Civil Engineering Reference
In-Depth Information
reduction in strength of the member. Ultimately the section
will fail when loads significantly below its original capacity
are applied.
cracking in the elevations. Movement joints should be incor-
porated to accommodate any expected movement, thereby re-
ducing unexpected cracking.
Brick units may also suffer from deterioration due to wea-
ther effects over a period of time. Depending upon the original
strength of the brick, any inclusions or defects within the clay
may lead to cracking thereby allowing moisture to penetrate.
Regular wetting and drying along with freeze/thaw action dur-
ing winter will eventually lead to spalling of the brick face
leading to section loss and strength reduction. This does not
necessarily lead to failure of the wall but will increase mois-
ture and damp penetration to the inner face of the brickwork.
Wall tie failure is also common in older cavity walls which
may be evident by bowing or bulging of the external elevation
in certain locations. Where this defect is present at or near a
floor level this may also present a failure of lateral restraint,
usually provided by the floor construction bearing and tied to
the wall.
9.5.5 Masonry failure
Masonry wall construction is designed to accommodate the
transfer of loads vertically down to foundation level and in
external conditions, resisting the secondary loading of lateral
forces due to wind and temperature.
Concentrating on all masonry construction, we can see that
the stability is required in both orthogonal directions and this
can be provided for in a number of ways, such as utilising
masonry shear cores to transmit the lateral loads at floor levels,
utilising the floors as diaphragms or by utilising the internal lat-
eral walls as shear walls buttressing the external walls, thereby
transferring the lateral forces to be distributed to ground level.
Critical failures of masonry wall construction are rare and
may generally only occur due to lack of care during refurbish-
ment (changing load paths) or by a sudden load condition such
as explosion. However, in these cases there are design criteria
established in the relevant codes and standards which the engi-
neer should follow to ensure that overall structural stability is
maintained. This includes the requirements for 'key elements'
in the Building Regulations along with the requirement for
horizontal ties at floor levels. In order to restrict lateral deflec-
tions of large masonry panels, piers or proprietary windposts
are introduced at suitable centres to stiffen the panel and trans-
fer the lateral load to the floor structure.
More commonly, masonry failures may be caused by sec-
ondary effects such as thermal and radiation, environmental
changes (creep and shrinkage), deflections due to applied
loads, ground movement/settlement and vibration effects. As
the brick is modular clay with mortar bedding the finish is brit-
tle and any element of movement will be visible in the form of
cracking. This cracking can follow either the mortar jointing
as shown earlier or can be seen as shear cracking through the
masonry unit. As Driscoll and Skinner (2007) point out, 'brick
walls are unlikely to have cracked unless there have been cen-
timetres of differential settlement across a typical domestic
building.'
The expansion of continuous lengths of brickwork will re-
sult in in-plane movement and cracking at wall returns and at
wall openings. It has been noted that the potential for crack-
ing at a wall return is greater when the depth of the return is
short. The expansion may also account for horizontal sliding
on a damp proof course (DPC). Relative movement may also
occur between materials of differing thermal coefficients such
as concrete and brick or even between block and brick cavity
walls. However, in most cases the use of flexible cavity wall
ties will accommodate any differential movement between dis-
similar leaves.
Care should also be taken where concrete roof or floor
slabs bear upon a wall as the expansion of the concrete slab
will transmit lateral forces to the masonry leading to vertical
9.6 Design
Very rarely can building failure be classed as unforeseen. As
engineers we are expected to undertake building design with
the relevant knowledge gained over many years with our past
experiences being disseminated throughout the profession
leading to changes in working practices. Critical failures such
as the disproportionate collapse at Ronan Point in 1968 led to
changes in structural design in practices around the world and
it is these practical experiences along with ongoing academic
research that lead to changes in the relevant design standards
and guides.
Society relies on the protection afforded by the Building
Regulations and Codes of Practice and also the engineer expe-
rienced in interpolating their requirements. It is the engineer
who must exercise engineering judgement to discharge his
or her responsibility to provide a stable structure, fit for its
intended purpose, that will not endanger its owner, occupants
or the public. As IStructE (1990) states, 'The standard of care
expected from a structural engineer, in whichever capacity he
[or she] is acting, is normally that of a prudent and reasonable
engineer, not of one inexperienced in the work he [or she] has
undertaken to do'.
This brings us on to the errors that may occur during the
design phase of a project including the preparation of struc-
tural details. The key to a safe building is the overall stabil-
ity of the structure. Failure to adequately assess the load paths
to ground level, the possible modes of failure and the struc-
tural behaviour of the system leads to inadequate provision of
restraints and ties necessary to ensure stability and robustness
of the structure.
Errors can occur in the interpretation of code requirements,
the assessment of loads and also the combination of load cases
which may prove more onerous to the design. These errors are
exacerbated by failure to recognise or cater for any secondary
effects such as residual stresses or fatigue.
