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Moving structures

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The time has come to stop being concerned with movement that does not threaten a building's use or safety

In the immediate post-war years, when we were grateful for any accommodation which had survived the Blitz, attitudes to odd cracks were relaxed. While redecorating, my father would summon us to see finger wide cracks discovered beneath the wallpaper, before ceremoniously plugging them with newspaper and filler. No panic attacks for him, whereas nowadays I am increasingly called out to pronounce upon hairline plaster cracks dramatised by white emulsion paint.

Expectations of building performance have become unreasonably high. Obviously, we must not assume that a mere building will last indefinitely. Regular maintenance and occasional structural intervention are essential to slow the process of deterioration and to extend the life of a structure. But it is time for reactions to be tempered by considering the issues.

This article deals with structural movement - relating to those parts of the building fabric which confer significant strength, stability and integrity. Roof carcassing, floors, walls, frameworks, and foundations form the principal structural elements. Non-structural fabric, such as plaster, render windows and doors, help stiffen a structure but their contribution is not to be relied upon in a significant way.

Subsidence, 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. We need be concerned only when movement threatens the use or safety of the structure.

New structures are designed so that strains are kept within reasonable limits.

Safety factors cater for variations in materials, design or construction inaccuracies, and random/accidental forces. In historic structures, detrimental movement results from inadequate design and construction, decay and ill-considered alterations.

Early historic structures succeeded because safety factors were incorporated by experience rather than calculation. Nevertheless, in medieval structures it is common to find secondary floor joists larger than they need be, while primary beams are undersized and sag excessively.

Apart from this, and some more singular problems, it is perhaps surprising that inadequate strength is generally not more of a problem.

From the start of the Industrial Revolution, the increasing involvement of engineers ensured more adequate sizing of structural members. Exceptions include domestic buildings with timber floors overloaded by subsequent office use.

The 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 outperformed the expectations of their constructors without the involvement of engineers and despite the ravages of two world wars.

However, as buildings relax and become frail with age, the single kindest way of increasing their longevity is normally to tie them together. Conversely the lack of continuity leaves the structures vulnerable to disproportionate damage.

Material decay

Water is the principal agency affecting the decay of most structural materials, causing: frost damage of masonry; rot of timber; and rusting of iron and steel.

The battle can largely be won by giving a building a good roof, and ensuring that driving rain is thrown clear of the building by generous drips, eaves, over-sailing copings, and cills; and by preventing rising damp with a damp-proof course.

Stone, brick, and concrete expand and contract and 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.

In most UK structures the principal loadbearing element is masonry. Different types of masonry move at different rates and sometimes in opposing directions.

This can give rise to differential movement and distortion (see sketch 1).

Fortunately most walls constructed before 1914 were set in lime mortar, which can accommodate considerable amounts of creep (continual strain under constant stress) without cracking, whereas more modern walls set in cement mortar require more frequent provision of movement joints (see sketch 2).

Subsoil and foundations In good ground, corbelling the base of walls - to provide a wider distribution of the load on the soil to reduce settlement - continued until the First World War, latterly with a shallow strip of concrete first cast into the trench, about 500mm below ground.

Movement of shallow spread foundations is commonly caused by normal constructional settlement, mining, leaking drains, shrinkable clay, tree-roots, changes of water-table, tunnelling and additional loads (see sketch 3).

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, show every crack.

Overall instability

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.

Notched 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 sketch 4).

Random forces and accidental damage are unpredictable. Explosions cause high pressures and suctions for very short durations. These dynamic loads cause overload, stress-reversal and dynamic rebound of structural elements. Ductile materials such as steel and reinforced concrete perform better than brittle materials such as timber, masonry and glass.

Fortunately, modern framed buildings have good natural resistance to explosions.

Assessment of stability

Against this background of potential causes of movement, it is hardly surprising that buildings seldom perform perfectly, and rarely acquire true stability.

But is this important? A stable structure is a system of pent-up forces. The odd distortion can be part of the charm, particularly for a historic structure.

Although engineers may be unnecessary for minor symptoms of movement, the need for equilibrium must be borne firmly in mind when exercising the '100year rule'. This says that if a building has stood for a century why should it not stand for another year or two and subsequent increments to infinity? While nothing may have apparently changed during the period of observation, structural fabric gradually degrades due to weather, thermal and moisture cycling and dynamic loads.

In so doing, structures can tiptoe towards disaster, and we must therefore be quite sure that a building which may be showing signs of previous movement has indeed acquired a new state of equilibrium and is not having its margins of safety eroded perilously close to failure.

Structural 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, For a cathedral, a much larger safety margin would be necessary, of perhaps 50 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 2).

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 structures and the intangible contribution of the non-structural fabric, such as the stiffening effect of horsehair in old plaster, or modern sheet flooring.

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.

If 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.

Hopefully 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 like me would break the glass for fun. The arsenal of equipment available today is vandalresistant, and when used wisely, gives meaningful results.

Once the causes have become clear, it is straightforward to eliminate them, and also make repairs.

Conclusions

Structural movement need not really 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.

The 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 cracks.

Clive Richardson is a structural engineer, visiting lecturer in structural movement at the Architectural Association and technical director of Cameron Taylor Bedford (tel 020 7262 7744)

TABLE 2: HOW LONG SHOULD BUILDINGS LAST?

Depending upon the financial, technical or material resources available, new structures or structural repairs may be designed to be serviceable for a specified minimum period. This might be:

between cyclical inspections;

a loan repayment period;

full repairing and insuring lease duration;

30 years (housing corporation rehabilitation cycle);

60 years (housing corporation new-build);

80-100 years (a lifetime - the layman's expectation); or ad infinitum (listed buildings and scheduled ancient monuments).

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