Charter Hall
121-127 Harrington Street
Fire Engineering Report
221298
Issue | March 2011
Arup
Arup Pty Ltd ABN 18 000 966 165
Arup
Level 10 201 Kent Street
Sydney NSW 2000
www.arup.com
This report takes into account the particular
instructions and requirements of our client.
It is not intended for and should not be relied
upon by any third party and no responsibility is
undertaken to any third party.
Job number
221298-00
Document Verification
Job title
Job number
121-127 Harrington Street
221298-00
Document title
Fire Engineering Report
Document ref
221298
Revision
Date
Filename
Issue
01/03/11 Description
Name
File reference
0001 Fire Engineering Report.docx
Issue to Charter Hall for comment
Prepared by
Checked by
Approved by
Ted Moyle
Marianne Foley
Marianne Foley
Prepared by
Checked by
Approved by
Prepared by
Checked by
Approved by
Prepared by
Checked by
Approved by
Signature
Filename
Description
Name
Signature
Filename
Description
Name
Signature
Filename
Description
Name
Signature
Issue Document Verification with Document
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX

Charter Hall
121-127 Harrington Street
Fire Engineering Report
Contents
Page
1
Introduction
1
2
Requirements of this report:
1
3
General building description and plans
2
4
Process and approvals pathway
4
5
Fire safety philosophy
5
5.1
6
6
Fire Safety Assessment
6.1
6.2
6.3
6.4
6.5
7
Stair pressurisation
Introduction
Occupancy
Simultaneous evacuation
RSET – Comparative Assessment
Implications for Fire Brigade
Conclusion
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
7
7
7
8
9
14
15
Charter Hall
1
121 Harrington Street
Fire Engineering Report
Introduction
Arup has been engaged by Charter Hall to determine a fire engineering strategy
for 121 Harrington St. The aim is to resolve the current fire stair pressurisation
system concerns and to provide an appropriate approvals route in order for a
current and certifiable Annual Fire Safety Statement to be achieved.
The building has been previously fire engineered, with the strategies and
associated fire safety measures required, stated in the following documents:
Fire Safety Study Report DLA-1C – Fire Modelling and Computing –
March 1999 (This report is the base build fire strategy)
Fire Safety Study Report DLA-1D – Fire Modelling and Computing –
April 2002 (This report is tenant fit out fire strategy)
The requirements of the previous fire engineering reports have been recently
reviewed by the author of the reports, via a site visit on the 2nd June 2010. It is
understood that any requirements from this site visit can be rectified and that the
author will be able to sign off the fire engineering design of the building, with the
exception of the stair pressurisation.
An Interim Annual Fire Safety Statement was provided by Knight Frank in
October 2010 and concluded that the required essential fire safety measures
complied, except for the following areas:
The café kitchen suppression system, signage and emergency lighting
were non compliant. These items can be rectified to meet the required
standards and do not form part of this fire engineering report.
An exception of the above is that the previous fire engineering report required a
stair pressurisation system to be installed in the building. The design of the stair
pressurisation and the sealing of the building is such that the current systems do
not comply, with air actually being forced into some of the stairs. Because of the
heritage nature of the building adding relief air louvers to the building facade is
not an option to overcome this issue. Instead, a separate fire engineered solution
is to be develop by Arup in regards to this matter and forms the basis of this fire
engineering report.
2
Requirements of this report:
Based on this fire engineering strategy, in regards to the stair pressurisation
system, the following fire safety measures are to be implemented to the building:
The 3 fire stairs do not require a smoke pressurisation system as per the
analysis and requirements of this fire engineering report.
Smoke seals are to be provided to all of the fire stair fire doors. The smoke
seal are to be of medium type that can withstand up to 200°C for 30
minutes and be applied to 3 sides of the self closing door i.e. top and sides.
The EWIS is to be amended to provide the simultaneous evacuation for the
whole building.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 1
Charter Hall
3
121 Harrington Street
Fire Engineering Report
General building description and plans
The Bushells Building at 121-127 Harrington Street is a 7-storey building.
The effective height is below 25 m.
The ground floor contains a car park (Class 7) and a café (Class 6).
Levels 1 to 6 are office use (Class 5).
