A survey of exit facilities in high office buildings

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A survey of exit facilities in high office buildings

fa

C A N A D A Ser TfIl B92 no.

64

c. 2 :BmE A S U R V E Y

OF

EIUT FACILITIES

IN

HIGH OFFICE BUILDINGS

by

M. Galbreath

D l V I S l O N O F B U F t O l N G R E S E A R C H

-

H A T I D N A P R E S E A R C H C O U N C I L a O T T A W A a C A N A D A

A _- SURVEY O F EXIT FACILITIES

IN

- - - - - - . -

HIGH QFFIC E BUILDINGS

M. Galbr eath

A survey has been m a d e of the exit stairs in ten office buildings

all of which are seven s t o r e y s or m o r e in height. Information on number

of occupants and time t o evacuate the buildings in practice drills was

p r o v i d e d by fir e authorities.

The National Building C o d e p r o v i d e s that on each floor of a n

office building there should be not m o r e than 6 0 _{per sons per unit -exit}

-

width, U n i t -exit -width is defined as 2 2 in. ; an increment of 12 in. is a s s u m ~ d to b e half a unit-exit-width. The width of stairs is d e s i g n e d

on this basis for the m u s t populated floor and is not cumulative when

t h e stair serves

a

number of f l o o r s . The minimum width of stairs

permitted by the

N.

B,

C.

is 3 6 in.

The number of occupants related t o the unit-exit-width i n the buildings surveyed, the height in storeys, t i m e to evacuate, and the

area of stair between f.loors*expressed as square feet per person are

given in Tables

I,

11, and

lII.

Buildings 1, 2, 4 and 6

_{a r e}

_{m o d e r n ring c o r r i d o r layouts with}

a central c o r e containing elevators, stairs a n d ancillary services and accommodation. Building 5 is similar but incorporates a s c i s s o r s

stair,

a

device that provides two separate paths of exit t r a v e l in the same general floor space. Building No. 3 is L-shaped in plan a n d has t h r e e stairs. Building No. 7 is an older building type with a central light w e l l and r i n g corridor. It has t w o stairs closed in by woad and w i r e d glass

partitions extending from the top f l o o r to the basement and t w o other

open stairs leading only from the second floor to the first. It w a s noted

by the fire w a r d e n that in evacuation d r i l l s persons leave the closed stairs at the second floor level when these become congested and u s e

t h e open stairs t o reach the main floor. Buildings Nos. 8 and 10 f o r m

part of the s a m e complex and a r e linked at the fourth and f i r s t floors

a n d at the basement. Both have long corridors in T or

H

arrangement

with a stair at the end of each c o r r i d o r . Building No. 9 has a l a r g e

rectangular plan and stairs w i t h corridors to the e*erior at each corner,

The area of stair between floor levels noted in Table J is

important because when this space is filled the group of occupants will elrtend beyond the entrance door from the floor immediately

below, This makes it difficult o r impossible for the people on the floor below to enter the stairs.

A study prepared for the London Transport Board has shown that when people are confined to about 2 sq ft per person a U f o r w a r d movement stops. This situation will occur when the number of

occupants on each floor exceeds the capacity of the stairs. The

British study gives approximate figures for speeds of groups of

people moving up stairs as follows:

Concentration, _- s q f t p e r p e r s o n Speed, f t p e r r n i n u t e

5. 5 132

(this is suggested

as

the optimum for design of footways)

10 160

[this is t h e practical maxhnum)

Speed on stairs down is s a i d to be a little higher than these figures.

In

Figure 1 the rate of exit travel f o r t h e ten buildings is shown,

based on

a

measure of the length of exit travel and the t i m e reported for evacuation of the building. Table I1 shows the travel distances from which the rate of travel is d e r i v e d . The line on the graph shows the

rate of travel developed by the British. W i t h four exceptions the rate

of evacuation appears to come close t o the British figures. The

circumstances a r e of course not the same and it may be that the optimum rate of travel. cannot be maintained over many successive storeys,

The maximum rate of evacuation in this survey is 132 f per minute.

In two buildings the r a t e of exit travel is noticeably l e s s . Both are

buildings with a very l o w occupant l o a d relative to the floor area.

In

one of these however, No. 3,

a

subsequent evacuation was much faster: the occupants believed that it was

a

real fire.

In

two buildings the space on the stair I s

2

s q f t p e r p e r s o n o r less. h o n e of these, No. 1, no

evacuation Bas been achieved in r epeat ed fir e alarms and in the other,

1

evacuation was completed

in

7, minutes,

In

this last building, most

of the occupants w e r e military- personnel a n d this may have contributed

to the higher net speed over

a

longer period.

Table 111 lists the same 1 0 buildings but gives the stair width

in terms of number of persons per unit-exit-width. Zn relation to

stair d e s i g n these buildings containless thanhalf the occupants contemplated by the NBC formula. It is apparent that i f the buildings w e r e occupied to the f u l l extent of the formula the concentration of people in the stairs would b e much gs eater and evacuation would be mor e difficult.

