Timber

Question 1

What is oversizing or

‘sacrificial timber’

Answer 1

Wood burns, but because it burns at a regular,
measurable rate it is possible to deliberately oversize
timbers, so that they can be used as structural
elements

Question 2

Describe the surface degradation or charring of wood

Answer 2

The surface degradation of the wood is normally in
the form of charring, and the flaming will occur
only with temperatures at the surface in excess of
350°C and the presence of a pilot ignition source

As the outer surfaces of a timber member char,
they tend to stay in place and the inner core of the
wood remains relatively unaffected and can retain
its stability and integrity.
The accepted rate of charring may 0.67 mm
min-1 is a widely accepted estimate for structural
species.

Question 3

timber is combustible and there
are various flame retardant treatments that can assist
in making wood more difficult to ignite. There are
two types of flame retardant treatments for timber

Answer 3

(a) Surface coatings - painted onto the timber
surface with little or no penetration into the
wood. Some doubt exists as to the permanency
over a period of time.
(b) Impregnation - using a combination of pressure
and vacuum this process ‘drives’ the
liquid into the timber. This is a type of permanent
treatment but it may affect the cosmetic
appearance of the timber in moist
atmospheres.

Question 4

the performance of timber in real fires is
frequently far superior to unprotected, non-combustible
materials such as steel and aluminium for
the following reasons:

Answer 4

(i) Timber does not expand significantly under
the influence of heat (in fact, it may shrink
slightly) and buildings reliant upon timber for
structural purposes are not likely to suffer
sudden collapse brought about by ‘unrestrained’
expansion.
(ii) British Standards 476: Part 5 comes to the
conclusion that timber in sizes normally used
for construction purposes is defined as ‘…not
easily ignitable’.
(iii) Timber has the inherent ability to protect
itself; the build-up of charcoal on the surface
of burning timber limits the availability of
oxygen thereby insulating the remainder of
the section.
(iv) It has been established that the burning or
charring rate is predictable and varies only
slightly with species of timber and not on
the severity of the fire. ‘Sacrificial’ timber
built into the construction may be consumed
by a fire before the structural core is
attacked.

Question 5

Detail the use of timber in building construction

Answer 5

1. Beams for floors
2. Framed walls
3. Load bearing timber walls. Timber framed houses
4. Roof supports