An atrium extends from ground floor up to level 6 and utilises a natural
ventilation smoke management strategy.
The required type of construction is A, however, its present structural
system is approximately related to type B (see previous fire engineering
report for details).
The 3 fire isolated stairs are provided on the office floor plates.
Sprinklers are provided with fast response heads to AS2118.
Building detection and alarm are provided to AS1670.1.
EWIS is provided to AS2220.2.
Hydrants and hose reels are provided to AS2419 and AS2441.
The office floors are smoke separated from each other and are also smoke
separated from the atrium.
The separation of the office floors from the atrium is largely by virtue of
the original external brick walls of the building, along with the original
(sealed) windows and newer doors to each floor from the bridges across
the atrium.
Note that the above list is not the full description of building’s fire safety
measures, but is shown to place this report into context.
Figure 1: Location of the building
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 2
Charter Hall
121 Harrington Street
Fire Engineering Report
Figure 2: Ground floor plan view
Figure 3: Level 1 plan view
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 3
Charter Hall
121 Harrington Street
Fire Engineering Report
Figure 4: Level 2 to level 6 general plan view
4
Process and approvals pathway
The currently installed stair pressurisation system was signed off by the Certifier
at the time, even though it had been tested and deemed non compliant (in relation
to air flows through the stair doors). Due the unavailable relief paths as described
in this report, a compliant stair pressurisation is unachievable.
It is recommended that Charter Hall request Council to provide a Fire Order to the
building, solely in relation to the stair pressurisation system but requiring the
work as described in this report (as all other fire safety aspects can comply with
the previous fire engineering reports, and comply with the BCA/ relevant
Standards at the time of construction).
Arup and Charter Hall are to liaise with the Council and communicate the fire
strategy proposed to omit the stair pressurisation system. Ideally the end result
would be a Fire Order requiring works to comply with this fire engineering report
by Arup.
Therefore with the rectification works to the café, a sign off of the original fire
engineering design by the previous author, and the alternative design from Arup
with deletion of the stair pressurisation system; a compliant Annual Fire Safety
Statement for the building should be achievable and a the required level of fire
safety for the building will be reached.
This approach is considered preferable to simply providing a fire engineering
report addressing the deletion of stair pressurisation to meet the performance
requirements of the BCA as there is a risk that neither the fire brigade nor a
Certifier would agree that the deletion of stair pressurisation met the Performance
Requirements for a new building when the base building design required it.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 4
Charter Hall
121 Harrington Street
Fire Engineering Report
Rather, the fire order approach acknowledges the constraints of the existing
heritage building and provides solution to give an acceptable level of fire safety
within these constraints.
5
Fire safety philosophy
The following discusses the general fire safety philosophy of the building and
demonstrates the main principles that provide sufficient safety to occupants and
Brigade during a potential fire event.
The atrium strategy utilises natural smoke ventilation based on a low fuel load at
the base of the atrium. The smoke is generally dilute due to the large volume and
roof venting available, with the doors and walls to the office floor plates separated
by smoke proof construction. This analysis is quantified in the previous fire
engineering reports.
The egress routes from the offices do not require occupants to enter the atrium and
the risk of fire and smoke conditions affecting office occupants during egress is
low. A worse case fire scenario would more likely be a fire in the office than in
the atrium. On this basis the risk of fire/smoke spread, occupant evacuation and
Brigade Intervention have to be considered below.
The offices utilise fast response sprinkler heads throughout to limit and control the
fire size. The floors are smoke sealed from each other (they are timber hence not
fire separated) and also to the atrium, and a requirement of this report is also to
provide smoke seals to the fire stair fire doors. Therefore sprinkler cooled smoke
has a low risk of migrating through the building, with the lifts being in the atrium,
reducing the risk further.
The building has low occupant numbers compared to the generous egress widths
provided by the 3 fire stairs, allowing for quick evacuation if occupants are given
early warning. The smoke detection system has quick detection due to close
spacing of detectors with the AS1670 system. The building also has lower premovement times compared with a BCA DtS complying design, due a pre recorded
verbal announcement alarm system (in lieu of just a toned alarm).
