Is it really a problem?
The machicolated battlements were last rebuilt in c1932 and are
now leaning again. Curved walls are particularly prone to leaning
outwards at their tops due to cyclical thermal and moisture movement
leading to creep distortion of the masonry.
last 50 years have seen the extremes of people's reaction to movement
in buildings. In the immediate post-war years, when we were grateful
for any accommodation which had survived the Blitz, attitudes to odd
cracks were relaxed. Whilst redecorating my father would summon us
children with glee to see finger-wide cracks discovered beneath the
wallpaper, before ceremoniously plugging them with newspaper and Polyfilla.
No panic attacks for him, whereas nowadays structural engineers are
increasingly called out to pronounce upon hairline plaster cracks
dramatised by white emulsion paint.
expectations of building performance have become unreasonably high
and everything is too precious nowadays. It is time for reactions
to be tempered by considering the issues.
forces of nature are capable of breaking down mountains, so we must
assume that a building will also not last indefinitely. Regular maintenance
and occasional structural intervention is essential to slow the process
of deterioration and to extend the life of its structure. Intervention
may be aimed at preserving the building indefinitely, but a more realistic
view may also be taken with finite expectations for both original
fabric and repairs.
WHAT IS STRUCTURE?
parts of the building fabric which confer significant strength, stability,
and integrity, such as roof carcassing, floors, walls, frameworks,
and foundations form the principal structural elements. Non-structural
fabric such as plaster, render, windows and doors can also help stiffen
a structure but their contribution is not to be relied upon in a significant
WHAT IS STRUCTURAL MOVEMENT?
settlement, heave, sway, bouncy floors, bulging walls, cracks, expansion
and contraction are all forms of structural movement. Such movement
occurs all the time, and usually its magnitude is so small it passes
unnoticed. Only when distortions and cracks threaten the use or safety
of the structure need we be concerned.
CAUSES OF STRUCTURAL MOVEMENT
structures are designed to carry their own weight and imposed loads
so that strains are kept within reasonable limits; safety factors
are included to cater for variations in quality of materials, design
and construction inaccuracies, and random or accidental forces. In
historic structures detrimental movement results from inadequate design
and construction, decay and ill-considered alterations.
the advent of calculus and 'modern' engineering, early historic structures
largely succeeded due to generations of craftsmen constructing buildings
in accordance with what they knew to have worked previously, and avoiding
construction that had failed to perform. In other words, safety factors
were incorporated by experience rather than calculation. Nevertheless
in medieval structures it is common to find that secondary floor joists
are often larger than they need be, whilst primary beams are undersize
and sagging excessively. Apart from this, and some more singular problems,
it is perhaps surprising that inadequate strength is generally not
the start of the Industrial Revolution, the increasing involvement
of the engineer, first with grand buildings and latterly more humble
structures, ensured more adequate sizing of structural members. Exceptions
include domestic buildings with timber floors overloaded by office
LACK OF CONTINUITY OR 'TOGETHERNESS'
vast majority of the nation's structures are low-rise unframed buildings,
where the individual components are predominantly held together by
friction and gravity. Most such structures (speculative Georgian and
Victorian housing for example) have out-performed the expectations
of their constructors without the involvement of engineers and despite
the two World Wars. However, as buildings relax and become frail with
age, the single kindest way of increasing their longevity is to tie
them together. Conversely the lack of continuity leaves the structures
vulnerable to disproportionate damage.
is the principal agency effecting the decay of most structural materials,
- timber decay
- rusting of iron and steel
- sulphate attack of cement and concrete.
battle against water can largely be won by giving the building a good
roof; by ensuring that driving rain is thrown clear of the building
by generous drips, throatings, over-sailing copings and bonnets; and
by preventing rising damp either through a damp-proof course (dpc)
or by ensuring that the ground is well drained.
crystalline materials (stone, concrete and brick for example - see Table 2) expand
and contract with changes in moisture-content and temperature. The
resultant strain must be accommodated by the structure, or permanent
deformations and cracks will occur. If movement is cyclical, then
such cracks may grow due to the 'ratchet' effect of debris in the
cracks preventing full closure.
most structures in the UK the principal load bearing element is the
masonry. Different types of masonry move at different rates, and sometimes
in opposing directions (Table 2). This can give rise to differential movement
and distortion (see illustration of Herstmonceux Castle). Fortunately
most walls constructed before 1914 were set in lime mortar, which
can accommodate considerable amounts of movement without cracking
due to creep (continual strain under constant stress), whereas more
modern walls require the frequent provision of movement-joints.
