Simple prescriptive rules for fire design of
timber members are available in the following references:
-
BS5268-4.1 (1978) – For timer members
-
BS5268-4.2 (1990) – For timber stud walls & joisted floor
constructions
-
EN1995-1-2 (2004) – For general timber structures
-
BRE Report 128 (1988) – For timber floors
In general, the design codes adopt the concept of calculating
timber member strengths on a residual non-charred section with
the charred portion considered as having no strength. The rules
for determining the charring depths are based on the standard
fire tests.
It is noteworthy that a large number of the standard fire tests
carried out to determine the rates of charring have been conducted
on panels subjected to a single face heating in a small furnace,
rather than on full-scale timber elements subjected to multi-face
heating in a normal size furnace (Purkiss 1996). As a result,
the charring rates obtained from the tests cannot accurately
represent the actual charring conditions of beam and column members
because the members’ geometry and the heat flux
on the members are quite different.
To take into account the increased rate of charring on multi-face
fire exposure, BS5268-4.1 makes the following assumptions:
-
Arrises will become progressively rounded during fire exposure
(see Figure 1).
-
Rates of charring for columns exposed to fire on all faces are
25% higher than those for beams.
Such compensating approaches are not adopted in prEN1995-1-2.
However, prEN1995-1-2 introduces the concept of effective charring
depth def as shown in Figure 2. In addition to the charring depth
d
char obtained from the standard fire tests, the constant d
0
= 7mm is calibrated from the tests. The coefficient k
0 (= 1.0)
depends on the factors such as the surface protection and the
required fire resistance duration.
 |
 |
Figure 1 Radius of Arris Rounding according to BS5628-4.1 |
Figure 2 Effective Charring Depth according to EN1995-1-2 |
The use of charring depths is suitable for large section timber
which will perform well in a fire. For smaller section timber,
protection from linings is required. For instance, prEN1995-1-2
states that in the walls and floors construction, timber frame
members with dimensions less than 38mm should be protected by
claddings throughout the required fire resistance time. Care
is required in ensuring the quality of fixings of the protecting
lining to the supporting timber on site, which must be the same
quality as the fixings used in the standard fire tests, from
which the tables were derived.
Tabular data
In the case of walls and floors, BS5268-4.2 provides the tabulated
data for various fire resistance periods in stability, integrity
and insulation including:
The tabulated data are limited to a fire resistance of 1 hour
or less, considering different joist sizes, floor boarding and
ceiling protection. The dimension limits include:
-
For loadbearing walls and partitions, the actual stud sizes > 62mm × 37mm
(depth × breadth) (see Figure 3)
-
For non-loadbearing walls and partitions, the actual stud sizes > 48mm × 37mm
(depth × breadth)
-
For joisted floors, the actual joist breadth > 37mm (see Figure
3)
|
Figure 3 Dimensions of Stud and Joist according to BS5628-4.2 |
In practice, timber structures are generally designed and built
in simple construction method, such as simply supported beams
and pin jointed triangulated trusses. The structural connections
can be in forms of nails, bolts, timber connectors and embedded
steel connecting plates. Special consideration is required in
ensuring the connection and detailing to achieve fire resistance
particularly with connections that transfer forces from one member
to another. Basically, charring of heated members may alter the
relative stiffness of various parts of a structure, and result
in a redistribution of forces. The redistribution of forces may
cause the yielding of connections made from metal fasteners and
components.
Where any part of a nail, screw or bolt is exposed to heating
during a fire, rapid heat conduction will lead to localized charring
and loss of anchorage. If this effect is likely to lead to the
failure of a structural member which is required to have fire
resistance, BS5268-4.1 requests the fastener to be fire protected.
For timber construction with no fire resistance requirements,
metal connectors and components can be used without restriction.
PrEN1995-1-2 takes the inherent fire resistance of various connections
as the minimum fire resistance. To achieve a fire resistance
more than the minimum value, depending on the types of connection,
the extra fire protection can be provided by:
-
Specifying extra cladding to metal fasteners or increasing
edge distances to fasteners
-
Increasing the number of fasteners or choosing stronger fasteners.
