NIST Technical Note 1623

Emergency Egress From Buildings

Part 1: History and Current Regulations for Egress System Design

Part 2: New Thinking on Egress From Buildings

Richard W. Bukowski
NIST Technical Note 1623

Emergency Egress From Buildings

Part 1: History and Current Regulations for Egress System Design

Part 2: New Thinking on Egress From Buildings

Richard W. Bukowski

Fire Research Division

Building and Fire Research Laboratory

January 2009

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Deputy Director 
Certain commercial entities, equipment, or materials may be identified in this

 document in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1623

Natl. Inst. Stand. Technol. Tech. Note 1623, 28 pages (January 2009) 

CODEN: NSPUE2
Emergency Egress from Buildings 

Part 1: History and Current Regulations for Egress Systems Design 

Richard W. Bukowski, P.E., FSFPE 

NIST Building and Fire Research Laboratory 

Gaithersburg, Maryland 20899 USA 

INTRODUCTION 

For most of history buildings were short enough that stairs provided for access were 

sufficient for rapid egress in the event of fire. Even in single stair (mostly residential) 

buildings, experience showed that this stair was sufficient for fire egress as long as the 

fire did not expose or block access to the stair. Fire resistant apartment doors shielded 

the stair from most fires and exterior fire escapes provided a second egress path 

beginning early in the 20th Century. 

The 1854 invention of the elevator safety brake enabling the passenger elevator is 

credited with facilitating increases in building height and the first so-called skyscraper in 

Chicago in 1885. These buildings utilized steel frames protected by masonry or tile and 

were dubbed “fireproof construction” providing a (possibly false) sense of security. By 

1914 authorities had begun to question these arrangements as evidenced by a move to 

change the term “fireproof” to “fire-resistive,” and description of egress provisions in 

regulations as “exceedingly deficient.”1

Model building regulations in the US started with the National Building Code published 

by the National Board of Fire Underwriters (NBFU) following the Great Fire of Boston 

(1872). Property loss claims from this fire resulted in more than 70 insurance companies 

being driven into bankruptcy, causing insurance interests to form the NBFU and to 

develop building fire safety rules aimed at reducing property losses in fires. These rules 

became the first model building code, called the National Building Code (NBC), and first 

published in 1905. The NBFU was able to tie compliance with their rules to their 

Municipal Grading Schedule on which insurance rates are based. Cities needed favorable 

rates to attract investment, so they were motivated to adopt regulations consistent with 

the National Building Code2

. The first (1905) edition of the NBC required exit stairs to 

have a minimum width of 20 in (510 mm)*. 

The purpose of this paper is to document current regulatory requirements for means of 

egress in fires, their origins and scientific basis, and the approaches used in other 

countries. Then the paper will present an argument for why these approaches and 

requirements should be re-evaluated to reflect changes both in buildings and in their 

occupants. Finally the paper will make some suggestions for reasonable revisions to 

 

*

 This paper will cite primary dimensions in the units found in the code cited. Thus U.S. codes will show 

English units first and non-U.S. will show metric first. Unit conversions are approximate because the paper 

cites the equivalent values found in the building regulations of various countries which are normally round 

numbers. 
design practice along with a more holistic philosophy that takes better account of human 

behavior and is based on a more appropriate performance metric. 

ORIGINS OF THE 44 INCH EXIT STAIR IN THE US 

In the 1913 National Fire Protection Association (NFPA) Proceedings3, the Committee 

on Fireproof (later, Fire-Resistive) Construction reported a number of recommendations, 

including one for minimum 44-in exit stairs (unobstructed width except that handrails 

would be permitted to intrude not more than 3 1⁄2 in on each side). That same year NFPA 

formed their Committee on Safety to Life. That committee’s first activity was to conduct 

a comprehensive review of fire safety issues and regulatory approaches found in building 

codes and local regulations in several, geographically diverse cities. At the NFPA’s 1914 

meeting they reported that, 

“... existing laws are exceedingly deficient in this very important matter of egress. A 

number of states report frankly that they have no real legislation upon the subject, 

many City Ordinances are of the most indefinite character, and in some the 

matter is simply left to the discretion of the fire department or other officials.” 

In the 1914 NFPA Proceedings, section on egress, the Safety to Life Committee cites the 

1913 NFPA Annual Meeting report of the Committee on Fire-Resistive Construction in 

which they said was presented, 

“... a splendid set of specifications for the construction of a standard building. Egress 

received detailed attention; -- specifications for smoke-proof towers, for stairs, 

for horizontal exits, and for the capacity of vertical and horizontal exits were 

included.” 

The committee also cites the 1913 laws of the New York State Department of Labor 

which, “... as regards fundamentals appear to agree entirely with the requirements of 

our Committee ...” Extracting from the referenced New York statute, they cite, 

“a. For buildings erected in the future, a minimum of 22 inches of stair width 

shall be required for not to exceed 14 persons on any one floor. 

 b. On buildings already erected this figure is reduced to 18 inches as a minimum. 

c. A 44-inch stair in new buildings permits 28 persons to be housed on each floor 

above the first one. 

d. In arriving at this decision the idea has been that all of the persons on all floors 

shall be able to remain in the stair tower without any movement, a person 

requiring about 22 inches in width, and one person to stand on every other stair.” 

They further characterize the New York laws’ stair geometry (7 3⁄4 in riser height by 10 in 

tread) as “good”, and that they recommend a minimum 44 inch wide stair for new 

buildings as this width is 

“reported sufficient to prevent three persons from forming an arch and blocking traffic.” 

EXIT CAPACITY 

The above explains why the US designs exits for “capacity” and why the capacity is 

based on the population of a single floor. The exit is sized to “store” people, motionless 
within the protected exit enclosure, such that the population of one floor will fit within 

one flight of the stair, with each person in a space 22 inches wide and standing on every 

other step. 

This philosophy was recognized in the 1935 National Bureau of Standards (NBS now 

NIST) publication, Design and Construction of Building Exits 4 . Developed by the 

Department of Commerce Building Code Committee, this report included survey data on 

exit sizes and configurations drawn from eight cities chosen, “... with a view to covering 

places varying in size and sufficiently distributed to give a fair cross section of building 

construction.” The survey included population counts on typical floors and compiled 

data on movement of people in buildings (railway terminals and schools). Studies of the 

flow of occupants of government buildings during fire drills were conducted and the data 

presented as discharge rates for stairs (as a function of width and stair geometry), ramps, 

and doorways. 

The data were used to suggest possible approaches to calculating minimum width of exits 

necessary to provide for occupant safety. These included (note that the descriptive text is 

paraphrased from the 1935 report): 

1. Capacity Method, which is based on the concept of storing occupants on the stairs 

within a protected stair enclosure, and allowing for the subsequent safe and 

orderly evacuation of the building. It recognized that travel down a long series of 

stairs in high buildings is exhausting even to normal persons. Objections of 

building owners over the loss of rentable space are noted as well as the comment 

by some authorities that people may not stand still in stairways, even in high 

buildings. 

2. Flow Method, which is based on the concept that people will move down the 

stairs at a typical flow, assumed to be 45 persons per 22 in unit of width per 

minute and 60 persons per minute through doorways. It is stated that this method 

is usually coupled with an assumed time in which it is safe to exit the building and 

that this method calls for considerably less stairway width than the capacity 

method. However they felt that it would be limited to a few occupancies and to 

buildings of low to moderate height since continuous movement down stairways 

in high buildings cannot be expected without serious effects on some occupants. 

