Civil Engineering Reference
In-Depth Information
be provided in Class 2B buildings regardless of whether verti-
cal ties or alternative loadpath analysis is used, unless there are
clearly justifiable reasons why this should not be the case. The
potential for the situation to arise whereby the robustness of a
Class 2B building may be permitted to be less than that of other-
wise similar Class 2A building is clearly unjustifiable.
building have been assessed in a systematic manner without
necessarily making compliance with the Building Regulations
more onerous. Examples of cases where such an approach
may be useful are buildings which are high-value client assets,
buildings for which the risk of terrorist attack is a foreseeable
hazard, or buildings which serve a critical function either to the
occupier or more widely. Where the risk assessment does result
in the identification of particular measures the incorporation of
which measurably reduces the risk associated with one or more
hazards, the designation of the structure as a Class 3 building
will have caused the risks to be better managed and the design
to be more satisfactory as a result. However, even if the risk
assessment concludes that no additional measures are required
over and above those indicated by Approved Document A, the
systematic risk assessment will have value in providing the cli-
ent with an audit trail which demonstrates that all foreseeable
risks have been rigorously identified and assessed.
12.6.4 Class 3
Car parks with more than six storeys, hospitals with more than
three storeys, and all other buildings exceeding fifteen storeys,
or to which the public are admitted and containing floor areas
greater than 5000 m² at each storey, are categorised as Class 3,
together with all buildings to which the public are admitted
in significant numbers, stadia accommodating more than 5000
spectators and buildings containing hazardous substances and/
or processes. For Class 3 buildings, a systematic risk assess-
ment of the building is required which takes into account both
foreseeable and unforeseeable hazards.
While not specifically required by Approved Document A,
it is indisputable that the design of a Class 3 building should
meet the requirements of Class 2B as a minimum. There may
be circumstances in which the application of the requirements
for a Class 2B building is not straightforward, for example, in
special structures, sculptures or structures that are not conven-
tionally framed. In such cases, the design should demonstrate
by alternative means that the structure is at least as robust as
that implied by the requirements for Class 2B buildings in the
context of framed construction.
In some circumstances it may be useful for the structural
engineer to recommend the designation of buildings as Class 3
which are not formally classified as such. The structural engi-
neer may explain to clients that it will rarely be detrimental to
the design to do so, as a minimum ensuring that the risks to the
12.7 Interpretation of building risk class
and design requirements
The definition of the building risk class is usually straightforward,
but in some instances may be open to interpretation, perhaps
because the building's use is ill-deined, is mixed use, has a vary-
ing number of storeys, incorporates mezzanine floors, incorporates
an unoccupied plant enclosure at roof level or due to the rules on
the classification of basement storeys. For buildings undergoing
alteration, modification or extension it is also sometimes unclear
what building risk class and therefore design requirements should
apply. The discussion below is written specifically with respect to
the application of Approved Document A in the United Kingdom
and is based on previous decisions of Building Control Officers on
compliance with the Building Regulations. Similar issues apply in
other jurisdictions although the thresholds may be different.
Consequence class
Example of categorisation of building type and occupancy
1
Single occupancy houses not exceeding 4 storeys. Agricultural buildings. Buildings into which people rarely go, provided no part
of the building is closer to another building, or area where people do go, than a distance of 1.5 times the building height.
2a Lower risk group
5 storey single occupancy houses. Hotels not exceeding 4 storeys. Flats, apartments and other residential buildings not
exceeding 4 storeys. Offices not exceeding 4 storeys. Industrial buildings not exceeding 3 storeys. Retailing premises not
exceeding 3 storeys of less than 1000 m
2
floor area in each storey. Single storey educational buildings All buildings not
exceeding 2 storeys to which the public are admitted and which contain floor areas not exceeding 2000 m
2
at each storey.
2b Upper risk group
Hotels, flats, apartments and other residential buildings greater than 4 storeys but not exceeding 15 storeys. Educational
buildings greater than single storey but not exceeding 15 storeys. Retailing premises greater than 3 storeys but not exceeding
15 storeys. Hospitals not exceeding 3 storeys. Offices greater than 4 storeys but not exceeding 15 storeys. All buildings to
which the public are admitted and which contain floor areas exceeding 2000 m
2
but not exceeding 5000 m
2
at each storey. Car
parking not exceeding 6 storeys.
3
All buildings defined above as Class 2 Lower and Upper Consequences Class that exceed the limits on area and number of
storeys. All buildings to which members of the public are admitted in significant numbers. Stadia accommodating more than
5000 spectators. Buildings containing hazardous substances and/or processes.
Note 1 For buildings intended for more than one type of use the 'consequences class' should be that relating to the most onerous type.
Note 2 In determining the number of storeys basement storeys may be excluded provided such basement storeys fulfil the requirements of 'Consequences Class 2b Upper Risk
Group'.
Note 3 Table A.1 is not exhaustive and can be adjusted.
Table 12.1
Categorisation of consequence classes (BSI, 2006). Permission to reproduce extracts from BS EN 1991-1-7:2006 is granted by BSI
