Civil Engineering Reference
In-Depth Information
Chapter 12
Structural robustness
David Cormie
Associate, Resilience, Security and Risk, Arup, UK
doi: 10.1680/mosd.41448.0183
CONTENTS
12.1
Introduction
183
12.2
Disproportionate and
progressive collapse
184
12.3
Basic approaches to design
for robustness
The design of a building to be robust is an essential part of good structural design.
Nominally, buildings may be stable under normal actions but abnormal actions - such as
accidental damage, unexpected structural movement or the effects of poor quality control
in construction - can produce consequences that are disproportionate to the initial event.
The design of buildings to be robust - to ensure that the extent of structural damage is in
line with the scale of the assault - has been a central facet of UK design since the late 1960s,
and has now been embedded in the Eurocodes. Similar, often more limited, measures have
been implemented elsewhere around the world. This chapter explains the basis of design for
structural robustness, gives practical guidance to the engineer undertaking a design against
disproportionate collapse in accordance with the UK Building Regulations and Eurocodes, and
discusses some of the issues that need to be considered when designing for robustness.
184
12.4
Historical development
of design for structural
robustness
184
12.5
UK/European regulations
and codes of practice
186
12.6
Building risk class and
design requirements
186
12.7
Interpretation of building
risk class and design
requirements
187
12.8
Existing buildings
189
12.9 Methods for design for
structural robustness 191
12.10 Systematic risk assessment
for design of Class 3
buildings
197
12.11 Terrorism and other
malicious risks 200
12.12 Achieving robustness in
design
200
12.13 Conclusions
203
12.14 References
203
12.1 Introduction
Structural robustness is a quality of a structural system which
describes an ability to withstand in a proportionate manner local
damage to which the building might reasonably be subjected.
Usually this is defined as the avoidance of an escalation of the
consequences of the local damage into a more widespread col-
lapse. Robustness enables a structural system to withstand a
degree of damage from an action which is beyond the design
basis of the structural design, but is nevertheless significant
enough for the limitation of damage and the risks to the build-
ing occupants or users to be of value.
All structural systems pose an associated risk to the users or
occupants of that structure, whether through error in design, sub-
standard material quality or errors during construction, or struc-
tural failure resulting from loads exceeding the design basis. It
is the aim of the structural designer to mitigate the risks to the
users of the structure so far as reasonably practicable. Quality
assurance during design and construction is one aspect of this
mitigation, while the designer takes reasonable steps to calcu-
late loads that will not be exceeded during the design life of the
structure, principally through the use of material and load partial
factors and a limit states design approach which seek to achieve
a tolerably low probability of failure of the structural system.
There does, however, remain a risk that a structural element
will fail during the design life of the structure. Similarly, while
the designer seeks to ensure that the design loads will not be
exceeded, there is a risk that the structure will be subjected
to an unforeseen action against which it is not specifically
designed. Robustness is the ability of the structural system to
limit the consequences of the abnormal failure of a structural
element, or to limit the damage caused by an unforeseen action
against which it has not been specifically designed.
Through the ability to limit the consequences resulting from
damage on a local scale, robustness therefore plays an import-
ant part in reducing the risks associated with the structural
system as far it is reasonably practicable to do so and is an
indisputably desirable quality in structural design. Buildings in
the United Kingdom are rarely designed against seismic loads
due to the low geographical seismic hazard, and the structural
columns of a framed building may not be specifically designed
to withstand the impact of an errant vehicle or the blast load
from a terrorist attack. Nevertheless, a robust building will be
better able to withstand the demands arising from such hazards
than a building designed merely for the design basis loads with
no consideration of beyond-design basis events.
Structural robustness is a form of ductility: a ductile response
in a material or structural system is one in which the system
can undergo significant deformation without suffering loss of
strength and in which failure is gradual and predictable, in
contrast to a brittle response in which the onset of deform-
ation results in a rapid loss of strength and sudden failure. A
robust structure is one in which at the system level it is able to
sustain damage and in which eventual failure is gradual and
predictable.
