Civil Engineering Reference
In-Depth Information
The intent of B3 is to ensure the
stability of the structure in fire.
Furthermore, there is a requirement
that the building or buildings
'shall
be subdivided into compartments
where this is necessary to inhibit the
spread of fire within a building'
.
Further requirements under B3 are
concerned with the provisions for the
subdivision of concealed spaces and
for walls between two or more
buildings.
B4 requires that
'the external walls
of the building shall offer adequate
resistance to the spread of fire over
the walls and from one building to
another, having regard to the height,
use and position of the building'
.
These simple functional
requirements are extremely broad.
The structural requirement under B3,
for example, states that
'the building
shall be so constructed that, in the
event of fire, its stability shall be
maintained for a reasonable period'
.
Guidance on how to achieve the
requirements of the Building
Regulations is given, for example, in
Approved Document B. There is no
obligation to adopt any particular
solution: if preferred a requirement
can be met in some other way. Even
to follow exactly the Approved
Document will only
'be evidence
tending to show compliance with the
Regulations'
. Nevertheless, it is
generally accepted that to follow the
guidance in the Approved Document
is to comply with the requirements of
the regulations.
All external walls must be fire-
resisting to some degree. So far as
loadbearing external walls are
concerned, as
Principles of modern
building
(6)
pointed out, they should
have a period of fire resistance
sufficient to ensure that they will
continue to act as loadbearing
elements during a fire, but if pierced
by numerous window openings
through which fire could spread out
of or into the building, it would be
unreasonable to require the non-
window parts of the wall to be of a
high standard.
So the concept of 'unprotected
areas' is applied to external walls in
relation to relevant boundaries of
properties, whether notional or not.
Such areas include doors, windows,
other kinds of openings, parts of
external walls having less than the
designated period of fire resistance,
and some areas of combustible
material. Scope for forming new
openings in walls may therefore be
limited by this consideration, and,
additionally, limitation of certain
existing openings may also be
appropriate, though small residential
buildings are allowed different limits
from other purpose groups.
A change in Approved
Document B was introduced in 1992,
with the deletion of many of the
previous provisions for non-
combustibility.
One curiosity with regard to
legislation may be noted. Although
all external walls need to be fire
resisting because they are elements
of construction, if, because of the
rules defining unprotected areas, the
wall is allowed to be 100%
unprotected, there is no area left
which needs to be fire-resisting. This
is why completely glazed buildings
are allowed.
Figure 1.54
Fire test on a mockup of the Dublin disco
fire
position of the element concerned,
both within the building and to the
boundary of the site. To quote one
example, there are specific
requirements for fire resistance of
wall areas adjacent to external stairs.
For the purpose of the Approved
Document, buildings are divided into
purpose groups that reflect their use.
Within a purpose group further
subdivisions are made depending on
the size of the building. In each case,
provisions are made with respect to
the nature of the exposed surfaces
and the fire resistance requirements
of elements of structure, and to the
spread of flame on the finished
surfaces.
It must be remembered that the
standard time temperature curve for a
fire test does not represent an attempt
to simulate the growth of an average
fire. Nor can the period achieved in a
fire test be related directly to the
expected survival time in a real fire.
The results obtained from fire tests
are relative assessments which
enable a quality judgement to be
made on a component of a structure.
Tests may also be carried out on
mockups after fires have occurred,
for forensic purposes (Figure 1.54).
The intent of B2 is stated as:
'In
order to inhibit the spread of fire
within the building, surfaces of
materials used on walls and ceilings:
●
s
hall offer adequate resistance to
the spread of flame
●
shall have if ignited, a rate of heat
release which is reasonable in the
circumstances'
.
Surface spread of flame
In addition to needing to meet
prescribed standards of fire
resistance, surfaces of walls may
also need to possess resistance to
the spread of flame. The actual
criteria to be met depend on the
location of the wall if internal, and
the size of the room which the wall
encloses. Although the
requirements vary according to
purpose group of the building, typical
examples, for most rooms, are Class
1, with Class 0 for circulation areas
and protected shafts. Parts of walls
may be of
Class 2 or Class 3, depending on
area of room and total size of wall
(52)
.
If the wall is external there may
be other requirements. For all
building types there is a general
requirement for external walls,
whether loadbearing or non-
loadbearing, to have suitable
periods of fire resistance if they are
close to a boundary. Depending on
the height of a building, if the walls
are within 1 m of the relevant site
boundary, they may need to possess
