Civil Engineering Reference
In-Depth Information
commercial office construction can frequently be anything up
to 13.5 × 18 m. As such, the designation of elements as key has
become commonplace and almost the norm, whereas the prin-
ciples were conceived with the assumption that key elements
would be the exception.
The second aspect the designer must consider follows from
this, namely that with the much more commonplace nature
of key elements the designer must consider whether a load of
34 kPa is appropriate for elements that have been shown to be
critical. 34 kPa is equal to 5 psi, a rounded estimate of the explo-
sion pressure estimated to have caused failure of the precast
concrete load-bearing flank wall panel at Ronan Point, based
on observational and experimental evidence (Moore, 2002).
Consequently it was recommended as a suitable design pres-
sure for load-bearing wall panels in large-panel structures for
which it could not be shown that the loss of the panel could be
sustained without resulting in a disproportionate collapse. The
load has remained enshrined in Codes of Practice ever since. It
is unfortunate that the numeric value in metric units suggests
a degree of precision which is unintended and undeserved. It
should be noted that previous versions of Approved Document
A referred to a design load of 'at least' 34 kPa applied from
any direction (Department of the Environment and The Welsh
Office, 1985).
When conceived, 34 kPa was a relatively onerous load in
most circumstances, certainly leading to an enhancement
of the element design over that required by other loadcases.
However, in modern construction columns are heavier due to
the greater loads resulting from the increased grid size. The
effect is further exacerbated in high-rise construction so that
the load is often a trivial loadcase that is bounded by other
loadcases. In addition, cladding typically spans loor-to-loor
rather than loading the column, decreasing the loaded width for
consideration in key element design and further decreasing the
impact of the requirement. The structural engineer should give
careful consideration to the selection of a design load for key
elements which is appropriate to the critical nature of the elem-
ents. The design load should normally be such that it results in
an enhancement of the element design, unless through a risk
assessment it can be shown that no reasonably foreseeable or
unforeseeable hazards will result in the failure of the element.
identifiable hazards should be implemented, in parallel adopt-
ing strategies based on limiting the extent of localised failure
through a minimum level of inherent robustness irrespective of
whether there are any hazards the designer can foresee. As a
minimum, a Class 3 building should be generally expected to
satisfy the Class 2B robustness requirements, sometimes also
incorporating additional mitigation measures which the risk
assessment finds to be necessary.
The design approach for Class 3 buildings therefore requires
a fundamental consideration of the hazards to which the struc-
ture might reasonably be subjected, and an assessment of the
risks to building occupants (and others who might be affected
by damage to the building, for example, the general public in
the vicinity of the building and the occupants of neighbour-
ing buildings or in close proximity) based on the likelihood
and the consequences associated with each hazard. The risk
assessment should not necessarily be limited to consideration
solely of accidental hazards: in some cases, malicious actions
are a foreseeable hazard. Two such examples are safety-critical
vandalism and the blast effects of a detonation due to explosive
terrorist attack. Where relevant such malicious hazards must
therefore be considered. (Consideration of terrorist actions
should normally be undertaken by a suitably qualified mem-
ber of the ICE Register of Security Engineers and Specialists,
www.ice.org.uk/rses.)
As discussed in Chapter 3:
Managing risk in structural
engineering
, risk may be defined as:
Risk
=
Likelihood
×
Consequence
(associated with a
particular hazard)
(or probability)
(or severity)
Each building differs in terms of its sensitivity to acciden-
tal hazards and the consequences of failure. A systematic risk
assessment for design of a Class 3 building should seek to
eliminate the risk of disproportionate collapse so far as reason-
ably practicable. This is derived from the designer's legal duty
under the Health and Safety at Work Act (1974) to reduce risk
within the scope of their undertaking so far as is reasonably
practicable. The requirement to exercise a legal duty so far as
is reasonably practicable is at the heart of health and safety
legislation in UK law, and acknowledges that it will be imprac-
ticable to eliminate all risk associated with foreseeable (and
unforeseeable) hazards. Thus the duty is not absolute, but con-
siders the progression in cost/benefit terms from proportionate
through to disproportionate action, implying that measures are
required up to a point of disproportion, but not up to the point
of gross disproportion.
The control of a given hazard is intended to be proportionate
to the risk posed. For further discussion of the management of
risk in structural engineering, refer to Chapter 3:
Managing
risk in structural engineering
.
12.10 Systematic risk assessment for design of
Class 3 buildings
12.10.1 Basis of a systematic risk assessment
For Class 3 buildings, Approved Document A recommends
that 'a systematic risk assessment of the building should be
undertaken taking into account all the normal hazards that
may reasonably foreseen, together with any abnormal haz-
ards'. Eurocode 1 goes further, calling for the systematic risk
assessment to take into account 'both foreseeable and unfore-
seeable hazards'. While it is difficult to assess and design for
unforeseeable hazards by their very nature, the concept of the
Eurocodes is that strategies based on the control of risk from