SUBSOIL AND FOUNDATION INADEQUACIES
medieval timber buildings had their main posts dug into the ground,
but almost all buildings which still survive had sill beams resting
on low masonry plinths. Medieval masonry buildings had walls which
were built straight into the ground without any attempt to disperse
the load over a broad foundation: latterly the walls were sometimes
stepped out, or 'corbelled', to provide a wider distribution of the
load on the soil.
good ground, corbelling continued until the First World War, latterly
with a shallow strip of concrete first cast into the trench, about
500mm below ground. In poor ground, short timber piles were sometimes
driven before commencing the masonry. With the advent of modern mild
steel and reinforced concrete at the turn of the century, foundations
became more sophisticated.
of shallow spread foundations is commonly caused by normal constructional
settlement, mining, leaking drains, shrinkable clay, tree-roots, changes
of water-table, additional loads and tunnelling. Flexible historic
buildings are often better able to cope with movement than modern
rigid structures, thanks to the prevalence of soft lime mortar, massive
walls, timber-frames, arches, and vaulted construction. Modern structures
with slender walls set in hard cement mortar with brittle plaster
and no cornices, will show every crack.
A lack of bracing can ultimately lead to collapse. Many a medieval church,
for example, has had a gable end rebuilt following movement of its
unbraced roof: this was prevented in more elaborate construction by
diagonal wind-braces which were inserted in the plane of the rafters.
Victorian shop-fronted terraces are also prone to falling over, being
perched on slender columns (see Sketch 2 above).
ALTERATIONS AND MISUSE
floor joists for services, doorways cut through trussed partitions,
partly-removed chimney-breasts and overloaded floors are the most
popular abuses of buildings. Many such alterations become obscured
over the years, and it is only investigative work that will uncover
the cause of the distortion (see Sktch 3).
ASSESSMENT AND CONCLUSION
this background of potential movement, it is hardly surprising that
buildings seldom perform perfectly, and rarely acquire true stability.
But is this important? If a building has sufficient commodity, firmness,
and delight then the odd distortion can be part of the charm, the
patina, of an historic structure.
intervention by engineers may be unnecessary for the odd symptom of
distress, it is too easy to rely on the assumption that a building
will last indefinitely simply because it has survived the last 200
years, while the building tiptoes to disaster.
movement is serious when the safety-margins of strength, stability,
or integrity have been significantly eroded, or the movement is progressively
leading to failure within a specified period. For a relatively modest
structure such as a house, no action may be considered necessary unless
the structure is likely to fail within a period of perhaps five years,
but for a cathedral a much larger safety margin would be necessary,
of perhaps fifty years due to its scale and the high cost involved
in carrying out major works. Expectations for the duration of a repair
may also vary (see Table 1).
An engineering assessment of the seriousness of any particular symptom
of structural distress is not just by calculation, but also through
an understanding based on practical experience of the performance
of old structures, and the intangible contribution of the non-structural
fabric, such as the stiffening effect of horsehair in old plaster.
The Building Research Establishment offers some guidance on the seriousness
of crack-widths but this must be used circumspectly. Cracks should
be examined to determine their cause, not rigidly filled in to see
if they reappear, as this may restrict cyclical movement causing the
problem to escalate. Careful examination can reveal the direction
of movement, and whether movement is ongoing. In particular:
at crack faces - how have they come apart?
the arrises fresh and clean?
there old paint or filler in the cracks?
old are the decorations?
the probable cause of the structural movement is still unclear, or
if the movement is suspected to be progressive, then movement monitoring
is warranted (see Table 3). Monitors are aids to diagnosis and prognosis, not a
substitute to understanding structures.
the days have long gone, when well-intentioned but misguided builders
stuck glass tell-tales across cracks with disfiguring blobs of resin,
in the vain hope that their demise would explain the cause. Mostly
the glass would come unstuck, or schoolboys would break the glass
for fun. The arsenal of equipment available today is vandal-resistant,
and when used wisely, gives meaningful results (see Sketch 4 above). Once the causes have
become clear, it is straightforward to eliminate them and make repairs (see flow chart, Table 4).
movement need not be a problem when considered rationally. Although
structures rarely acquire true stability, cracks and bulges are not
always serious, and crack-monitoring is not automatically necessary.
What needs to change is people's expectations.
Victorians had the right idea; cornices to conceal movement between
ceiling and wall junctions, woodwork painted chocolate brown to camouflage
joint shrinkage, and stretchy Lincrusta wallpaper to obscure random
- C Richardson, The AJ Guide to Structural Surveys, AJ:26.6 - 24.7.85
- C Richardson, Bulging Walls: Survey, Assessment and Repair, AJ:
- JE Gordon, Structures, or Why Things Don't Fall Down, Penguin
Books Ltd, London, 1978
Renovation of Traditional Buildings, CIRIA report no 111:
- HJ Eldridge, Common Defects in Buildings, Dept of the Environment, Property Services Agency, HMSO, London, 1976
This article is reproduced from The Building Conservation Directory, 1996
RICHARDSON BSc, CEng,
FICE, FIStructE, ACIArb is a Chartered Engineer and Technical Director of URS and their Conservation Team Leader. At the time this article was written he was Associate Director
of JAMES Consulting Engineers, Engineer to
the Dean and Chapter of Westminster Abbey, and Visiting Lecturer
in building conservation at The Architectural Association School
of Architecture, London.
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