The London Transport s t u d y shows that a 3-ft wide stair c a n

h a n d l e little more than one traffic lane. From the figures given it is

appar ent that

a

12 -in. h c r ernent adds one quart ex t o the unit -exit -width a n d n o t one half as provided in t h e National Building Gode. The stair

width recommended in the London Transpart s t u d y S a r two traffic lanes

is 42

in, between handrails

plus 4 in. on each s i d e above the handrail ta p r o v i d e width for s h o u l d e r s ,

On the basis of t h e rates of t r a v e l shown -in Figure 1 it should b e possible to evacuate a 20-storey building in 1 2 minutes if t h e r e a r e

f u l l units of exit width a n d the area of a t a i x is not less than 3 s q ft per person.

h

Building No. 1 wikh its present population this would require

the equivalent of

one more stair of t h e

same area as these existing

stairs

.

F o r higher buildings, evacuation the would b e pro gr e s s i v e l y

Isager, It is therefore not realistic t o attempt to provide for evacuation of high buildings by stairs in a Limited time. An alternative i s to provide

areas of refuge ox waiting spaces, connected d i r e c t l y to stairs ar other

means of egress, in which the population of each floor c a n stand until evacuation becomes possible.

1.

Second Report of the Operational Research T e a m on

the Capacity of Footways., Research Report No. 9 5 ,

London

Transport Board, London, August

1958

Also d e s c r i b e d in:

Information Sheet k 194 Internal Cir cdation,

Architects Journal Information Library, 20 March

19

6 3 Architectural P r esg Ltd.

,

9

-

13, Queen

A n n e b

Gate,

London, S . W.

I ,

England.

2. National Building Code of Canada, 1965.

Issued by The Associate Committee on the National Building Code*

TABLE 1

HIGH BUILDINGS: SPACE

IN

STAIRS REIATED

TO

NUMBER

OF

OCCUPANTS

Area of

No. af

No,

of Persons St air s Stair Area,

TABLE

ll

HIGH BUILDINGS:

R A T E

O F EXIT TRAVEL

Travel Travel Total :$ R a t e of

D i s t a n c e Distance Travel Time to Exit

per Floor, incorridors, Distance, Evacuate, Travel,

Building No. f t assumed, ft f t min £t/min

100 1 0 0 100 100 100 L O O 100 100 100 100

xC Travel distance per floor x number of f l o o r s

+

travel. distance in corridors

%%:: _{Revised to}

4

_{min in circumstances} that w e r e thought to

be a real emergency

TABLE UI

HIGH BUILD EJIJGS: NUMBER O F OCCUPANTS RELATED T O UNIT -EXIT-WIDTH

*

W i d t h Totalunits P e r s o n s * *

of Stair s , U n i t

-

No. of of Exit per U n i t

-

Building No. in Exit - W i d t h Stairs W i d t h E x i t - W i d t h

*

Unit - E x i t - W i d t h a s s u m e d to be 2 2 in.

I

I

I

I

I

_I

1

_I

I

-

R a t e o f

T r a v e l

A c c o r d i n g

t o

B r i t i s h

S t u d y

-

-

-

-

Q

-

:

O p t i m u m

b y

-

B r i t i s h S t u d y

-

9 7

O 0 9

10

0 -

l 6

-

0 3

04

- d

Q

5

-

-

...,

-

I

I

I;

I

- I

I

P

I

1

C O N C E N T R A T I O N

O N

S T A I

RS,

S Q F T I P E R S O N

F I G U R E

1

R A T E O F E X I T

T R A V E L R E L A T E D

T O

C O N C E N T R A T I O N

OF

O C C U P A N T S

I N

S T A I R S .

CNWMERALS

R E F E R

T O

B U I L D I N G S L I S T E D

I N

T A B L E S

I, I 1

& 1 1 1 . )

*

_{R e p e a t}

_Evaluation

_{o f}

_{B u i l d i n g N o .}

_{3 i n}

C i r c u m s t a n c e s

t h a t

W e r e Thought

t o

b e a

R e a l

E m e r g e n c y .

B R 4 Z Z C

PHYSICAL MEASUREMENT

OF

STAIRS

(1

3

Area of Stair per F l o o r

The plan area of stair per floor w a s measured within

the exterior walls. Projections such as pipe ducts were

deducted from the area, The area of the space that occurs

in some stairwells between the centre handrails was also deducted from the plan area. This area was found by m e a s u r i n g the length of the stair in plan by the distance

between the ends of the steps,

These dimemions have been u s e d in the previous

studies r e f e r r e d t o in the Bibliography. It has been assumed that width of traffic lane at shoulder height

( 2 f in. or 22 in. per l a n e of t r a f f i c ) is critical unless t h e width at floor level is less by 4 in. on each s i d e of the stair.

(2) Travel Distance

T r a v e l distance per floor w a s m e a s u r e d following the centreline of the _{stair treads a n d landings.} On the stairs the distance is measured following the slope. The British studies

r e f e r r e d t o also measured path of travel on stairs, following the slope of the stairs.