Civil Engineering Reference
In-Depth Information
structures such as retaining walls when, although designed
to accommodate lateral loads from soil and water over the
retained height, the increase in pore water pressure can cause
rotational and sliding failure. As saturated soils are more com-
mon close to oceans, lakes and rivers, failures of harbour quays
and bridges due to liquefaction are common during seismic
events.
manufacturing methods are being used for the first time with
the associated unknowns. In these cases, issues such as the
true allowable material stresses or the inclusion of faults dur-
ing the production process come to light. This highlights the
reliance of the design engineer on adequate industry testing
and academic research or analysis, and on bringing this to
the engineering practitioners. As Hossain (2009) points out,
'well designed structures, coupled with the hard effort of the
experts and correct materials can ensure the structure a com-
plete success'.
As has been discussed previously, the long-term deterior-
ation of
in situ
materials is a common fault in buildings and
this ties in with the requirement for proper management of
the structure. Ongoing regular inspections of the structure to
ascertain deterioration or degradation of the material is cru-
cial, along with adequate material testing where required to
ensure that the change of material properties does not affect
the requirements of the structural system, as was shown in the
Pipers Row MSCP collapse.
It is not just with new structures that we should be aware
of possible long-term material faults. In the UK, we know
of deleterious materials used in building construction in the
past, which we would no longer regard as acceptable today.
Examples of these materials are:
9.4 Modes of failure
In order to prevent disproportionate collapse, as previously dis-
cussed, it is essential that the building structure is adequately
connected thereby providing the required restraint and robust-
ness in the event of loss of support. These requirements are
now well established in the Building Regulations (Section 5
A3) and advice is also given in the relevant design standards
and construction guides.
Within the Building Regulations, key elements are high-
lighted, namely those structural elements which, if removed,
would result in the instability of the building or damage in
excess of the limits provided. These key elements are required
to withstand accidental design loading, both vertically and
horizontally, simultaneously with one third of the normal char-
acteristic design loading.
The use of precast concrete panel systems has become more
prevalent, especially following the Egan Report
Rethinking
Construction
(Egan, 1998), and the drive to reduce waste and
streamline the construction process has led to an increase in
modular systems and off-site manufacturing. The key now,
however, is the requirement for effective tying of structural
members to prevent disproportionate collapse.
With traditional masonry wall construction, the floors and
roofs provide some restraint to the external walls by either
direct support/embedment into the wall or by the use of galva-
nised straps to provide the lateral restraint. Bulging may be the
consequence of differential expansion/contraction between the
inside and outside of the wall, sometimes in conjunction with
wind pressures acting on unrestrained panels causing the outer
leaf (of a cavity) to displace laterally, usually at floor levels.
Here, the engineer should ensure there are adequate straps and
cavity ties wall at these levels.
When designed to the relevant design standards, structural
members should be able to resist the stresses due to applied
loadings, whether static or dynamic (wind/seismic). These
loadings may be applied to the structure in any direction and
the effects of bending, shear or torsional failure of the struc-
tural member should be assessed to ensure the material is not
subject to overstressing.
High alumina cement or concrete
■
Woodwool slabs as permanent formwork or in structural
■
elements
Concrete or mortar additives containing calcium chloride
■
Aggregates for use in concrete (plain or reinforced) which do not
■
comply with the relevant British Standard Specification
Calcium silicate bricks or tiles
■
Asbestos
■
Lead
■
Of this list, some are materials found to be extremely danger-
ous to human health. For example, asbestos (present in insula-
tion boards/cladding/tiles), if damaged, can release fibres into
the air causing damage to the lining of the lungs and can lead
to asbestosis or cancerous malignant mesothelioma, for which
there is no cure. It is not purely a historical legacy of the con-
struction industry as asbestos could be present in any building
built or refurbished before the year 2000. However, in good
condition, undisturbed and properly managed, asbestos does
not pose a significant health risk (HSE, 2010). It is, therefore,
essential that engineers, technicians and other surveying staff
are aware of the risk of asbestos and should always ensure that
if a building does not have an existing asbestos register that a
suitable testing programme is undertaken before any investiga-
tion or construction is undertaken.
Also in the list, however, are deleterious materials which
can cause or lead to structural failure in buildings. Specifically,
high alumina cement/concrete (HAC) has been the cause of a
9.5 Material failure
Structural failures are commonly linked to the materials used in
construction either due to long-term deterioration of the mater-
ial or by overstressing the material in its permanent state.
Lack of knowledge of the material by design engin-
eers is a common fault, especially where new materials or
