Download ARC5 Atria in Buildings - Lawrence Webster Forrest

LWF PROFESSIONAL BULLETIN
Atria In Buildings
Introduction
The Roman Empire was the first civilisation to
introduce atria into buildings. Some 2000 years later
the British Standards Institution published a code of
practice for the incorporation of atria in buildings as
part of the BS 5588 series of Standards.
Category C - Occupants who constitute a sleeping
risk, and
BS 5588 Fire precautions in the design, construction
and use of buildings, part 7: Code of practice for the
incorporation of atria in buildings took 10 years to
write and was finally published in 1997.
Whilst concentrating primarily on the subject of life
safety, the code also gives limited guidance on
measures that may be taken to reduce the potential
for loss or damage to the contents and finishes in the
event of a fire.
The rather loose definition of an atrium used in the
code is as follows;
Atrium (plural Atria). A space within a building, not
necessarily vertically aligned, passing through one
or more structural floors.
Enclosed lift wells, escalator wells, building service
ducts and stairways are specifically excluded from
this definition. The stated aim of the code is to
‘ensure that the incorporation of an atrium into a
building does not present an increased risk to life as
a result of fire and smoke spread’ and it is only
concerned with the additional measures that may be
necessary to compensate for any increased risk
resulting from the inclusion of the atrium. It does
not provide a fire engineered solution for any
particular design and does not replace the
recommendations contained in Approved Document
B - Fire Safety, issued in support of the England and
Wales Building Regulations.
The code deals only with those aspects of the design,
construction and management that relate specifically
to the presence of an atrium and any other aspects
not atrium-related remain the subject of other
published codes and technical guidance. The code
bases its range of design solutions on four different
building occupancy types. These are;
Category A - Occupants who are awake and familiar
with the building layout,
Category B - Occupants who are awake but
unfamiliar with the building,
Category D - Occupancies with a significant number
of people requiring medical or nursing care.
There are certain types of building to which this
code does not apply. These include;
1) Prisons or other buildings intended for the
confinement of persons,
2) Auditoria of theatres etc. to which part 6 of BS
5588 applies,
3) Shopping malls to which part 10 of BS 5588
applies,
4) Certain small premises covered by part 11 of BS
5588, and
5) Buildings in Occupancy categories A & B
containing a 2 storey atrium, one storey of which is
at ground level, provided that the building is
designed for simultaneous evacuation.
As always, there is no legal obligation to adopt the
recommendations contained in the code if it can be
demonstrated that an equivalent level of safety can be
achieved in another manner. This opens the way for a fire
engineered solution for which it may be necessary to
obtain the services of a qualified and experienced fire
engineer.
Problems Associated with Atria in Buildings
The presence of an atrium in a building can provide
a route by which fire and smoke may spread through
the building with greater ease than in an equivalent
non-atrium building.
The quantity of smoke
produced by a fire is a function of both the energy
possessed by the fire and the height through which
the rising fire plume can rise. An atrium, resulting
as it does from the absence of one or more floors,
provides for an unusually high fire plume and,
consequently, an increased volume of smoke and fire
effluent. Because of this open spatial planning, the
fire effluent can spread through the building and may
cause damage that is out of all proportion with the
actual size of the original fire.
load within the atrium and the adoption of an
appropriate evacuation strategy for the building.
It is obvious that the unhindered spread of smoke
from floor to floor will greatly increase the numbers
of occupants that may be put at risk during the initial
stages of the fire. The time available for the
occupants to escape from the floors affected will be
significantly reduced and also the conditions with
which the fire crews will be presented on their
arrival will be considerably worsened.
The first method used is a decision tree process that
takes the user through a series of questions and
recommendations for determining the most
appropriate design solution. Each decision tree
comprises of two elements;
As the smoke from a fire rises through an atrium, it
will entrain large quantities of cool air from within
the atrium. This reduces the temperature of the
smoke plume but increases the mass of smoke
produced. As it continues to rise, the plume will
cool until its buoyancy is equal to that of the
surrounding air. At this point stratification will
occur and this may result in a semi-stable layer of air
forming some distance below the roof of the atrium.
This relatively cool layer of smoke may then spread
horizontally into any storeys that are open to the
atrium.
2) A recommendation box which provides a solution
for that aspect of the design.
It is obvious, from the above description, that
enclosing the atrium using an imperforate screen will
reduce the likelihood of any smoke spreading into
the storeys remote from the floor on which the fire
originated. For aesthetic and functional reasons,
however, it is often required that the atrium be either
open to the associated floor areas or, if enclosed, for
the enclosing structure to be largely of glass. If the
fire is large enough, the temperature of the smoke
plume is likely to cause the failure of most nontoughened glasses and if the temperature continues
to rise, the heat build-up within the atrium may
eventually lead to the failure of any non-fire resisting
glass associated with the atrium. However, the
provision of fire resisting glazing around the atrium
may not be financially viable nor entirely justified
when the actual risk is assessed.
The code,
therefore, provides guidance on other methods that
can be adopted in order to minimise the flow of hot
smoke and gases from a fire, either in a floor
associated with the atrium or in the atrium itself,
from spilling into other floor areas remote from the
atrium. The suggested methods make use of, what is
termed ‘smoke retarding’ construction, fire resisting
construction and ventilation and smoke extraction
from each floor along with certain specified
management procedures such as restriction of fire
Application of the Code
The code makes extensive use of three graphical
methods for illustrating the recommended design
solutions.
1) A question box containing a question fundamental
to the design, and
The user will be guided to further questions and
recommendations as they move progressively
through the decision tree process.
Following on from the decision trees are a number of
Tables that identify a range of design solutions
appropriate to each occupancy category.
The
decision trees and associated tables are related to the
main technical issues that need to be resolved in
order to arrive at an appropriate level of fire life
safety. These are;
• The evacuation procedure,
• the fire detection and alarm system
• the degree of separation between the atrium and
associated accommodation
• the type of smoke management system to be
provided
• the use of the base of the atrium, and
• the provision of an automatic fire suppression
system.
At the end of the design process, the user is directed
to a series of ‘exemplar; buildings contained in
Appendix A to the code. These are simplified
generic sketches that identify the appropriate design
solutions applicable to the different categories of
occupancy and atrium types.
The layout of the code may actually encourage
‘cherry picking’ from the range of suggested
solutions, but it must be emphasized that the
decision tree process is fundamental to the design
process and should be followed through from
beginning to completion and solutions should not be
adopted that do not form part of the decision tree.
This does not, however, make the adoption of a fire
engineered solution any less acceptable.
ARC 5 – Atria in Buildings
Conclusion
The code of practice is an easy to use and fairly
comprehensive guide to designing the fire safety
aspects of buildings containing an atrium. The
design process is represented graphically in a series
of decision trees and tables supplemented by a
number of simplified building exemplars.
Whilst it does provide a range of possible solutions
for relatively straightforward cases, the code does
not provide guidance in more complicated or novel
situations, such as buildings that are designed to
include more than a single atrium. The problems
associated with adhered plumes, that is, plumes that
originate within the floor area associated with an
atrium and which emerge into the atrium as a spill
plume and rise up through the building adhering to
the walls of the atrium, are not dealt with at all.
The code does make a substantial contribution to the
problems of fire safety design in modern commercial
buildings. It does not provide all of the answers but
it does provide the building designer, regulator and
fire engineer with a collection of design solutions
that may be appropriate in all but the most complex
of building designs.
Lawrence Webster Forrest Limited
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ARC 5 – Atria in Buildings