3. Combined Method suggests the flow method for lower buildings shifting to the 

capacity method for taller buildings also accounting for type of construction and 

use. Once again they point out that tall buildings would require a disproportionate 

amount of space devoted to stairways as compared with useable floor area. 

4. Probability Method considers only the population of the six, most densely 

populated floors since it is improbable that simultaneous evacuation of all floors 

of a large building is needed. This is the first time that phased evacuation (as 

currently practiced in tall buildings) is suggested. 

5. Floor Area Method relates area to units of exit width needed as a function of 

construction type and use. Like the probability method, simultaneous evacuation 

of all floors is not considered but the number of floors considered varies with 

occupancy. 
In the end, the 1935 report suggests that the needs of the vast majority of buildings can be 

met with the provision of two, two-unit-width (44 in) stairs. The capacity method 

(occupants waiting within the exit enclosure) is appropriate for low buildings with a 

gradual shift to the flow method for taller buildings where people will be less comfortable 

waiting in the stair. For tall buildings the floor area method has some application as these 

are of fire resistive construction and only those near the floor of origin are initially at risk 

but six floors is not sufficient for larger buildings. They suggest half of the floors should 

be considered. 

EARLY THOUGHTS ON ELEVATORS AS A MEANS OF EGRESS 

Both the 1914 NFPA ProceedingsError! Bookmark not defined. and the 1935 NBS report discuss 

the possible use of elevators for egress from tall buildings. In 1914 the Committee on 

Safety to Life expressed the opinion that, “... elevator shafts properly enclosed and with 

openings adequately protected have decided value from an escape standpoint, and are 

absolutely necessary in high buildings.” They cited as “... loss of life possibilities in 

many modern so-called fireproof buildings ...” the common practice of unenclosed stair 

and elevator shafts that might permit a fire in lower stories that, “... spread with 

unexpected speed could result in a loss of life which would stagger the civilized world.” 

The Committee called for enclosing elevator shafts, improving the fire resisting powers 

of elevator doors, ensuring the integrity of the electric current applied to elevators, and 

“drilling” elevator operators in emergency procedures, including that persons in the upper 

stories shall first be taken to the ground. 

The 1935 NBS report discusses a credit for elevators against required aggregate exit 

width. They discount automatic elevators as unsuitable as their “...capacity and rate of 

speed is not great.” And “... they are not subject to a single will as in the case of an 

elevator operator, but to demands from many tenants.” While there was a suggestion in 

the formulae of the flow method that five elevators might be equal to a single unit of exit 

stair width for some construction types and use, in the end they concluded that the 

uncertainties were such that no direct credit be given for elevators but to recognize their 

availability in high buildings. 

EARLY REGULATORY APPROACHES IN THE US 

The 1914 report of the Committee on Safety to Life included detailed recommendations 

for the design and arrangement of egress stairs and fire escapes with the intent that this 

material would be incorporated by others into building regulations. No code or standard 

was produced by the Committee until the 1927 publication of the first edition of the 

NFPA Building Exits Code (NFPA 101-T)5

 which later became the Life Safety Code. 

The 1927 edition of NFPA 101-T defined stairs as Class A, B, or C. Class A stairs were 

the main stair of a newly-constructed Assembly occupancy, and were 44 in (1100 mm) 

wide (handrails could intrude not more than 3 1⁄2 in (89 mm) on each side) with a rise of 

not more than 7 in (178 mm) and a tread of not less than 10 1⁄2 in (267 mm). Class B 

stairs were for new construction of all stairs not required to be Class A, and for existing 
construction where Class A stairs would be required if new. Class B stairs were the same 

width as Class A but the rise was permitted to be not more than 7 3⁄4 in (197 mm) with a 

tread of not less than 9 1⁄2 in (241 mm). Class C stairs covered existing stairs in existing 

buildings and were at least 36 in (0.9 m) wide (not less than 32 in, 0.81 m between 

handrails, but stairs less than 44 in (1100 mm) wide only required a handrail on one side). 

Occupant load on a floor dictates required capacity (total width of stairs in number of 22 

in units) required in a minimum of two stairs located “as remote as practical.” 

The 1935 NBS report included recommended code language in an appendix that did not 

follow any of the five methods for calculating minimum exit widths discussed previously. 

They explained that tentative requirements were drawn up and compared against the 

results of the field studies. Eventually a consensus of the Committee was reached and 

was presented in the recommended code language. 

The suggested code requirements largely followed the capacity method for at least two 

stairs of two (22 in, 550 mm) units of exit width each, with the floor area method used 

(by means of occupancy load factors consistent with those found in current regulations) 

to determine aggregate width. No suggestions of maximum egress time (including no 

references to fire resistance times associated with construction types, building height, and 

use) that might facilitate the use of the flow method, and no mention of partial evacuation 

of tall buildings as discussed in the probability and floor area methods was made. These 

recommendations were consistent with those in the 1927 edition of NFPA 101-T, but this 

is not surprising since the Committee on Safety to Life was represented on the NBS 

Committee. The requirements suggested in the NBS report and NFPA 101 were adopted 

in the model codes and building regulations throughout the US until the mid-1980’s when 

the 22 in (550 mm) unit of exit width was abandoned for assessing exit capacity in units 

of people per inch, but retaining the 44 in (1100 mm) minimum width. This method 

provides similar results for aggregate exit width but provides more capacity credit for 

fractions of the 22 in unit. 

SCIENTIFIC STUDIES OF FLOW RATE 

The 1935 NBS report included field surveys of discharge rates down exit stairs and 

through doors for various government buildings during drills and for subway and rail 

terminals at rush hours. The data collected were discussed by the Committee and a 

consensus reached that there was a clear correlation between width and flow. The 

committee agreed that, “... rates of 45 persons per 22-inch unit per minute for travel 

down stairways, and 60 persons per 22-inch unit per minute through doorways, which 

had been in use on the basis of earlier observations, were sufficiently confirmed to 

warrant their retention in connection with the requirements under development.” 

Almost from the start there were issues raised with the assumed flow rate on stairs of 45 

persons per minute per (22 in) unit of exit width. Togawa6 in Japan conducted research 

in the 1940’s and 50’s, that showed for densities above 1 person per square meter (10 

square feet) that flow rates decreased significantly. His data suggested a flow rate of 26 

persons per minute per (22 in) unit of exit width. 
Pauls has published extensively on this topic and continues to be the scientific conscience 

of stair design in the US codes. Pauls7

 and Fruin8 both discussed the concept of effective 

width of a stair, which is generally 0.3 m (1 ft) narrower than the actual width due to the 

natural tendency of people to keep a distance from walls and handrails. Fruin further 

spoke of the personal space (buffer) around people that increases their effective space 

requirement. Pauls found that for people walking on stairs their body sways from side to 

side and they desire sufficient space so that they do not make contact with the person 

beside them. Pauls work confirmed that of Togawa finding that flow rate in stairs at 

typical densities is about 27 persons per minute per (22 in) unit of exit width. Extensive 

studies in Russia also confirmed the effects of density on flow rates including the values 

suggested by Pauls and Togawa, as reported in a 1969 book by Predtechenskii and 

Milinskii9. 