Brigade intervention is provided by easy access on both sides of the building from
the main road and being situated in the city centre (short arrival times). The floor
plates are relatively small with generally clear sightlines for fire fighting activities
and search and rescue operations. A compliant hydrant system is provided for fire
fighting activities and is located outside the choice of fire stairs on each office
floor.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 5
Charter Hall
5.1
121 Harrington Street
Fire Engineering Report
Stair pressurisation
As the building is less than 25m in height, the stairs in the office floors are
required to be pressurised due to the associated risk with the atrium (as per the
BCA). If no atrium were provided then the fire stairs on the office floors would
not require stair pressurisation. Stair pressurisation is also a provision of the
existing fire engineering report of the building.
A stair pressurisation system has been installed into the 3 fire stairs. The stair
pressurisation system as it is currently would provide a worse smoke environment
when operating in a fire event, as the air velocity coming out of one stair would
force smoke into another stair, adversely affecting egress and fire brigade access.
The fire stairs do not comply with AS1668.1 as the required airflows of 1m/s
through the fire stair doors during testing are not achieved. The issue is that there
are insufficient air relief paths through and out of the building, due to a
combination of the following factors:
the fire engineering report requires all office floors to be smoke sealed
from each other and smoke sealed from the atrium.
the heritage nature of the building requiring the current non openable
façade windows to be maintained and also preventing provision of new
openings and penetrations within the heritage fabric.
no mechanical relief is provided on the office floors as air handling
systems are required to shut down in fire mode.
The impact is that smoke can enter the stair in a fire event, spread to non fire
floors and also effect egressing occupants and arriving fire brigade. Due to the
restrictive elements of the building that do not allow a relief air path for the stair
pressurisation system, a fire engineered solution is required to be provided.
The fire engineered solution proposed is considered to provide an improved level
of safety to that of the originally certified non compliant stair pressurisation
system.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 6
Charter Hall
121 Harrington Street
Fire Engineering Report
6
Fire Safety Assessment
6.1
Introduction
To understand the risk of omitting stair pressurisation and the associated risk of
occupants evacuating the building, the following two assessments are provided:
1. For a simultaneous evacuation, the assessment demonstrates that all office
occupants can fit into the 3 fire stairs at once. This reduces the risk that
they will be queuing on a non-fire floor if smoke or fire spreads to that
floor. Occupants can be protected in a fire rated shaft with smoke seals to
the fire doors.
2. A comparative assessment to show that the current building design will
have a shorter overall evacuation time (RSET) than a building designed in
accordance with the current DtS provisions. This is performed by
assessing the travel distances and occupant flow times into the 3 existing
fire stairs versus a DtS case using compliant travel distances and the
associated occupant loading and flows using a DtS scenario that also uses
3 fire stairs.
6.2
Occupancy
The net floor area of the offices ranges from 995m2 to 1018m2 (as shown in the
Denny Linker floor plans dated 24-11-2000) The occupant load in each storey
would be approximately 100 persons based on the occupant load density of
10m2/person as per BCA.
Floor
Total office NLA
Occupant density
Occupant load for offices
L1~L6
6000m2
10m2/person
Total occupant load of 600 people
100 persons per floor
34 persons per floor/per fire stair
In terms of occupancy and egress from the other non office floors the following
assumptions are made, which exclude their numbers for the evacuation
assessment:
The carpark is generally a low populated space. Occupants are mostly
employees from the offices (taken into consideration for the office
populations); however the public is able to use the carpark as well.
Occupants in the carpark can egress through into the atrium and then to
outside, or through the carpark entrance (which is likely to be open during
occupied hours) or into a fire stair that discharges at street level.
The café area includes a public seating space, a kitchen and back of house
office area. Occupants are likely to egress straight to outside via the main
entrance (as that exit is closest and occupants are likely to have entered
that way). There is also an alternative egress path through the carpark and
into the carpark fire stair.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 7
Charter Hall
121 Harrington Street
Fire Engineering Report
Occupants in the atrium would likely be transient and employees of the
building (taken into consideration for the office populations). They would
be able to evacuate through the front entrance or through into the carpark.
