Civil Engineering Reference
In-Depth Information
and the designer must therefore be aware of the differences that
exist between the individual material codes.
In the United States, historically the requirement for design
against disproportionate collapse has been minimal, limited to
the high-level requirement that a building should be designed
to sustain local damage with the structural system as a whole
remaining stable and not being damaged to an extent which is
disproportionate to the original local damage. Individual State or
City building codes are free to define their own requirements, and
the New York City Building Code quickly implemented meas-
ures similar to the United Kingdom (City of New York, 1973).
Elsewhere in the US, the introduction of design require-
ments for structural robustness was only triggered for federal
and defence buildings by the collapse of the Alfred P. Murrah
building in Oklahoma in 1995, and much more recently, the
International Building Code introduced prescriptive measures
for civilian buildings modelled on UK requirements in the
2009 edition of the code (International Code Council, 2009).
For Department of Defense buildings, UFC 4-023-03 (United
States Department of Defense, 2010) outlines a systematic
design approach based on prescriptive tying, alternative load-
path analysis and key element design according to the building
risk classification which is closely modelled on and an enhance-
ment of the approach defined in Approved Document A.
ensure the level of robustness achieved in the design is at least
equal to that implied by the Approved Document.
The requirements given in Annex A of BS EN1991-1-7:2006
are closely modelled on those in Approved Document A and
the corresponding publications for Scotland and Northern
Ireland, and it is anticipated that Approved Document A will
be updated to reference the Eurocodes as approved standards
for design. While the Annex is officially informative, it is to all
intents and purposes rendered normative by the UK National
Annex to BS EN1991-1-7:2006, which states that the 'guid-
ance … should be used in the absence of specific requirements
in BS EN1992-1-1 to BS EN1996-1-1 and BS EN1999-1-1
and their National Annexes'. Consequently it is anticipated and
expected that requirements given in Annex A will be adopted
as standard practice in Eurocode design.
12.6 Building risk class and design requirements
The design requirements for structural robustness contained in
Annex A of BS EN1991-1-7:2006 are based on the building
risk classes given in
Table 12.1
.
12.6.1 Class 1
For single-occupancy houses not exceeding four storeys, agri-
cultural buildings and unoccupied buildings into which people
rarely go, no specific measures are deemed necessary provided
the building has been designed and constructed in accordance
with the rules given in BS EN1990 to BS EN1999.
12.5 UK/European regulations and codes of
practice
The requirements for design of structures in the UK for robustness
are defined in Eurocode 1 (Annex A of BS EN1991-1-7:2006)
(BSI, 2006). The Annex is informative rather than normative,
the legal requirements being those set down in the Building
Regulations for England and Wales, Scotland and Northern
Ireland, each of which contain a similarly worded and broad
requirement that a building 'shall not be constructed so that in
the event of an accident the building will not suffer collapse to an
extent disproportionate to the cause' (2004, 2010a, 2010b). More
detailed guidance is given in Approved Document A (Office of
the Deputy Prime Minister, 2004) for England and Wales and in
similar publications for Scotland and Northern Ireland (Building
Control Northern Ireland, 2009; Scottish Building Standards
Agency, 2010a, 2010b) which describe a number of building
risk classes and sets out the rules (design requirements) for each,
and a critical appraisal of the requirements set down through the
Building Regulations is given by Arup (2011). The Approved
Documents contain official guidance and are not mandatory, hav-
ing the same status as codes of practice in the United Kingdom.
However, the Eurocodes and the British Standards which pre-
ceded them have been written to comply with the guidance given
in the Approved Documents and compliance with the Approved
Documents is normal in design except for structures which are
not easily classified using the building risk classes given in the
Approved Document or to which the rules given for the relevant
risk class do not readily apply. In such instances, it is normal to
12.6.2 Class 2A
All other buildings are categorised as Class 2A or higher, for
which the provision of effective horizontal ties or effective
anchorage of suspended slabs to walls is required.
12.6.3 Class 2B
Hotels, flats, apartments and other residential buildings with
greater than four storeys, office buildings with greater than four
storeys, retailing premises with greater than three storeys, hospi-
tals with three storeys or fewer, educational buildings with greater
than one storey, car parks, and buildings to which the public are
admitted and containing floor areas exceeding 2000 m² at each
storey are categorised as at least Class 2B. For Class 2B build-
ings, either horizontal and vertical ties are required, or the build-
ing should be checked to ensure that upon the notional removal
of each beam, column or nominal section of load-bearing wall,
the building remains stable and the area of floor at risk of collapse
does not exceed the smaller of 15% of the storey area or 100 m²,
and does not extend further than the immediate adjacent storeys
(
Figure 12.1
). Where the notional removal of such an element
would result in an extent of damage in excess of this limit, it
should be designed as a 'key element'. It should be noted that
while the wording of the current edition of Approved Document
A3 does not require horizontal ties to be provided if the alterna-
tive loadpath approach is adopted, horizontal ties
should
always
