SUSTAINABLE BUILDINGS FOCUS

CUSTOMER CARE
Fathi Tarada and Mike Deevy describe the fire safety
design behind Hackney Council’s new customer service
centre in northeast London
ACKNEY COUNCIL’S new customer service centre In proposing a fire safety strategy for the building, a number

H (CSC) will have one of the largest smoke extract

capacities of any building the UK, and its façades
have been engineered to protect neighbouring buildings in
of significant challenges had to be overcome. At an early stage
in the project, and with the approval of the Council’s insurers,
it was decided not to install sprinklers. Yet the office floors are
case of a fire. largely open to the atrium, with the potential for fire and
smoke spread within the building. In addition, the CSC is
Construction of the CSC is nearly complete. With 15,850m2
adjacent to council and other properties, and has a largely
gross internal area, the five-storey building, which is located
glazed façade. The risk of fire spread to adjacent buildings
behind Hackney’s grade-II listed Town Hall, will be the
therefore had to be contained to acceptable levels.
centrepiece of the Council’s public building campus. There
will be a ‘one stop shop’, along with a public reception Smoke management
area and a call centre on the ground floor, with council staff
offices and meeting rooms on other floors. In case of a fire in the base of the atrium or within the office
floors, it is important to draw the smoke upwards within the
The CSC is designed with environmental considerations high
atrium and away from the occupied spaces. This strategy
on the agenda, and incorporates the latest in sustainable
maximises the tenability of the environment for human
design. The large internal atrium will form a natural
survival, improves the visibility, and provides opportunities
ventilation system, reducing the requirement for mechanical
for escape and firefighting.
air supply. Natural lighting will be provided by the glass roof
and walls, with energy-efficient interior lighting providing The calculation procedure provided by BS 7346-4: 2003:
the balance. The building’s energy supply will be partially Components for smoke and heat control systems. Functional
provided by solar panels, and hot water and heating will come recommendations and calculation methods for smoke and
from a wood-burning, carbon-neutral boiler. Construction heat exhaust ventilation systems, employing steady-state
materials have been selected to be non-polluting, recycled, or design fires. Code of practice has been followed, and the
made from renewable sources, wherever possible. guidance in BRE 368: Design methodologies for smoke and

Fire engineering challenges for the CSC included office floors being largely open to the atrium, and the centre’s close proximity to other properties

Hopkins Architects

July 2009 www.frmjournal.com 41

 FOCUS SUSTAINABLE BUILDINGS

Location of fire

Figure 1: CFD calculations of smoke spread in the atrium Figure 2: Rectangle to rectangle radiation – after Ehlert and Smith