It is noted that the two assessments below have a margin of safety to allow a
scenario where occupants on ground floor utilise the carpark fire stair as well as
full capacity office floor evacuation. These include:
Additional holding capacity of the fire stair that extends to the carpark,
even with all office occupants inside.
The building provides half the evacuation time necessary when compared
with the DtS case, so a number of additional occupants from ground floor
would not affect the conclusion of DtS case being a worse case.
6.3
Simultaneous evacuation
If the 3 fire stairs can accommodate all the occupants in the building, the
occupants on the other non-fire floors can evacuate into the stairs without waiting
on the floor for extended periods of time. This reduces the risk that they will be
queuing on a non-fire floor if smoke or fire spreads to that floor. Once all
occupants are in to the stairs, then the doors will be closed behind them and the
smoke seals will minimise any smoke spread into the stairs.
Smoke and fire spread to the other non-fire floors is not likely to result in
untenable conditions due to the sprinkler protection and the reduction in smoke
spread via the major pathways of the stair (provided with smoke seals), the lifts
(smoke separated from the office and located in the atrium) and the smoke sealing
of each floor. As the floors are timber, simultaneous evacuation is considered
preferable to reduce the risk to occupants in the unlikely case that the sprinklers
failed to control the fire.
The maximum number of people that can be accommodated in one fire stair at any
time is given in Chapter 4, Means of escape and human factors, CIBSE Guide E
Nc(max) =pAS
= 3.26 x 12 x 6 = 235 person capacity per fire stair in this building
Where,
‘Nc(max)’ is the maximum number of occupants that can be accommodated in
one stairway at any an time;
‘p’ is the maximum occupant density of the stair, which is approximately
3.26persons/m² based on a high density as per the SFPE handbook and used in the
assessment above. Further, CIBSE Guide E recommends the use of 3.5
persons/m² for people in the stairway without suffering extreme discomfort.
‘A’ is the horizontal area of stair and landings per storey (m²) which is
approximately 12m²; for a standard 1m wide stair, and
‘S’ is the number of storeys (this includes levels 1 to level 6)
Based on this methodology, the capacity of the 3 fire stairs is 235 x 3= 705
occupants. Therefore as the office floors have a maximum population of 600
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 8
Charter Hall
121 Harrington Street
Fire Engineering Report
occupants (see section 6.2 above for details on occupant numbers), all office
occupants are able to fit into the 3 fire stairs simultaneously.
It is noted that the fire stairs within the current building are wider than 1m in
width. This would allow a much greater holding capacity of occupants than that
assessed above. Therefore the assessment is conservative.
6.3.1
Assessment Conclusion
With amendments to the EWIS, a fire anywhere in the building would trigger the
alarm and simultaneous evacuation would occur. The assessment shows that the
all office occupants can utilise the fire stairs at the same time as a place of relative
safety as they can all fit in the stair that is provided with smoke seals. This means
that they are protected from potential fire and smoke conditions. As they are all
able to get into the stair quickly, and in the early stage of a fire, then the stair
pressurisation is less crucial than for a DtS case where the queuing times would be
longer.
A DtS case could allow occupants to queue to enter the fire stair on multiple
floors for a simultaneous evacuation (due to larger DtS floor plates and occupant
numbers than in the existing building), and hence they would be more vulnerable
to fire and smoke conditions on the fire floor or on non fire floors. If the
evacuation were simultaneous, the air flows from the stair pressurisation would
not meet the 1m/s (due to the amount of doors open simultaneously), hence smoke
would not necessarily be kept out of the stair for this extended evacuation time.
Alternatively a DtS case could evacuate floor by floor with a phased evacuation,
which would allow the air flow of 1m/s to be maintained In this case however,
occupants on the non fire floors would be evacuating at a much later stage of the
fire and there is greater potential for smoke to have spread between floors. This is
particularly of concern in this building due to the timber floors and lack of zone
pressurisation on the office floors (required in a new building due to atrium
provisions) that could allow smoke to spread up to the floors above the fire floor.
Therefore due to the risk of smoke spread through the building when taking into
consideration the above factors and the potential impact to occupants if delayed
by phasing the building evacuation, utilising a simultaneous evacuation is
considered a safer evacuation procedure. As simultaneous evacuation would
prevent a stair pressurisation system from working efficiently (due to the number
of door open at once), the requirement for it to protect egressing occupants is
reduced.
