Civil Engineering Reference
In-Depth Information
extended recognised. The first such step change was the intro-
duction of the robustness requirements in the Fifth Amendment
in the wake of the Ronan Point collapse, and the second was in
2004 when the requirements were extended to apply to almost
all buildings. As such, there may be some circumstances in
which the robustness requirements make the proposed change
of use or extension untenable, and past determination letters
have shown that vertical extensions are likely to be held sub-
ject to the current requirements of Approved Document A3
(Secretary of State for Communities and Local Government,
2006, 2008): if the addition of a storey to a Class 2A building
raises the classification of the building to Class 2B, either the
existing building must be strengthened to meet the Class 2B
requirements or the proposed extension must be abandoned.
Exceptionally, particular circumstances may justify an alterna-
tive approach consistent with Regulation 8, which states that
the Building Regulations shall not require anything to be done
except '… for the purposes of securing reasonable standards
of health and safety': for example, the case may be argued for
a loft extension to a four-storey block of residential flats for
not assuming the building classification be raised to Class 2B,
provided the occupancy of the building does not significantly
increase (i.e. the extension will provide additional accommo-
dation for the present occupants, rather than being additional
flats). It must be remembered that it is ultimately the responsi-
bility of the Building Control Body to interpret and apply the
Building Regulations.
The so-called 'Camden ruling' is sometimes propounded as
a suitable approach for upward extensions where the addition
of one or more storeys to a Class 2A building raises its classifi-
cation to Class 2B. The Camden ruling, formalised in the mid-
1980s by Camden Building Control, suggests that if the design
incorporates a strong floor at the original roof level designed
(including its supporting structure) to be capable of taking the
debris load from the storeys above, the building may be classi-
fied as Class 2A rather than Class 2B, meaning horizontal and
vertical ties are not required. Consideration of dynamic effects
was required, with a dynamic load factor stated as 3.0 for roof
loadings and 2.0 for walls (Heyne, 2006). This approach has
been accepted in the past by Building Control Bodies as dem-
onstrating a reasonable level of robustness, should the new
construction collapse, to prevent further collapse through the
entire building which would be considered disproportionate.
However, while DCLG do not comment on matters unless
prescribed by the Approved Document, the Camden ruling
is not favoured by DCLG (Carpenter, 2007) and it has gen-
erally now fallen out of favour amongst Building Control
Officers. As such, in considering whether to base a design
on the Camden ruling, the structural engineer must consider
whether he or she would still argue in the aftermath of a col-
lapse affecting the original part of a building following new
construction of additional storeys above that the risks to occu-
pants of a building were no worse than before the extension,
and hence that compliance with Class 2B requirements was
unreasonable. Whether a risk is significantly greater depends
on the perspective of the observer: had the extension not been
built, there would have been nothing to bring about the col-
lapse of the building.
12.8.3 Extensions resulting in no change of building
risk class
Where there is no change in the building risk class as a result
of the extension, the extension is usually considered under
Regulation 4(3), which states that building work shall be
carried out so that after the work is completed, the building
complies with the robustness requirements, or, where it pre-
viously did not comply with the requirements, is no more
unsatisfactory than it was before. The extension must com-
ply with the robustness requirements regardless of the condi-
tion of the existing building. This principle that a building is
made no more unsatisfactory than it was before the extension
is an important one, which should be judged both in terms of
the structural considerations and the occupancy of the build-
ing. The diligent structural engineer should, however, seek to
apply regulation A3 as far as reasonably practicable: where
measures can be straightforwardly achieved that improve the
structural integrity of the building as part of the renovation at
relatively little cost, for example, incorporation of anchorage
details between slabs and load-bearing walls if the façade of
the building must be demolished for construction access and
subsequently rebuilt, it would be remiss of the engineer not
to do so.
12.8.4 Material alterations
The final category of building work on an existing building
that might trigger consideration of the robustness require-
ments is a material alteration. An alteration is
material
if a
building that previously complied with the robustness require-
ments would no longer comply, or if a building that previ-
ously did not comply with the requirements were made more
unsatisfactory. In both cases, consideration is given to the
state of the building at any stage of the building work, rather
than merely the final condition of the building. Alterations
may or may not incorporate an extension, discussed above.
Alterations not incorporating an extension which might
adversely affect the robustness of the building include, for
example, the incorporation of a lift core into a block of flats
to provide level access, the breaking out of the irst-loor
slab to create a double-height lobby or the construction of an
atrium in the building.
Structural alterations without extension may or may not be
accompanied by a change of use which results in an upward
change in the building risk class. A building whose class
increases through the change is likely to be made less satisfac-
tory in relation to Approved Document A than it was before,
because the change in risk class brings with it a requirement
for a higher level of robustness measures for the building
(National House Building Council, 2010).
