FIRE ENGINEERING AT THE GLA BUILDING
Tony O’Meagher & Anthony
In order to offer clients increased robustness plus some cost savings, Arup
Fire is pursuing a performance based approach to structural fire resistance
on several major building projects in London. The first of these was the GLA
building, designed by Foster and Partners, which was opened last year. Now
known as City Hall the building provides offices and committee rooms for the
Greater London Authority, as well as public assembly and exhibition spaces.
The use of a fire engineering approach enabled
the exposed steel columns to be fire-protected with a thin-film
intumescent coating, giving a high quality finish. Natural ventilation
was designed to limit the temperature of smoke layers, allowing
glazing that did not have a fire rating to be used in compartment
elements. Extensive discussion with the building control [London
Borough of Southwark] and fire [London Fire and Emergency Planning
Authority] authorities, enabled appropriate evacuation procedures
to be worked out for the mixed population of public and office
Figure 1: section showing the main components of the
This is an office building for the Mayor’s administration, with a council
chamber for the GLA and several meeting rooms. It is also a public building
with general access to the council chamber and meeting rooms, and to the top
floor which is a multi-purpose area. Access is at ground and lower ground levels.
The council chamber overlooks the Thames and is the base of an atrium that
rises past all the upper floors. A spiral ramp serves every floor. Above chamber
level the ramp is inside the atrium. The upper section of ramp is separated
from the section serving ground and lower ground, as it passes the chamber
level. Ground and lower ground floors are linked by an elliptical atrium. The
night photo below shows the lighted upper atrium façade with the ramp
visible inside it.
Structural fire engineering
Modern office buildings, retail facilities and the like typically incorporate
a high proportion of glazed or non fire-rated elements for facades and atria
walls. For this type of construction a performance based approach, allowing
for ventilation of heat through sections of façade penetrated by fire,
generally shows that the guidance on fire resistance in Approved Document B
is conservative. The fire resistance periods can typically be reduced to 60
minutes using a Performance Based approach.
Reducing the fire protection on structural steel
members can result in a significant cost saving and other benefits,
Background to Structural fire engineering: Traditional
- Reduced cost of materials, labour and equipment
- More rapid building construction, particularly if fire
protection is applied off-site
- Less bulky structural members that reduce building height
or increase floor to floor height
- Architectural form of the structure can be more freely
In England and Wales the recommendations for structural fire resistance are
given in Approved Document B, and other countries have similar prescriptive
guidance. This regulatory guidance can trace its origins to fire tests done
by S Ingberg (1928) in the USA which related the fire load to the fire resistance.
Post war compartment fire studies refined Ingberg’s values for fire resistance.
Traditionally the fire resistance of structural
members has been determined in Standard Fire Tests. The time-temperature
environment in the Standard Fire Test represents a more severe
heating condition compared to that in many typical natural fire
compartments. In a well-ventilated compartment the duration and/or
the severity of the time-temperature environment is generally
less than in a Standard Fire Test. The effect of ventilation
and fire load on fire severity is illustrated in Figure 2. Fire
tests were conducted in compartments where the fire load and
the natural ventilation were varied. The well ventilated compartments
experienced lower temperatures and fires of shorter duration.
In Figure 2 the numbers identified with each curve indicate the
fire load density in kg/m2 (ie 60, 30 or 15) and the
ventilation area as a proportion of the façade area (ie ½ or ¼).
The compartments used in the tests were small
by modern standards but the results are indicative of the influence
of fire load and ventilation on the time-temperature environment
generated within fire compartments.
Figure 2: typical time temperature curves of compartment
test fires compared to standard [ISO] fire resistance furnace
test curve ©Arup
Key Factors for Time-Equivalent Analysis
When a fire reaches a stage where there is full involvement of the combustibles
within a compartment (known as flashover), the intensity of the heat in the
hot smoke layer will cause glazing and non-fire resisting facades to fail,
allowing hot gases to escape (see Figure 3). Similarly, openings to atria will
also allow hot gases to escape. The temperatures reached in a compartment and
the duration of a fire depend on natural ventilation through openings to atria
and glazing or non-fire resisting facades that fail in a fire.
Figure 3: Natural Ventilation for a Fully Developed
Factors that affect the intensity and duration of a fire include:
fuel load (quantity and type), geometry of the compartment, the
thermal insulation provided by the linings and the natural ventilation
following glazing failure.
The principles of the time-equivalent analysis
have been understood for many years. Technical papers have been
published by Law (1978), and Pettersson (1976). To date, time-equivalent
analysis has been used on a limited number of projects to determine
the effect of fire on structural members within individual compartments.