6.4
RSET – Comparative Assessment
Egress of the occupants from the building can also be compared with a deemed to
satisfy building, with the maximum number of occupants catered for by the width
of fire stairs, to further assess the necessity for stair pressurisation.
6.4.1
Methodology
The required safe egress time (RSET) from a point on a floor is the time needed
for all occupants to evacuate to an exit in an emergency scenario. RSET can be
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 9
Charter Hall
121 Harrington Street
Fire Engineering Report
expressed by the following equation:
RSET = talarm + tp-m + ttravel + tmove
Where:
talarm
is the detection and alarm time
tp-m
is the pre-movement time
ttravel
is the occupant movement time to reach an exit
tmove
is the time for occupants to queue and flow through the exit
The existing building has 3 fire stairs and 100 people per floor (due to the small
floor plate area). A DtS design that has 3 fire stairs is able to hold 320 people per
office floor. The following assessment will quantify the difference between the
DtS RSET and the existing building RSET, showing that the existing building can
evacuate its occupants quicker than a comparative DtS case.
The nature of the comparative assessment means that the alarm time, premovement time and travel speeds will be the same for both the DtS design and the
building design
6.4.2
Assessment of Detection/Alarm time
The building is provided with a detection system to AS1670, as the building
contains an atrium. The following assesses the time taken for the alarm system to
activate with various fire growth rates.
Considering a generally open plan office and modelling a range of fire growth
rates, the detection/alarm time for the existing AS1670 system is calculated using
the program FPETool. The calculation uses a nominal 2.75m ceiling height and
assumes the fire is midway between detector heads (as the worst case fire location
i.e. 7.2m horizontally from the detector).
The following table compares the detection times for different fire growth rates.
Times are calculated using a temperature rise of 13°C and an RTI of 1 for smoke
detectors.
Activation time, Sec
Fire growth rate
Slow
Medium
Fast
AS1670 (10.2m Spacing)
210
106
54
An alarm time of 106s is taken for both the DtS building case and for the existing
building case for input into the RSET timeline, as it is expected that an office fire
would produced medium fire growth rate. This is shown in the figure below which
depicts tests carried out by NIST for work station fires.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 10
Charter Hall
121 Harrington Street
Fire Engineering Report
It is noted that travel distances in the existing building and current fitout are
assumed to be DtS (no fit out plans have been provided to Arup and there is no
discussion of extended travel distances in the base building fire engineering
report), so no additional analysis is required for extended travel distances.
6.4.3
Pre-movement time
The comparative nature of the assessment means that the pre-movement time for
the DtS design and the building design will be the same. For completeness, it is
anticipated based on data from Section Table 3-13.1 of the SFPE Handbook; that
the pre-movement time for occupants in the small space who can clearly see the
smoke will be approximately 30 seconds. For occupants remote from the fire with
a non-directive alarm a pre-movement time of up to 3 minutes may be expected.
Therefore it is considered that as the building has a directive alarm as part of the
previous fire engineering strategy, the pre-movement time would be less than 3
minutes (more in the range of 2 minutes). However for the total evacuation time
of occupants 3 minutes has been used to be conservative.
6.4.4
Movement time to the stair exits
The movement time, to and through an exit, consists of travel time, which is the
time required to reach the exit, and flow time to pass through an exit into a place
of safety, which could be the fire stair or could be taken to the final exit, usually a
road or open space.
Depending on the distance of travel and the numbers of people, the limiting factor
may be travel distance or flow through an exit, which can be determined and used
in the RSET assessment. Furthermore, it may be the flow through a door or the
flow in a stair that limits the overall evacuation time. The following section
investigates the dominating flow.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 11
Charter Hall
6.4.4.1
121 Harrington Street
Fire Engineering Report
Travel Time
For the comparative assessment a typical occupant walking speed of 1.2m/s (taken
from Chapter 4.7.1, CIBSE Guide E) has been used.