From the data collected by NIST in the investigation of the World Trade Center (WTC) 

collapse on September 11, 2001, the flow rate in the stairways can be estimated10. For 

WTC 1, there were approximately 7900 survivors who exited the building over the 102 

minutes between the aircraft strike and the collapse. The building had three stairs, two at 

44 in (1100 mm) and one at 56 in (1400 mm). Assuming the occupants used the stairs 

equally, there were 2630 people in each 2-unit wide stair who exited in 102 minutes, or 

13 people per minute per unit of exit width. The NIST study found that the egress flow 

decreased by about 80 % in the last 20 minutes before collapse. If one assumes the 2630 

people exited in 82 minutes, the flow rate would be 16 people per minute per unit of exit 

width. These estimates support the argument that current flow rates may be significantly 

less than the rate suggested by Pauls and Togawa in the 1970’s and one third the rate 

proposed in 1914Error! Bookmark not defined.. 

SCIENTIFIC STUDIES OF EXIT WIDTH 

Clearly, the current 44 in (1100 mm) minimum exit stair width is intended to support two, 

22 in queues of occupants either standing still (capacity method) or moving down the 

stair (or a single queue of occupants moving down and being passed by firefighters 

moving up, known as counterflow). The 22 in dimension for the width of a person was 

offered in 1914 as originating with soldiers standing in a lineError! Bookmark not defined.. 

Challenges to the adequacy of the 22 in dimension include the need to provide for body 

sway as people move down the stair (Pauls), and the need to allow for some personal 

space (Fruin, Predtechenskii and Milinskii). Recently, the adequacy of the basic 22 inch 

dimension is being questioned in light of the increasing size and weight of the typical 

person, especially in the US. The 22 inch dimension refers to the width of a person at the 

shoulders, which is assumed to be the widest part. Predtechenskii and Milinskii suggest 

that 4 in (100 mm) be added to each side to allow for a personal buffer except that for 

low obstructions (like handrails) the additional space is not needed since one’s shoulders 

are at a higher level and will extend over the obstruction. 
From anthropometric data for 

modern Americans, the width at 

the hip is approaching the width at 

the shoulder, and it seems that this 

exception may no longer be valid. 

Thus, with the shoulder width of 

the 97.5th percentile adult male 

reaching 20 in (510 mm) 11 and 

allowing the 4 in on each side for 

handrail and personal space, the 

new unit of exit width should be 

28 in (700 mm) and the minimum 

stair width 56 in (1400 mm), see 

Fig 1. 

Arguably the most comprehensive 

studies of movement on stairs were 

conducted by Templer, beginning with 

his doctoral research 12 and including 

work at NBS13 in the 1980’s. Templer 

observed the movement of many 

individuals up and down stairs of varying width and tread geometry, tabulating variables 

ranging from quantitative (speed, number of stumbles) to qualitative (perceived comfort). 

From this work Templer concluded that the minimum width of an egress stair should be 

56 in (1400 mm). 

Figure 1 Anthropometric data for adults 

Reprinted with permission from NFPA 101® the Life Safety 

Code®, copyright © 2006 National Fire Protection Association

SCIENTIFIC STUDIES OF TREAD GEOMETRY 

One of the earliest studies of stair geometry was conducted by an architect in France 

named Francois Blondel14. Blondel was primarily interested in comfort rather than safety 

and observed that the main stairs of classic cathedrals were comfortable to use and 

accommodated large numbers of people attending services. He made measurements and 

found that the ratio of stair height to tread depth was a constant, and he related this 

dimension to the length of the human gait. His formula was 2R+G=24 in, where R is the 

rise and G is the going (or run). Templer adjusted Blondel’s formula for the use of the 

old (pre French Revolution) inch and a modern gait more like 28 in (71 cm) and arrived 

at a modern (metric) formula, 2R+G=710. The so-called 7/11 stair geometry commonly 

required in US codes meets the relation 2R+G=635. 

Templer15 summarizes a number of research studies of stair geometry and safety. Many 

such studies were conducted by observing people moving up or down stairs in buildings. 

Observations in subway or train stations at rush hours provided data for higher population 

densities. A few studies were conducted in laboratory settings on specially constructed 

stair sections where the geometries and stair angle could be varied systematically. 

Templer himself conducted several of these studies, including some at NBS. 
Most of the studies reviewed concluded that the measure of Total Energy Cost per Meter 

Rise is a useful metric for the evaluation of stair design for normal use and comfort; 

however stair safety is more closely related to the likelihood of missteps which is a 

function of how the stair relates to the human gait. In both cases the effect is different for 

ascent and descent, with descent being more hazardous. 

When ascending a stair a person walks on the ball of the foot with less of the foot placed 

on the step. Shorter treads (goings) and higher risers produce fewer missteps. When 

descending a stair the heel and most of the foot needs to be placed on the tread. Too 

much of the front of the foot extending over the nosing results in rotation of the foot and 

a fall, or a distorted gait while trying to place more foot on the tread. Tread depths of at 

least 11 in (280 mm) are recommended to accommodate the 95th percentile foot, but 

considering only gait and accident history treads (goings) of at least 9 in (230 mm) are 

required. Riser heights of 6.3 in to 7.2 in (160 mm to 183 mm) had the fewest missteps. 

Other dimensions apply to curved stairs16. 

Other factors relevant to stair safety include lighting, slip resistance, single steps (most 

codes prohibit flights of fewer than three steps), handrails, and inability to detect the edge 

of the tread due to lack of contrast. 

REVIEW OF INTERNATIONAL CODE REQUIREMENTS FOR EGRESS 

STAIRS 

The building regulations summarized below are from countries that are widely disbursed 

and culturally different, and should be representative of any range of methods used to 

treat egress. Where the original code documents are published in languages other than 

English, the codes used were official translations published in English, so they can be 

presumed accurate. 

Characteristics of egress stair design include the factors previously discussed as well as 

the minimum number of egress stairs provided, maximum travel distance to a stair, width 

of doors and passageways, interior finish, lighting and marking, headroom, and handrails. 

All of the codes examined address all of these characteristics and all are reasonably 

consistent in requirements. For example, all require handrails on both sides of an egress 

stair and a center handrail when stair width exceeds 1800 mm (71 in). It is interesting to 

note that, while all the codes require a minimum of two egress stairs from every floor, 

many of the codes address specific conditions in which a single stairway is permitted, and 

some are quite liberal in this regard. Design occupant loads are generally consistent 

among most building regulations in the world. 

United States 

In the United States model building regulations are developed by private sector, nonprofit organizations and are adopted by State and local governments with modifications 

that reflect local needs and practices. Two such model codes exist but the requirements 

of both are reasonably consistent, especially with respect to egress system design. A 
detailed review of current US building code requirements for egress system design is 

provided by Bukowski and Kuligowski17. 

In the US codes, the minimum width for an egress stair is 1100 mm (44 in) except that a 

space served by no more than 50 people can have a 900 mm (36 in) stair. The capacity of 

a stair (number of people served per floor) is 0.3 in per person unsprinklered and 0.2 in 

per person sprinklered. Thus, for the 1100 mm (44 in) stair the capacity is 147 

unsprinklered and 220 sprinklered. As an example, the design occupant load for offices 

specified in US building regulations is 10 m2 per person (100 ft2 per person). 

 

Australia 

In Australia the legal responsibility for regulation of buildings rests with the States and 

Territories. Since 1996 a national model building code, the Building Code of Australia 

(BCA) has been developed under a mutual agreement (and funding) by the Australian 

Building Codes Board, and is adopted with local modifications by the States and 

Territories. The local modifications are published as individual annexes in the BCA 

document. 