Publication of the analysis method in Eurocode 1 (ENV 1991-2-2:1995)
some years ago, made the approach more accessible to the broader
The principle of time equivalence is that the member is exposed to an equivalent
heat dose. The equivalent fire severity can be stated more formally as: “the
time of exposure to the standard fire test that would result in the same maximum
temperature in a protected steel member as would occur in a complete burnout
of a fire compartment”.
The thermal inertia of the compartment linings
also affects, but to a lesser extent, the intensity and the duration
of a fire. Linings that are more insulating, or have a high thermal
inertia such as gypsum plaster, slow down heat transfer from
the compartment to the walls and ceilings, with the result that
the temperatures and the fire duration in a well insulated compartment
Practical Application of structural
fire engineering: the Greater London Authority Building
One of the first applications of the time-equivalent analysis technique to
a complete building structure was at the new GLA building. As the illustrations
5, 6, show this is a fully glazed building with the potential for a high heat
loss rate to the exterior in case of fire. The time-equivalent analysis method
is appropriate for compartments where the fuel load can be characterised, ie
offices, retail areas, schools, hospitals, residential apartments and hotels.
Where sprinklers are installed a reduction factor
is applied to the calculated value for fire resistance, recognising
that sprinklers reduce the intensity of a fire.
A factor of safety for consequence of structural failure is also applied to
the calculated value of fire resistance, introducing another degree of conservatism
for taller buildings.
When the Structural Eurocode mentioned in 2.2 above was published in the UK
a National Application Document [NAD] was included. The NAD gives UK values
for parameters in the time-equivalent time analysis, to enable the safety objectives
of the Building Regulations to be met. This was the approach used at the GLA
building. The use of the analysis method generally results in reductions in
the structural fire resistance compared to the values recommended in the Approved
Document B, where the building is potentially well ventilated in a fire through
a high proportion of unprotected facade.
In the case of the GLA project a 60 minute standard
was agreed for the structural elements. The building is over
30m in height and under AD B would have been expected to have
120 minutes. The lower period opened up a wider range of options
for fire protection the structural steel framework with architectural
benefits as well as economic ones.
Interestingly, when the method is applied to
the style of office building common in the first half of the
20th century [when Ingeberg was working], such as figure 7, the
fire resistance can be as high as 4 hours. This may be justified
by the lower percentage of glazed openings and high thermal inertia
of the construction.
Figures 5 and 6, the Greater London Authority building, showing the high proportion
of exterior glazing [the spandrel panels are fritted glass].
Temperature control ventilation
Although the floors are constructed as fire
resisting compartment floors, the public and private spaces are
not separated by fire-rated construction everywhere. The green
line on figure 8 represents the boundary between the upper and
lower atria. In many places this separation is formed by glazed
screens using ordinary laminated glass.
Automatic vents open in the top of the atrium
to remove hot smoke, while vents near the bottom admit cooler
make-up air from the lower atrium which has a series of automatic
opening vents that double either as inlets, in this scenario,
or as smoke outlets in the case of a fire in the lower atrium.
The complicated geometry of these spaces makes it impossible
to illustrate the air paths precisely on a 2D representation.
Figure 8: line of separation between upper and lower
artia, and between offices and public accommodation adjoining
the upper atrium ©Arup
Means of escape
The public areas of the building are evacuated
simultaneously, but the office areas have a phased evacuation
arrangement. Depending on the fire location the office levels
may not be evacuated in the first instance. A voice alarm system
is programmed to provide the appropriate messages.
There are two firefighting pressurised stairs
in the core, sized on the basis of use by the top [public] storey,
two office storeys and the council chamber [with up to 300 occupants]
at the same time. Allowance was made for the time delay of people
from the top floor reaching the council chamber level, in assessing
A fire in the council chamber should be limited
by the fire load. The furniture and fittings were designed to
keep the fire load density low.
In the event that a fire on an office level
penetrates the atrium enclosure the temperature control ventilation,
described above, should preserve the integrity of the atrium
enclosure indefinitely, assuming a heat release rate into the
atrium of up to 2.5MW.
The building is fitted with sprinklers so it
is unlikely that an office fire would grow large enough to penetrate
the atrium in the first place.
The use of fire safety engineering
- ensured the architects could meet a complex brief while
realising an unusual programme that has created a new landmark
- demonstrated the benefits in addressing structural response
to fire explicitly
- demonstrated that designers can make use of the building
design and systems to enhance the fire safety performance.