Time (at 1.2m/s)
DtS ( up to 40m)
Design Case (up to 40m)
33 sec
33 sec
The analysis is conservative as it uses 40m for the existing building design,
whereas with the ability for office occupants to use 3 fire stairs on a relatively
small floor plate (to a DtS design), can provide occupants with much shorter
travel distances.
6.4.4.2
Flow Time
The queuing time in front of a doorway to stairs depends on the flow rate of the
doorway and the flow rate in the stairs, should the stairs be full. The following
flow rates given in Table 3-13.5, Chapter 3-13, Section 3, SFPE Handbook
Edition 3rd can be used to predict queuing.
2
Crowd condition in front of doorway
Density (persons/m )
Flow rate (persons/min/m)
Moderate
1.09
55.94
Optimum
2.39
85.48
Crush
3.26
49.34
Table 1: Flow rates and Densities for Doorways from SFPE handbook
2
Crowd condition in stairs
Density (persons/m )
Flow rate (persons/min/m)
Minimum
0.54
16.45
Moderate
1.09
45.90
Optimum
2.07
59.19
Crush
3.26
39.50
Table 2: Flow rates and Densities for Stairs from SFPE handbook
Using the worst case scenario of crush conditions for a fire affected floor (as this
provide a slower flow rate for evacuation) and would be consistent for a
simultaneous evacuation strategy, Table 4 provides the following results.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 12
Charter Hall
121 Harrington Street
Fire Engineering Report
Elements
DtS (3 x 0.8m doors and 3 x 1m
stairs))
Existing Building - (3 x 0.8m door
and 3 x 1m stair)
Occupant load in each
floor
320 persons (maximum capacity
for the three stairs)
100 persons
Doorway capacity
118 persons/min (49.34 x 2.4m)
118 persons/min (49.34 x 2.4m)
Stair Capacity
118 persons/min (39.50 x 3m)
118 persons/min (39.50 x 3m)
Table 4: Flow time comparison through doors
The table above illustrates that both the flow rate into the stair door and down the
stairs is equivalent considering the associated flows and widths available.
Elements
DtS (3 x 1m stairs)
Existing Building - (3x1m stair)
Total population of building
1920 persons (6 floors x
320 persons)
600 persons (6 floors x 100
persons)
Flow rate (see above)
118 per/min
118 per/min
Flow time down stairs assuming
total population (worst case)
16.3 minutes (978 sec)
5.1minutes (305 sec)
Table 5: Comparison of whole building evacuation time in DtS building and in the
existing building
Under the worst case scenario of crush condition, the total flow time down the
stairs would be 5.1 minutes, approximately 3 times less than a DtS complying
building with up to 320 people on a floor and three fire stairs.
Comparison of overall evacuation time for DtS and existing designs
The building features a small floor plate. In order to assess the relationship
between the floor plate and the building design, Table 6 below lists the egress
time for a building meeting the DtS provisions compared with that in the current
building using the 3 fire stairs provided.
Elements
DtS building
Exiting building
Number of exits
3 stairs with each width of 1m
3 stairs with width of 1m
3 doorways with width of 0.8m each
3 doorway with width of 0.8m
320persons
100persons
Occupant load in each floor
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 13
Charter Hall
121 Harrington Street
Fire Engineering Report
Elements
DtS building
Exiting building
Detection time
106sec (AS1670) med growth
106sec (AS1670) med growth
Pre-movement time
180 sec
180 sec
Movement time to reach the
fire stairs
33 sec
33sec
Flow time in Stairs
978 sec
305 sec
RSET (sec)
1297 seconds (22 mins)
624 seconds (11 mins)
Table 6: Total Evacuation times comparison
6.4.5
Assessment Conclusion
It can be seen from the RSET comparison that the current building design
achieves half the evacuation time when compared with the DtS case being
assessed. Therefore it is considered that occupants would be exposed to fire and
smoke conditions for a shorter period of time in this building than in a compliant
DtS design, reducing the importance of the stair pressurisation for maintaining
smoke free stairs during egress. Also as, the egress time is much lower, egress is
occurring at an earlier stage of the fire hence less smoke is being produced, and it
is not as hot
It is noted that previous fire engineering reports for the building utilised an
ASET/RSET methodology on the office floors to show that occupants could
evacuate to the fire stair before untenable conditions occurred. Therefore a similar
assessment is not performed in this report as it has already been included in the
base build and fit out reports. The assessments in this report do not adversely
affect or contradicts the previous analysis and conclusions.