The BCA 200618 is a performance-based code that includes prescriptive requirements 

drawn from the predecessor code as deemed-to-satisfy requirements within the published 

code document. While it is common to use performance analysis to address such issues 

as travel distances, remoteness requirements, or fire resistance requirements for shaft 

enclosures, stair geometries and minimum widths generally comply with the prescriptive 

rules. 

The minimum clear width (between handrails) of a required exit stair serving a storey or 

mezzanine accommodating up to 100 people is 1000 mm (39 in). Stairs serving a storey 

that accommodates more than 100 but not more than 200 must have an aggregate width 

of at least 1000 mm (39 in) plus 250 mm (10 in) for each 25 people (or fraction thereof) 

in excess of 100, and stories accommodating more than 200 must have an aggregate 

width of 2000 mm (78 in) plus 500 mm (20 in) for every 75 people (or fraction thereof) 

in excess of 200 unless the stair or ramp is steeper than 1 in 12, when the additional 500 

mm (20 in) is for every 60 people in excess of 200. Design occupant loads are similar to 

the US codes. For example, for offices the design load is 10 m2 per person which is 

identical to the US 100 ft2 per person. 

Stair geometry is specified as a range for both the riser (rise) and going (tread), along 

with a range of the ratio of twice the riser plus the going (Blondel’s formula). The values 

specified are a riser between 115 mm and 190 mm (4.5 in and 7.5 in), a going between 

250 mm and 355 mm (10 in and 14 in) and the ratio (2R+G) between 510 and 700. 

United Kingdom 

The UK building regulations apply to England and Wales but not in Scotland or Ireland. 

In 1985 UK adopted a performance-based code and converted its prescriptive rules to 
Approved Documents which represent deemed-to-satisfy rules. Approved Document B19

deals with Fire Safety. 

Section 5 deals with Design for Vertical Escape for buildings other than Dwellings, with 

minimum stair widths for simultaneous evacuation on Table 7 and for phased evacuation 

in Table 8. Table 7 lists the maximum capacity (number of persons served) as a function 

of stair width and number of floors. Stair widths range from 1000 mm (39 in) to 1800 

mm (71 in) with the provision that stairs wider than 1800 mm must be divided by a 

handrail. Thus the minimum permitted width is 1000 mm (39 in). For a stair of 1100 

mm equivalent to the US minimum 44 in stair, 260 people can be accommodated on two 

floors, increasing by 40 people for each additional floor. For phased evacuation 120 

people per floor can be accommodated by a 1100 mm (44 in) stair with an additional 10 

people for each 100 mm (4 in) in width. Design occupant loads in Approved Document 

B are somewhat more conservative than in other codes, for example offices are 6 m2

 (60 

ft2

) per person. 

Some insights into the development of the table appear in the supporting text in 

Approved Document B. Provision 4.25 presents an alternative to using Table 7 for stairs 

1100 mm (44 in) or wider to determine number of persons served (P) for simultaneous 

evacuation by applying the formula P=200w+50(w-0.3)(n-1). Note 5 to this formula 

explains that 200w represents the number of persons estimated to have left the stair after 

2.5 min of evacuation and the second term represents the number accommodated on the 

stair at that time. This implies that they are assuming a flow of 45 persons per minute per 

510 mm (22 in) of width and a storage capacity of 50 people per (meter minus 0.3, which 

may represent Fruin’s boundary space) in each story of stairway. This appears to follow 

the 1935 NBS recommendation for a combination of the flow method and the capacity 

method with the assumption that the first 2.5 min of evacuation time is safe. 

It is also interesting to note that Approved Document B suggests that buildings in excess 

of 30 m (100 ft) in height be designed for phased evacuation. This has somewhat 

different stair capacity requirements but also suggests that the capacity should be 

provided by the stairs remaining after any one is discounted as being used by the fire 

service for fire attack. Appropriate arrangements are to be worked out for specific 

building conditions in consultation with the fire brigade. This is in addition to a 

requirement that buildings in excess of 30 m (100 ft) in height be equipped with a 

firefighting shaft including a protected elevator dedicated to emergency response service. 

Spain 

Spain has a national building regulation developed by the Ministry of Housing and issued 

by Royal Decree. Like Australia, Spain recently adopted a performance-based building 

regulation that includes deemed-to-satisfy requirements within the code that are based on 

the traditional rules. Because Spain is a Member State of the European Union (EU) they 

fall under the Construction Products Directive of the EU which requires coordination of 

construction regulations so that the regulations do not become a barrier to free trade 

within the Union. 
The Spanish regulations20 (Section SI 3, table 4.2) assign evacuation capacity (number of 

occupants who may use the stair) of protected stairs as a function of stair width and 

number of floors to be traveled. For unprotected stairs they assign different values for 

travel upward and downward as a function of width. Stair widths ranging from 1000 mm 

(39 in) to 2400 mm (95 in) are covered, implying a minimum width of 1 m (39 in). A 

1100 mm (44 in) wide stair corresponding to the US minimum, can accommodate 248 

people on two floors, increasing by 36 people for each additional floor. 

Occupant loads in the Spanish code are consistent with other countries; e.g., offices are 

10 m2 (100 ft2) per person. Stair geometry is regulated in the code section on protection 

from falling (Section SU) where stair treads must be at least 280 mm (11 in) and risers 

130 mm (5 in) and no more than 185 mm (7 in), with the tread to riser ratio of 4:1 for the 

entire length of the stair. 

Hong Kong 

The Hong Kong building regulations are prescriptive and derive from the British rules. 

Following the return of Hong Kong to Chinese sovereignty their designation as a Special 

Administrative Region (SAR) permitted them to continue using prior regulations. There 

is a separate document for Means of Escape in Case of Fire 21 (MOE). Table 5 

(unsprinklered building) and Table 6 (sprinklered building) are similar to the Spanish 

table, providing a discharge value (number of occupants who may use the stair) as a 

function of width and number of stories. Widths range from 1050 mm (41 in) to 1900 

mm (75 in) in 150 mm (6 in) increments, and values for widths exceeding 1900 mm are 

permitted by linear extrapolation (without limit). A 1100 mm (44 in) wide stair 

corresponding to the US minimum, can accommodate 242 (unsprinklered) or 452 

(sprinklered) people on two floors, increasing by 32 people for each additional floor. 

Occupant loads in the Hong Kong code are only slightly greater than other countries; e.g., 

offices are 9 m2 (90 ft2

) per person. Stair geometry requirements specify treads at least 

225 mm (9 in) and risers not more than 175 mm (7 in). 

The Hong Kong MOE includes a unique requirement for refuge floors in buildings 

exceeding 25 stories in height. Every 25 stories an unoccupied floor (normally a 

mechanical floor) must arrange at least 50 % of its area as an area of refuge (2 h fire 

separation) for occupants to rest or to cross between stairways. They are required to be 

open on at least two sides (with water curtains) above a safe railing height to provide 

natural smoke control. They must be served by a fireman’s lift to facilitate rescue 

assistance. These requirements have also been adopted in other areas of China and in 

some other Asian countries. 

Peoples Republic of China 

Regulations for building fire safety in China are contained in the Code for Design of 

Building Fire Protection22 (GBJ 16-87) and published as a National Standard that is 

locally enforced. Section 5.3 deals with Safety Evacuation of Civil Buildings. 
The total width of stairs is specified per 100 people for buildings of specific height (in 

stories) and construction class (1 through 4) along with a minimum width of any stair of 

1100 mm(44 in). While the definitions of the Chinese construction classes do not align 

completely with those specified in US codes, an approximate correlation is presented 

below. 