heat exhaust ventilation (1999) has been used to calculate the tenability on the top floor. This solution allowed for the
extract requirements for the smoke ventilation system. design of open floors on the ground floor and first three
storeys, while achieving the required level of fire safety.
The smoke volume flow rate into the atrium has been
calculated as 250m3/s, provided by ten fans, each of 25m3/s Neighbouring buildings
capacity. The ten extract points were designed to prevent
Two buildings, Christopher Addison House and 2 Hillman
plug-holing, where fresh air is extracted in addition to
Street, lie to the north of the CSC, and are owned and
smoke. The basis for the smoke production is an 8MW fire,
operated by London Borough of Hackney. In accordance
as recommended by Table 3.3 of BRE 368.
with the guidance in Volume 2, Clause 13.6 of Approved
In addition, the inlet area required for the smoke venting Document B (ADB) to the Building Regulations in England
system was designed to be 65m2, provided by automatic and Wales, and due to the fact that these buildings are not
openings. The fire alarm system, designed as a Category L1 used for residential or assembly and recreation purposes,
system to BS 5839-1: 2002: Fire detection and fire alarm systems the risk of fire spread by radiation has been discounted for
for buildings. Code of practice for system design, installation, these buildings, since they are located on the same site.
commissioning and maintenance, will drive all make-up air This interpretation of building control guidelines was
sources to open in case of fire. In the case of a power failure, all confirmed by building control.
make-up air sources will fail-safe to the open position. Radiation calculations were undertaken to assess the risk of fire
Working closely with the project architects, computational spread through radiation from the CSC to the Town Hall,
fluid dynamics (CFD) was used to consider the effects which is located to the east of the centre, and also to the
of different locations for the design fire, including the properties to the south. The upper limit considered for
effect of breaking of the glazed balustrades and windows radiative flux was 12.6kW/m2, corresponding to the piloted
near the fire. The general-purpose CFD code, ANSYS ignition of cellulosic products, as per ADB. The radiation
CFX10, was used to model the fire scenarios. A three- intensity from all unprotected openings was assumed to be
dimensional computer-aided design model, developed by 84kW/m2, equivalent to a ‘black body’ temperature of 1,105K.
the project architects, was imported into the CFD mesh A number of approaches to estimate the radiative heat flux
generation software and used as a basis for the unstructured based on the methods described in ADB were attempted, but
mesh. The design fire was modelled using the eddy- none of them were found to be sufficiently reliable or accurate
dissipation combustion model, the k-ε model was used to for this particular task. Instead, two alternative approaches
model turbulence, and the equations were discretised using were developed. The first of these approaches was to model
a second-order upwind biased convection scheme. Figure 1 the buildings within Airpak, a CFD tool developed for
shows an iso-surface of smoke from a fire on the second heating, ventilation and air-conditioning applications, and
floor. Smoke within this surface has concentration thereby extract the radiative view factors. The overall heat flux
equivalent to less than 10m visibility. at a ‘target’ located on a neighbouring building could then
The results of the CFD analysis showed that for certain fire be calculated in a spreadsheet, with all the radiation
scenarios, smoke was likely to spread to the floor above the contributions from the individual floors summed up.
fire, and hence fire spread was also possible. Glazing was The CFD approach initially indicated that the heat flux
installed to separate the top floor from the atrium, thus at the buildings to the south of the CSC was likely to
increasing the size of the smoke reservoir and assisting exceed 12.6kW/m2. A solution was proposed based on

42 www.frmjournal.com July 2009

compartmenting out the fourth floor from the atrium, using
impact glazing (rated to Class A of BS 6206: 1981:
Specification for impact performance requirements for flat safety
glass and safety plastics for use in buildings) installed around the
atrium edge. Since the fourth floor will be isolated from
smoke flowing upwards through the atrium, the risk of
vertical fire spread to the fourth floor was considered remote.
In addition, the extent of unprotected areas on the ground-
floor south-facing façade was reduced by the design of larger
upstands and downstands. Based on these approaches, the
maximum radiative heat flux at the target buildings was
reduced to 12.6kW/m2. This solution was approved by the
London Fire and Emergency Planning Authority.
In order to confirm the CFD results, radiation calculations
were undertaken using the view factor formulae developed
by Ehlert and Smith (Ehlert, J. R. and Smith, T.F., View
Factors for Perpendicular and Parallel, Rectangular Plates,
J. Thermophys. Heat Trans., Vol. 7, no. 1, pp173-174,
1993). A 1m2 target was located on adjacent buildings at
the closest location to Hackney CSC, approximately mid-
way in terms of building height.
The finite-to-finite radiation configuration factor for two
parallel planes (see Figure 2) is given by:

where

The spreadsheet calculations based on the analytical

configuration factors confirmed the CFD results to within
1%, and were accepted by the independent checkers acting
on behalf of building control.
Hackney’s CSC has been designed to satisfy high standards
of sustainability and fire safety. Its large atrium, acting as
its lungs during normal operation, emphasises the need for
special precautions in terms of smoke extract and façade
engineering, in order to ensure the safety of occupants and
to prevent fire spread to other buildings I

Fathi Tarada is the director of, and Mike Deevy

a consultant in, the fire safety engineering team
at Halcrow Group Limited

July 2009 www.frmjournal.com