6.5
Implications for Fire Brigade
With regard to fire fighting intervention the faster building clearance means that
once the fire brigade arrive and set up it is more likely in this building than in a
DtS designed building that the stair will be clear of occupants allowing for faster
fire brigade response to attack the fire. The provision of three stairs also gives
them more options for fire attack.
A sprinkler system with fast response sprinkler heads has been provided
throughout to reduce the risk to life safety of occupants and NSWFB and allowing
additional property protection. The sprinklers will aid to control or suppress the
fire before and during fire brigade intervention.
The building has a compliant hydrant and hose reel system (at the time of
installation). As there are 3 fire stairs, there is a choice of hydrant and hose reels
to utilise in a fire event. With a much smaller floor plate than that allowed by DtS,
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 14
Charter Hall
121 Harrington Street
Fire Engineering Report
the likely fuel load is lower and also provides better sightlines for fire fighting and
less area for search and rescue.
If the fire is on an office level, the smoke seals to the fire doors will help minimise
smoke spread into the stairs and the sprinklers will help keep fire sizes, and hence
smoke production, low. It is acknowledged that after occupants have evacuated
the building, a compliant stair pressurisation system would be beneficial for
brigade intervention and would also be efficient (as the number of doors open at
one time would be limited, allowing the required airflows to be maintained).
However as the stairs are required to be pressurised due to the risk created by the
provisions of the atrium, it is considered that the office floors where the fire stairs
are located do not provide conditions worse than a DtS office building under 25m
in height, that has no atrium, and also no requirement for stair pressurisation.
The base building fire engineering design naturally vents the atrium smoke to
outside and the intrinsic nature of the atrium reduces the risk of smoke spread on
multiple office floors due to the external wall and separation of offices by
predominantly masonry construction with fixed glazing. The fire engineering
report also required limiting the fuel load in the atrium to help minimise potential
fire sizes. Therefore the risk of smoke spread to multiple office levels from the
atrium and then into the fire stairs (with smoke seals) is considered low.
7
Conclusion
Based on the fire engineering assessments above and the required fire safety
measures of this report, it is considered that omission of the stair pressurisation
does not significantly impact on the life safety of occupants and the Fire Brigade,
such that it would need to be retrofitted to comply. In summary, the main points
are as follows:
The heritage and restrictive nature of the building fabric restricts the
penetrations required for relief air to ensure a stair pressurisation system
would work. Therefore a solution is required, as the current stair
pressurisation conditions are hazardous to occupants and brigade.
Occupant numbers are low, on small floor plates and are provided with
generous stair capacity that enables all occupants to fit in the fire stairs
simultaneously and allows for quick building evacuation.
The timber floors, lack of zone pressurisation and separation from the
atrium on office floors; concludes that simultaneous evacuation of the
building is the safer approach than phased evacuation
Simultaneous evacuation would prevent the stair pressurisation from
working efficiently due to the amount of doors open at once
The quick occupant evacuation and close proximity of Brigade Stations
allows the Brigade to enter and fight a fire in a short period of time.
The stairs require pressurisation as per the DtS provisions only due to the
atrium, however smoke and fire spread from the atrium is seen as a low
risk. As such it is considered that potential smoke and fire conditions
during Brigade intervention on office floors would be no worse than a DtS
complying building, less than 25m in height and without an atrium (and
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 15
Charter Hall
121 Harrington Street
Fire Engineering Report
indeed is better with the sprinkler protection an AS1670 smoke detection,
plus three stairs).
Considering the above, the recommendation is to decommission the stair
pressurisation systems, retrofit smoke seals and amend the EWIS for simultaneous
evacuation. A fire order is considered the best way to gain approval for the works
as they are being done to address an existing system that cannot perform as
required by the building design and BCA.
221298 | Issue | 1 March 2011 | Arup
J:\221298-00 121 HARRINGTON STREET\05 REPORTS\0001 FIRE ENGINEERING REPORT.DOCX
Page 16