Class 1 or 2 buildings (similar to US Type IA and IIIA) require a minimum aggregate 

width of 650 mm (26 in) per 100 people for 1 and 2 story buildings, 750 mm (30 in) per 

100 people for 3 story and 1000 mm (39 in) per 100 people for 4 story or higher . Class 3 

buildings (similar to US Type IIIB) require a minimum aggregate width of 750 mm (30 

in) per 100 people for 1 or 2 story, 1000 mm (39 in) for 2 story and 1250 mm (49 in) for 

4 story or higher. Class 4 buildings (similar to US Type IV heavy timber) require 1000 

mm (39 in) per 100 people for 1 and 2 story buildings. Class 4 buildings are not permitted 

taller than 2 stories. 

Japan 

Japan regulates building safety through a national law that is promulgated by the national 

government, through the Ministry of Land, Infrastructure, and Transport (MLIT). The 

Building Standard Law of Japan23 (BSL) is utilized nationally without local amendment 

and is enforced by local officials who are empowered to determine compliance or lack 

thereof, but not empowered to issue variances or determination of equivalencies. These 

can only be determined by the MLIT. Unlike most building regulations the BSL 

combines building and zoning regulations. 

Beginning in 2000 the Building Standard Law was revised to a performance-based 

structure with the prescriptive rules moved into the Building Standard Law Enforcement 

Order to facilitate updating and interpretation. 

Chapter II, Article 23 deals with stairs which may be through stairs or escape stairs. This 

article establishes a minimum stair width of 1200 mm (47 in) with stair treads of 240 mm 

(9.5 in) or more and a rise of 200 mm (8 in) or less. 

Chapter V of the BSL Enforcement Order deals with Evacuation Facilities. Article 120 

addresses through stairs provided for access and egress, and article 121 requires that at 

least two through stairs be provided. Article 122 requires that certain through stairs be 

designated as escape stairs and be constructed in accordance with article 123. None of 

these articles address stair width or geometry. Article 124 requires that the aggregate 

width of escape stairs in stores (mercantile) be not less than 600 mm (24 in) per 100 m2

(1000 ft2) of floor area of the largest floor served by the stair. 

It does not appear that the Japanese code incorporates the concept of occupant load but 

rather specifies required stair capacity in terms of the floor area served, which is the 

equivalent of applying a uniform occupant load to all use categories in the group. The 

Japanese code typically groups assembly type uses, educational, mercantile, and 

residential uses when specifying minimum widths of stairs, doors, and corridors. 
Part 2: New Thinking on Egress from Buildings 

BUILDINGS ARE TALLER 

At the start of the 20th Century the 

tallest building in the world (Park Row 

Building in Manhattan) stood just 391 

feet (119 m). By 1913 the record 

height had doubled with the completion 

of the Woolworth Building at 792 feet 

(241 m). Record heights crossed the 

thousand foot mark in 1930 with the 

Chrysler Building (1046 feet, 319 m)) 

and went to 1250 feet (381 m) with 

completion of the Empire State 

Building the following year (1931). 

The World Trade Center (1368 ft, 417 

m in 1971), Sears Tower (1450 ft, 442 

m in 1974) and Petronas Towers (1486 

ft, 448 m in 1998) ended the century 

with the tallest building record just 

short of 1500 feet (457 m). 

The early 21st Century has seen a rapid 

surge skyward, first in Asia and then in 

the Middle East. The height record (as 

of this writing) is Taipei 101 at 1671 ft 

(510 m), but several taller buildings are 

under construction. The Burj Dubai 

Tower is estimated to top out at no less 

than 2624 ft (800 m) and there is said 

to be a building planned for elsewhere 

in the Middle East at 3280 ft (1000 m). 

Prior to the World Trade Center 

Towers, nearly all the tall buildings 

were tapered (or stepped) for a number 

of reasons, ranging from reducing 

structural loads to not casting shadows on 

neighboring buildings (see Fig 2). This meant 

that the occupant load decreased on the higher floors, decreasing the cumulative load on 

the stairs, unless there was an assembly use at the top such as an observation deck or 

restaurant. Today, most of the taller buildings are uniform with height with no such 

decrease in the number of occupants served by the stairs. 

Figure 2 - Timeline of World's Tallest Buildings 

Source: Skyscraper Museum (used with permission) 
In tapered buildings the number or width of the egress stairs can be increased for the 

lower floors to accommodate the increased occupant load. However, for tall buildings of 

uniform cross section the cumulative occupant load can lead to congestion, and the 

number of stairs needed to accommodate simultaneous egress would require so much 

space that the building would not be economical. Here, emergency plans rely on phased 

evacuation; but this should theoretically include fire endurance for any building elements 

that might impact egress at least as long as the required egress times. 

Up to and including the Empire State Building these tall buildings were typically Type 

1A (4 hr) construction. Thus a reasonable limit on egress times would be 2 h (the 4 h fire 

resistance time with a safety factor of 2). From a flow and egress time perspective, using 

a conservative 1 min per floor descent rate would limit heights of Type 1A buildings to 

100 floors. A rate of 50 floors per hour is used to allow for notification, pre-movement, 

and horizontal travel times as well as some rest stops in the stairs. 

Beginning with the World Trade Center Towers it became common to permit Type 1B 

construction (3 hr). With an occupant load of 390 per floor (110 floors) and three egress 

stairs (6 1⁄2 units of total exit width) total (simultaneous) evacuation times were estimated 

at about four hours (including congestion, queuing, and transfer corridors), which is 

consistent with the egress times observed in the 1993 bombing. Applying the criterion of 

a total egress time of half the fire resistance time, one would want the building to 

withstand a fire for 8 h or until complete burnout of all combustibles, whichever occurs 

first. 

PEOPLE ARE BIGGER AND LESS FIT 

Morbid Obesity Rate (BMI>30)

0

5

10

15

20

25

30

35

1980 1985 1990 1995 2000

Year

Percent

Japan

Norway

France

Sweden

Finland

Spain

Canada

United Kingdom

Australia

United States

In the early 20th Century the 22 

in (1.1 m) unit of exit width 

was sufficient for the 95th

percentile US adult male. In 

the 21st Century this is no 

longer the case. The American 

male is larger with a 95th

percentile shoulder width of 20 

in (510 mm), which requires a 

28 in unit if exit width and a 56 

in (1.4 m) stair (applying 

Templer’s 4 in on each side to 

account for body sway and 

personal space). Templer did 

not allow the 4 in to a handrail 

since people were narrower at the waist and hip than at the shoulder, but this is no longer 

true. 

Figure 3 - Obesity rates by country 

Data: International Obesity Task Force, EU Briefing paper, 2005 

The physical condition of the average occupant is such that the exertion of descending 

many flights of stairs is no longer possible without frequent rest stops and a slower pace. 
High percentile (95th) average weights have increased substantially in recent years (Fig 3), 

and the number of people reporting that they need assistance due to physical conditions 

has also increased. 

The WTC Investigation found 6 % of occupants reporting the need for assistance in 

traveling down stairs. These increases can be attributed to the widespread adoption of 

modern accessibility regulations that have made it easier for people with mobility 

limitations to be more active in society, and to the recognition that there are many 

conditions not generally considered to be disabilities that can limit the ability of people to 

move down many flights of stairs. 

