Olsson Fire and Risk are featured in the summer edition of Timber in Construction Magazine.

1. www.timberinconstruction.co.uk
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FIRE PROTECTION
Burning issue
As new technologies allow the creation of ever taller timber structures questions have been raised
about the fire safety challenges that such buildings pose. Danny Hopkin, Olsson’s structural fire
engineering lead talks about managing risk and overcoming preconception
imber structures
are undergoing a
resurgence due
to their aesthetic
appeal, indubitable environmental
credentials and economic speed
of construction. New technologies,
such as Cross Laminated Timber
(CLT), have made timber structures
viable in markets typically dominated
by steel and concrete. As such, the
complexity of schemes for which
timber is adopted and the scale of
building, is rapidly shifting.
Timber was traditionally a
material for low-rise construction but
timber towers are now frequently
conceived and the burgeoning high-
rise market is polarising opinion.
The aesthetic, environmental and
construction programme credentials
are without dispute. Nonetheless,
timber is combustible and the
perception of performance in fire
is rooted in a long and distressing
history. After all, the various Great
Fires, such as London (1666),
Chicago (1871), Baltimore (1904)
and San Francisco (1906), all
devastated timber neighbourhoods.
Over a century ago, building
resilience in fire began to enter
the consciousness of the world’s
developed societies. The destructive
aftermath of the Great Fires
instigated an important and familiar
concept – what we understand
as fire resistance. In the wake of
these fires, the need for fire proof
materials became apparent. To
assess the veracity of a product’s
fire proof credentials, a consistent
means of benchmarking was
required and a standard fire test
was necessary. This test needed to
be robust as ultimately, once it was
implemented, it would be difficult to
change. The test heating regime was
intended to be more severe than any
fire that could be imagined. During
this time, the fire brigades were the
authority on fire development and
so, based upon anecdotal evidence,
a heating regime was proposed.
Nowadays, the tests that define
the fire resistance of an element
of construction are consistent with
those that emerged in a period
where fire science was in its infancy.
Ironically, these tests are now
adopted to assess the fire resistive
performance of combustible
materials too.
It can be argued that the fire
resistive principle has limited
relevance for exposed wooden
structures, what cannot be ignored
is the influence that the principle
has on design today. Shortly after
the emergence of a standard fire
test, it was quickly appreciated that
fires did not behave according to the
adopted test heating regime.
In 1928 this led to a concept
that allowed real fires to be
correlated with the idealised test
heating regime according to the
non-permanent fire load (furniture,
fixtures, stored goods, etc). This
concept allowed fire resistance
durations (or periods) to be defined
according to a building’s use, but
on the pretence that the structural
frame/enclosure does not contribute
to the available fuel.
The same principles define
what fire resistance is required for
elements of structure in different
buildings today, with values
proposed in guidance documents
such as Approved Document B.
One of the key challenges that
has to be acknowledged for
timber structures is the role that a
combustible building fabric has in
fire development (and fire severity).
Fortunately, the necessary data is
emerging.
The perception of performance
in fire is often seen as a more
difficult obstacle to overcome.
There is a misplaced presumption
that inert materials like concrete
and steel suffer no damage under
fire conditions and ride out a fire
intact. That is a gross simplification.
All principal construction
materials degrade when heated to
temperatures synonymous with fully
developed fires. The consequences
and manifestations of this
degradation differ and so a universal
concept, such as fire resistance,
cannot be a panacea.
Design based upon science,
appreciation of uncertainty and risk
mitigation is central to overcoming
preconception and the design of
timber buildings that perform safely
in fire. Designer competence is
a core mandatory prerequisite.
Fire safety engineers must have
the ability to distinguish between
circumstances where the status-quo
compliance approach has credence
and when it doesn’t. Only then, can
they manage the risks and develop
appropriate solutions for ambitious
timber buildings.
Timber is once again an exciting
material. To use it appropriately
in modern structures, the historic
understanding of this most ancient
of materials has to be coupled with
state-of-the-art knowledge and
design skills.
T
Olsson Fire & Risk (OFR) has advised Urban Splash on fire protection at its hoUSe development in New Islington, Manchester
Pic: Urban Splash