacuation but also shorten the duration of evacuation in some scenarios. This paper examines occupant evacuation scenarios in fire exposed high-rise buildings. A parametric study is carried out on evacuation strategies in a 32-story typical office building during different fire exposure scenarios. The movement of occupants with and without situational awareness is simulated. The effect of critical parameters such as number of stories, width of the egress paths, location and number of exits on the evacuation process is evaluated. The time required for occupants to evacuate the building is estimated under normal conditions (to simulate fire evacuation drill) and under realistic fire exposure. Results from the study indicate that the two most significant factors that influence evacuation time are the location of stairway within the building and the floors at which fire starts. When fire starts at the lower levels of the building, the evacuation time is the highest. More importantly, if situational awareness is incorporated in emergency evacuation procedure, it can improve the evacuation efficiency in a fire exposed high-rise building; wherein up to 24% reduction in evacuation time is achieved.

Appendix

The computation of evacuation time of the building evaluated in this study using the hydraulic model is carried out in the following steps. Refer to Fig. 4 for dimensions of the egress components.

1.1 Flow Capacity of Corridors

Effective width, $W_{e} = 1981 - \left( {2 \times 152} \right) = 1677\,{\text{mm}} (5.5 {\text{ft)}}$

Density, $D = 125\, {\text{persons/245 m}}^{ 2} (125\,{\text{persons/2635 ft}}^{ 2} ) {\text{corridor area}}$

$= 0.51\,{\text{persons/m}}^{ 2} \left( {0.047\,{\text{persons/ft}}^{ 2} } \right)$

As the density is less than 0.54 persons/m² (0.05 persons/ft²), all occupants will have a speed (S) of 1.19 m/s (235 ft/min) [5].

Specific flow, $F_{s} = SD = 0.607\,{\text{persons/s/m}} (11.139\,{\text{persons/min/ft) effective width}}$ (governs)

Maximum specific flow, $F_{sm} = 1.3\, {\text{persons/s/m}} (24\, {\text{persons/min/ft) effective width}}$ [5]

Calculated flow for the corridors, $F_{c} = F_{s} \times W_{e} \approx 61\, {\text{persons/min}}$

The calculated flow (Fc) is an initial value for the corridors and can be sustained only if the next transition point (stairway doors) can accommodate this flow.

1.2 Flow Capacity of Stairway Doors

Effective width, $W_{e} = 813 - 305 = 508 \,{\text{mm}}\, (20\, {\text{in.)}}$

Specific flow, $\begin{aligned} F_{{s\left( {door} \right)}} & = \frac{{F_{{s\left( {corridor} \right)}} W_{{e\left( {corridor} \right)}} }}{{W_{{e\left( {door} \right)}} }} = \frac{0.607 \times 1677}{508} \\ & = 2\, {\text{persons/s/m (}}36.76 \,{\text{persons/min/ft) effective width}} \\ \end{aligned}$

Maximum specific flow, $F_{sm} = 1.3\, {\text{persons/s/m}} \,(24\, {\text{persons/min/ft) effective width}}$ [5] (governs)

Calculated flow for the corridors, $F_{c} = F_{sm} \times W_{e} \approx 40\, {\text{persons/min}}$

As $F_{{c\left( {door} \right)}} < F_{{c\left( {corridor} \right)}}$, queuing of occupants occurs at the doorway entrance.

Rate of queue buildup is $61 - 40 = 21\, {\text{persons/min}}$

1.3 Flow Capacity of Stairways

Effective width, $W_{e} = 1118 - 305 = 813\,{\text{mm}} \,(32\,{\text{in.)}}$

Specific flow, $F_{{s\left( {stairs} \right)}} = \frac{{F_{{s\left( {door} \right)}} W_{{e\left( {door} \right)}} }}{{W_{{e\left( {stairs} \right)}} }} = \frac{1.3 \times 508}{813}$

$= 0.822\, {\text{persons/s/m}} \,(15.04\, {\text{persons/min/ft) effective width}}$ (governs)

Maximum specific flow, $F_{sm} = 1.01\, {\text{persons/s/m (1}}8.5\, {\text{persons/min/ft) effective\,width}}$ [5]

Density, $D = 1.1\,{\text{persons/m}}^{ 2} \left( {0.1\,{\text{persons/ft}}^{ 2} } \right)$

Speed, $S = k - akD = 1.08 - 0.266 \times 1.08 \times 1.1 = 0.76\,{\text{m/s}}\, (149.6\,{\text{ft/min)}}$

Length of stairways on each floor, $L = 9.67\,{\text{m}} (31.73\,{\text{ft)}}$

Time to descend from one floor to another is $\left( {\frac{9.67}{0.76}} \right) = 0.21\,{ \hbox{min} }\left( {13\,{\text{s}}} \right)$

After 13 s, 8 $\left( { \approx 0.21 \times 40} \right)$ occupants will be in each stairway to produce a total of 256 $\left( { = 8 \times 32} \right)$ occupants in all the floors. The remaining 117 $\left( { = 125 - 8} \right)$ occupants will remain in queue in front of each stairways.

In each of the floors, merging of stairway flow and stairway entry flow occurs.

Merging flow, $F_{{s\left( {out-stairs} \right)}} = \frac{{\left[ {F_{{s\left( {door} \right)}} W_{{e\left( {door} \right)}} + F_{{s\left( {in-stairs} \right)}} W_{{e\left( {in-stairs} \right)}} } \right]}}{{W_{{e\left( {in-stairs} \right)}} }}$

$$= \frac{{\left( {1.3 \times 508} \right) + \left( {0.822 \times 813} \right)}}{813} = 1.6 3\,{\text{persons/s/m}} (30.0 8\,{\text{persons/min/ft) effective width}}$$

Maximum specific flow, $F_{sm} = 1.01 {\text{persons/s/m}} \,(18.5\, {\text{persons/min/ft) effective width}}$ [5] (governs)

1.4 Calculation of Evacuation Time

Under normal conditions, a duration of 0.5 min is assumed to be required for the flow of occupants to reach the stairway door [5]. This is a conservative estimate as the exit stairways are placed at an average distance of 22.86 m (75 ft) from all the occupants and the occupants are moving at a speed of 1.19 m/s (235 ft/min). At the end of 43 s (30 s + 13 s), 256 occupants will occupy the stairways.

Time taken by the end of flow to reach the 31st floor

$$= 43 + \left( {\frac{117}{40}} \right) \times 60 + 13 = 231.5\, {\text{s}}$$

Similarly, the time taken for the flow to reach the each of the lower floors can be added to obtain the total time required for all the occupants to exit the building.

${\text{Evacuation time}} = 231.5 + \left[ {\left( {\frac{117}{0.813 \times 1.01}} \right) + 13} \right] \times 31 = 5052\, {\text{s}}$ (84.2 min)