NEW TECHNOLOGIES 

A number of new technologies are available to address limitations of the past. One of the 

most promising is the ability to design and operate elevators safely and reliably during 

fires. Here the ability to provide reliable power, sophisticated operational protocols, and 

real time monitoring of critical functions, permit the use of protected elevators as a 

primary means of egress in fires. Further, fire departments have recognized the need for 

protected elevators to provide logistical support to operations in tall buildings. 

Modern fire alarm systems combine reliability, flexibility, and advanced functionality 

that permits real time monitoring and tactical support for incident management not 

previously possible. With the advent of the industry 

standard24 fire service interface in the U.S., it is 

possible for the incident command to actively 

manage the evacuation process for improved safety 

and efficiency. For example, it is possible to 

monitor conditions in the stairways in real time and 

to advise occupants to change stairs to avoid 

congestion, especially where crossover corridors or 

refuge floors are provided. Inexpensive cameras 

and digital image processing software make it 

possible to present images for action to fire service 

personnel only when issues arise, permitting a few 

personnel to monitor many locations. 

Advances in photoluminescent materials now 

permit issues of lighting levels and contrast within 

stairs to be addressed without the need for complex 

emergency power systems(Fig 4). When fully 

charged by continuous ambient light these materials 

provide more light for longer periods and can be 

applied to highlight stair nosing and handrails and 

as path lighting in transfer hallways. 

Figure 4 - Photoliminescent materials can be 

used to illuminate stairways 

Source: NRC Canada (used with permission) 
REFUGE FLOORS 

The incorporation of refuge floors in tall 

buildings in Asia also represents a new 

approach worthy of review. As 

discussed earlier, these are arranged 

every 20 to 25 floors (generally on 

mechanical floors) to provide a protected 

area for occupants to rest temporarily on 

their journey down the stairs and to 

cross over from one stair to another. 

Refuge floors are also intended as 

protected space in which people with 

disabilities can await rescue by the fire 

department (fire service elevators are 

generally arranged to be able to stop at 

the refuge floors). Figure 5 - Refuge floor in a Hong Kong high-rise during a 

fire drill Source: Arup (used with permission) 

Since these requirements are fairly new there have been no emergency evacuations of 

buildings so equipped, but there have been evacuation drills that have shed some light on 

performance (Fig 5). One issue identified in drills is that many occupants decide to wait 

on the refuge floor for the “all clear” which fills the available space, preventing 

additional people to enter the floor from the stair. Without wardens or fire service 

personnel stationed on the floor to keep people moving, the purpose is defeated. It is not 

clear in a real emergency if this will be a problem since occupants may be motivated to 

get completely out of the building. 

PERFORMANCE METRICS FOR EGRESS SYSTEMS 

In the U.S., Australia and Japan, the design of egress systems are based on the population 

of the largest, single floor. In U.K., Spain, and China the number of floors served by the 

stair impacts the total number of people served by a stair of a given width. Yet in any 

performance analysis of an egress system in these or other countries, regulators require a 

timed egress analysis to estimate Required Safe Egress Time (RSET) which is compared 

against Available Safe Egress Time (ASET). ASET is generally determined by fire 

modeling to estimate conditions in the egress path that might lead to injury or death. 

Clearly the appropriate performance metric is time, yet there is no regulatory 

performance objective nor design criterion for egress systems in terms of time in any of 

the codes examined. As early as 1914 the Safety to Life Committee recognized that 

designing egress stairs on the basis of flow (of occupants down the stair) required some 

“assumed time in which it is safe to exit the building” which is the ASET mentioned 

above. 

For compliance with the life safety objective of any building regulation, it is reasonable 

to determine ASET as the time to reach potentially incapacitating or lethal conditions 
anywhere within the means of egress. Considering the fact that the means of egress is 

designed to protect people within it from exposure to fire or smoke, it is also reasonable 

to assume that a limiting ASET would be the time to fire-induced partial structural 

collapse. If burnout occurs before collapse, there is no theoretical limit to the ASET 

though a reasonable value may be prescribed instead. Currently, the fire resistance time 

of structural components and assemblies rated by a standard fire exposure in units of time 

has not been shown to correlate with the time to structural failure of the component, 

assembly, or system as a whole for an arbitrary real fire. Clearly, some factor of safety 

that is appropriate for the level of uncertainty is needed in order to approximate ASET by 

the fire resistance rating in cases where local collapse is estimated to occur before 

burnout. 

Based on these arguments an appropriate performance objective for egress systems 

design may be that the time needed for total evacuation of the building be less than the 

required fire resistance time for the primary structural frame (i.e., the columns and other 

structural members including girders, beams, trusses, and spandrels having direct 

connections to the columns and bracing members designed to carry gravity loads). 

Where the fire resistance time is determined by test against a standard fire curve (i.e., 

ASTM E119 or ISO 834) a safety factor could be applied that is large enough to account 

for the variability of egress performance and the uncertainty in the fire resistance time 

representing the time to failure of at least one element of the primary structural frame. 

Where the fire resistance time is determined by engineering analysis following the 

Natural Fires or other similar method which accounts for the actual design level fire event, 

some lower safety factor could be applied 

since the predicted time to structural failure 

would be expected to be less uncertain 

although the egress variability would be the 

same. For the sake of this discussion factors 

of safety of 2.0 and 1.5, respectively will be 

used; however, the actual values represent 

policy decisions that need to be established 

through the model code development and 

regulatory adoption processes. 

Since high-rise buildings in any of the codes 

examined for this paper are required to 

have a primary structural frame of not less 

than 3-h fire resistance as determined by 

standard test, the total evacuation time would be required to be no more than 1 1⁄2 h, 

reflecting the safety factor of 2.0 discussed above. Recent analyses have shown that 

evacuation times of 1 h may be achievable with stairs in buildings up to about 50 stories 

and with a combination of protected elevators and stairs for most buildings of any height, 

without increasing the number or size of the stairs or elevators above current practice; 

this objective is clearly practicable25. 

Figure 6 – Interior of skybridge connecting the 

Petronas Towers at mid-height Source: Bukowski 

(used with permission) 
These same arguments can be applied to the issue of a performance objective for fire 

department access. Since U.S. codes require provision of 30 min of local water supply in 

high rise buildings, it is reasonable to say that it should be possible for the fire 

department to be able to put water on a fire at any height within 30 min. The 30 min 

local water supply requirement includes both sprinkler and standpipe flows, and in that 

initial 30 min there would be no standpipe flow, so this results in a safety factor. Using 

protected elevators fire departments can meet the 30 min objective for buildings of any 

height. Using fire department response time and a conservative estimate of 2 min per 

floor for ascent with equipment it is straightforward to determine the height threshold for 

fire service access elevators. 

RETHINKING EGRESS SYSTEMS DESIGN 

Some will argue that, other than some specific extreme events like the World Trade 

Center attacks, there have been no reported failures to evacuate even the tallest buildings. 

Therefore there is no reason to change what has been done for more than 100 years. 

There are several things wrong with this argument. 

First, there have been some failures in tall building evacuations. A real evacuation of 

both Petronas Towers for a bomb scare in 2001 resulted in an evacuation time of several 

hours when the skybridge (Fig 6) jammed with occupants crossing to the opposite tower 

in accordance with the original evacuation plan. By incorporating elevator egress for 

floors above the skybridge the total time for total (simultaneous) evacuation of both 

towers was observed in a drill to be 20 min26 . For Taipei 101 an evacuation drill 

conducted prior to opening showed a total 

evacuation time of about 2 1⁄2 h. The fire brigade 

reported being uncomfortable with this time. 

Incorporating protected elevators for egress from 

the upper floors reduced that time to just under one 

hour27. 

As discussed previously, buildings have become 

much taller and heights continue to increase beyond 

the ability of anyone from the upper stories to 

egress down stairs. Also, these buildings are much 

less tapered with height compared to early 

skyscrapers, with larger occupant loads in the upper 

floors. People today are larger and less fit, and 

accessibility regulations have resulted in a growing 

fraction of occupants with mobility limitations 

requiring egress assistance. Increasingly, there are 

people in buildings who cannot be moved down 

stairs under any circumstances (Fig 7). Following the collapse of the WTC buildings, 

occupants of tall buildings are reluctant to delay egress and are not comfortable with long 

egress times. 

Figure 7 –Typical powered wheelchair with 

attached ventilator is used by people with 

severe disabilities such as Christopher 

Reeve. He could only survive away from his 

chair for a few minutes, and he and the 

chair weighed 300 kg (660 lb) (photo used 

with permission) 
STRAWMAN EGRESS SYSTEM PERFORMANCE METRICS 

Based on the prior review and discussion the following suggestions are provided to 

facilitate the needed rethinking of the philosophy and details of egress system design for 

buildings. Thresholds and performance levels are public policy decisions that need to be 

made in conformance with the methods in place for making regulatory decisions in an 

individual country. These usually require public consultation and a legislative or 

administrative process. The suggestions provided in the following sections include 

provisions that would be formatted as code requirements (in bold) followed by 

explanatory material. 

Performance Objectives 

Buildings shall be designed and arranged such that the responding fire brigade can 

access a fire on any floor and begin suppression operations within 30 minutes of the 

transmission of the original alarm, 95 % of the time. 

It is expected that this objective will require the provision of a fire service access elevator 

in buildings with occupied floors more than 30 m (100 ft) above the level of fire 

department access. The 95 % criterion is intended to recognize that conditions may exist 

that prevent the objective from being met on occasion, but that most expected conditions 

should be considered. The 30 min limit is based on the U.S. requirement for local water 

supply for automatic suppression systems. 

Buildings shall be designed and arranged such that 98 % of the expected occupants 

are able to evacuate the building without outside assistance in a time not exceeding 

half of the required fire resistance time of the primary structural frame. 

In the U.S. model building codes buildings taller than 4 stories are required to be Type I 

construction with either 3-h (Type IA) or 2-h (Type IB) fire resistance as determined in 

ASTM E119 for elements comprising the primary structural frame. Thus the maximum 

total egress time (applying a safety factor of 2.0) would be either 1.5 h or 1h, respectively. 

The 98 % reflects an expectation that there may be some occupants who will require the 

assistance of the fire brigade for egress, but that even most occupants with disabilities can 

either self-evacuate or require only assistance from other occupants. It is expected that 

this requirement would result in occupant egress elevators in Type IA buildings taller 

than 80 stories and Type IB buildings taller than 50 stories. 

Protected Fire Service Access Elevators 

Where required, at least one elevator serving every floor shall be designated for use 

by the fire service in emergencies. This elevator shall be powered by normal and 

emergency power, with both power and control wiring protected by fire resistant 

construction at least equal to the fire resistance requirement applicable to the 

primary structural frame. Any alarm transmission to the fire brigade shall result in 

the designated fire service elevator being taken out of normal service and recalled to 
the designated level. The designated fire service elevator shall open on every floor 

into a protected lobby with direct access to a building stair containing a standpipe 

and any other required equipment for fire department use. The elevator equipment 

shall be protected from compromise by water from sprinklers or firefighting. 

Reliable communication with fire service personnel using the elevator and 

monitoring of critical functions in the fire command center shall be provided. 

Fire service access is not normally considered part of the egress system design except to 

the extent operations within the building can have an impact on the evacuating occupants. 

The only code found where this is explicitly discussed is in UK Approved Document B 

which suggests that in tall buildings one stair may need to be discounted because it will 

be blocked by operations on the fire floor and one or two floors below. It should also be 

recognized that this “attack stair” may be compromised on any floor(s) above the fire due 

to smoke leakage into the stair from the fire floor when the hose is advanced onto the 

floor, since the standpipe is located within the stair. Only where a vestibule is provided 

(in the U.S. called a smokeproof tower or fire tower) would the stair itself be kept mostly 

smoke free. However, the evacuation assistance provided by the fire service, especially 

by the fire service elevator, will have a significant impact on the success of the 

evacuation. 

Protected Occupant Egress Elevators 

Where required, all elevators except any designated fire service elevator(s) shall be 

designed and arranged to permit their safe use for occupant evacuation. These 

occupant elevators shall be powered by normal and emergency power, with both 

power and control wiring protected by fire resistant construction at least equal to 

the fire resistance requirement applicable to the primary structural frame. 

Occupant egress elevators shall operate in a hoistway protected from the adverse 

effects of water and opening into a protected lobby on each floor that serves as an 

area of refuge while awaiting the elevator. The lobby shall be sized to accommodate 

75 % of the occupant load of the floor at 0.5 m2

 (5 ft2) per person. Elevator lobbies 

shall have direct access to an egress stair and be provided with two-way 

communications to the fire command center and approved means to provide real 

time information to waiting occupants. 

On a fire alarm the elevators shall begin evacuating occupants of the fire floor and 

two floors above and below the fire floor, taking them to the level of exit discharge 

before returning for another load until all 5 floors are evacuated. On a decision for 

a full building evacuation by the official in charge the elevators shall evacuate all 

remaining occupants from the highest floors and proceeding downwards, shuttling 

occupants to the level of exit discharge before returning for another load. 

Information systems on all floors except the level of exit discharge shall 

communicate to occupants the status of the system and the estimated wait time. 

Information systems on the level of exit discharge shall indicate that the elevators 

are out of service and people should not enter. 
Occupant egress elevators are by far the fastest means of evacuating a tall building. In 

normal service the number, size, and speed of passenger elevators in most buildings are 

designed to be able to move approximately 10 % of the total population of the building 

from random floors to the level of exit discharge in 5 min. This means that any building 

of any height can be totally evacuated by elevator in one hour or less without increasing 

the number, size, or speed of the elevators normally provided. Modern elevators utilize 

computerized controllers capable of sophisticated operational protocols, and the addition 

of real time monitoring and information systems can add the necessary reliability and 

guidance to users to permit their use for egress during fires. In addition, occupants use 

the building elevators every day for normal ingress and egress. The use of the elevators 

for egress in fires is not appreciably different as long as the people are provided with 

sufficient information to make decisions. 

Considerable effort by NIST, the elevator industry, the American Society of Mechanical 

Engineers (ASME), and the building code organizations is being put into the 

development of standards and code requirements for protected elevators both for fire 

service access and for occupant egress. The language suggested above for both purposes 

is consistent with that effort but the activity needs to play out to arrive at a consensus on 

requirements for the elevator equipment. Similarly, proposals are being considered by 

the model building code organizations for the building code related parts of the systems. 

Again, the performance objectives above are consistent with those proposals but the 

process should play out to arrive at a consensus of the involved parties. 

Stair Width 

Where stairs are the primary means of vertical egress in fires and other emergencies 

such stairs shall be a minimum width of 1400 mm (56 in). Where protected 

elevators are provided as the primary means of vertical egress in fires and other 

emergencies stairs shall be a minimum width of 1100 mm (44 in). 

Wider stairs are needed to accommodate the increased body size of occupants but this 

additional width is not needed where most occupants would be expected to egress by 

elevator. Further, any building provided with occupant egress elevators would also have 

fire service access elevators, eliminating the issue of counterflow except for the fire floor 

and one or two floors below in the attack stair. 

Stair Capacity 

Where stairs are the primary means of vertical egress in fires and other emergencies 

sufficient stair capacity shall be provided to accommodate the maximum number of 

building occupants on all floors except any with direct access to the outside, within 

the stairways. Where protected elevators are provided as the primary means of 

vertical egress in fires and other emergencies sufficient stair capacity shall be 

provided to accommodate at least half the maximum number of building occupants 

on all floors except any with direct access to the outside, within the stairways. Any 

floor containing an assembly space that results in a higher occupant load when 
provided with an area of refuge sized to accommodate 100 % of the occupant load 

of that floor with direct access to a stair and an occupant egress elevator, shall be 

permitted to neglect the additional occupant load for the purpose of determining 

stair capacity. 

While the philosophy of “storing” stationary occupants in stairs is not intended, the stairs 

remain a more protected space within the building. Some occupants may choose to use 

stairs (particularly on the lower floors) and the ability to find refuge in stairs is an 

important redundancy. Where occupant egress elevators are provided it is expected that 

most occupants will use them, but some minimum capacity for refuge is needed. 

Likewise, the provision of assembly occupancies (restaurants, bars, conference facilities, 

observation decks, ...) that result in local concentrations of additional people traditionally 

required additional stair capacity that continued through the building to the level of exit 

discharge. With occupant egress elevators most of these people will egress by elevator 

and it is only necessary to accommodate them temporarily while they await the elevators. 

The capacity of an egress stair is defined as the number of occupants who can 

descend the stair in a time equal to one half the fire resistance time of the primary 

structural frame, at a flow rate of 47 occupants per minute per meter of stair width 

(26 occupants per minute per unit of exit width). (The design flow rate is a policy 

decision among (81, 47, or 30) occupants per minute per meter with 47 being used in 

this example). 

The number of occupants that could be served by a stair should be based on the flow or 

discharge rate (Flow Method as defined in the 1935 NBS report) over 1h (Type IB) or 1 

1⁄2 h (Type IA). Table 1 below shows discharge rates per hour, per (22 in) unit and per 

meter, and for a 1100 (44 in) and 1400 (56 in) stair, based on assumed flows of 45 

occ/min/unit (which has no specific scientific basis but is used in the UK Approved 

Document B), 26 occ/min/unit (based on the work of Togawa and Pauls), and 16 

occ/min/unit (based on estimates from the NIST WTC report). If one used a flow rate of 

26 occ/min/unit, a single 1100 mm (44 in) stair could serve 2160 people (4320 for two 

stairs) and a 1400 mm (56 in) stair could serve 2750 people (5500 for two stairs). This 

would be the total number of occupants served by the stair(s) on all floors except those 

with direct egress to the outside (who would not use the stairs). This would also be 

limited to 50 or 80 stories since it would take longer than (1 or 1 1⁄2) hours to descend 

from greater heights. 

Table 1 – Total Number of Occupants Served by an Egress Stair Based on Flow 

Flow 

occ/min/unit 

Flow 

occ/min/meter 

Dis Rate 

occ/h/unit* 

Dis Rate 

Occ/h/meter*

Dis Rate 

Occ/h/1.1 m* 

Dis Rate 

Occ/h/1.4 m* 

45 81 2700 4860 5350 5830 

26 47 1560 2820 3100 3380 

16 30 960 1800 1980 2160 

* Multiply by 1.5 for Type IA Construction where ASET is 1 1⁄2 h 
In a system designed by the flow method it is important to ensure that the flow through 

doors is equal to or greater than the stair flow so that flow restrictions and congestion is 

avoided. This raises an interesting issue. There seems to be consensus that flow through 

doors is about 60 occupants per minute per door regardless of width. If a design stair 

flow rate of 26 occupants per minute per unit is selected, the stair flow in a 2-unit (44 in, 

1100 mm) stair is 52 per minute which can be accommodated by a single door. In a 2 1⁄2 

unit (56 in, 1400 mm) stair the flow is 65 per minute which will theoretically result in an 

accumulation of 5 people per minute at the upstream side of a single door. It needs to be 

determined if 56 in stairs need double doors to prevent congestion, as this can be a 

significant cost issue in design. 

Stair Geometry 

Current requirements in the building codes reviewed are consistent with the research 

recommendations. 

Refuge Floors 

Horizontal transfer corridors designed as means of egress components shall be 

provided every 25 floors (generally on mechanical floors) to link all egress stairs and 

to provide the ability to safely move between stairs. 

The initial experience with refuge floors indicates that the provision of a means to 

transfer stairways has merit but the provision of a large space to “rest” encourages delays 

in evacuation that may be counterproductive to safety. The protected lobby with direct 

access to a stair provided as part of the occupant egress elevator system can provide for 

rest stops if needed, but most occupants would use the elevators and not need to rest. 

Thus, it is recommended to not provide refuge floors but to consider a protected 

horizontal transfer corridor linking all stairways on the mechanical floors. The 25 floor 

increment is based on the Asian requirement for refuge floors but could be flexible where 

only stairway crossovers are provided. 

Evacuation Management 

Video cameras shall be installed every 5 floors in every egress stairway and in every 

egress elevator lobby with the images displayed in the fire command center. Image 

analysis software shall be employed to minimize the fire department burden for 

monitoring these images. 

The potential for changing conditions impacting the safety and efficiency of the 

evacuation suggests that full building evacuations need to be actively managed. The 

addition of cameras in the stairs and egress elevator lobbies which can be monitored in 

the fire command can facilitate such management. Using available software developed 

for the security industry the monitoring burden in fire command can be minimized. 

Images from cameras in stairs would be kept in background except when no movement is 

detected for some time interval, indicating no people or no movement of people. Images 
from lobby cameras would be kept in background except when movement is detected, 

indicating there are people in the lobby needing pickup. If that floor has not yet been 

evacuated this information can be registered with the elevator controller. If the floor has 

already been evacuated the presence of stragglers would be noted to the fire department. 

Placement every 5 floors is considered reasonable for the purpose. More frequent is 

probably not necessary for active management of egress but less frequent might permit 

unobserved blockages. 

CONCLUDING REMARKS 

The increasing height of buildings coupled with changing demographics and public 

concerns about the safety of tall buildings have led to a call for a fundamental rethinking 

of egress systems. This paper provides a review of the approaches currently found in 

building regulations internationally, and attempts to identify the origins of these 

specifications including the extent to which they may be based on scientific data or 

consensus opinion. The case for moving to a performance metric of time is presented and 

a set of criteria for evaluating egress systems against safe egress time is suggested. 

Performance criteria based on practical objectives are suggested but these and suggested 

regulatory thresholds need to be vetted through the existing consensus process of model 

code development and regulatory adoption followed in the adopting jurisdiction. The 

result should be a design approach that addresses the needs of occupants and buildings of 

all heights with criteria based on sound engineering principles. 

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