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Trial and error

technical & practice: From Roman times to Ronan Point, it is a good thing that building designers have risked failure as a means to an end

About 1247, work began on the choir of Beauvais Cathedral. It was completed up to the eastern piers of the transept by 1272. Then on 29 November 1284 at 8pm, the great vaults of the choir fell, and the divine service ceased for 40 years.

Medieval masons did not have the benefit of mathematical methods to design structures; they worked to rules of geometric proportion.When something collapsed, they tried new avenues. If successful, the masons were bolder the next time. So it goes.

Gothic cathedrals, according to Jacques Heyman 1, stand by virtue of a more or less delicate balance of forces; the thrusts are taken through the flying buttresses to the main buttresses, and so to the ground.

Beauvais had collapsed because the lateral thrust of the 48m-high vaults did not have sufficient abutments, exacerbated by buckling of an eccentrically loaded supporting pier. The fraternity of masons learned the lesson, and at the cathedral of Troyes, the lateral buttressing was twice consolidated in the 1360s and in 1402.

Between 1564 and 1569, a 153mhigh stone tower was added to the transept of Beauvais. About two years later, the crossing piers were beginning to lean. The chapter delayed for two years and repairs were not started until 17 April 1573.

Some 13 days later, the tower fell.

Although no one was killed, the tower was never replaced, and Beauvais became what it is today, a choir and transept without a nave.

Cathedrals continued to be built and extended in post-medieval times. Improved mathematical skill and engineering principles began to be applied to other grand structures, such as bridges, railways, and canals; but trial and error remained.

Made from girders In the 1840s, Robert Stephenson chose cast-iron girders for his railway bridge over the River Dee. Cast iron had long been known to be weaker in tension than compression, so he compensated by designing larger bottom flanges.

Similar girders had been successful before, so Stephenson simply made them bigger. But he did not realise that such linear scaling up would erode the girders' safety margins, where buckling instability became critical.

The bridge opened in 1846, and vibrated under passing trains, but the absence of trouble with similar structures forestalled closer scrutiny.

2The following year, five inches of ballast was added to cover the timber planking on which the rails were laid. The driver of the first train across the newly ballasted bridge felt it 'sinking' under him, and so opened the throttle to rush across. Only the locomotive reached safety; its five carriages and the girders crashed into the river.

The most likely cause of failure was buckling of the tall cast-iron girders with their unequal flanges, after the small safety margin had been eliminated by the addition of the ballast.

Five people were killed and 18 injured.

The loss of life called for a coroner's inquest, which in turn led to a Royal Commission to look into the application of iron in railway structures.

Isambard Kingdom Brunel gave evidence to the Commission. He argued against rigid rules for bridge building and even called the investigating body 'The Commission for Stopping Further Improvements in Bridge Building'. He believed that with proper care in eliminating nonhomogeneous aspects and other imperfections, reliable iron castings could be made 'of almost any form, and of 20 or 30 tons weight'.

But Brunel and many other engineers did not bargain for the variability of the tensile strength of cast iron, as well as its low strength.

Nor could they cater for the deceit of some unscrupulous foundries, who would disguise poor castings with Beaumont's Egg: a mixture of beeswax, fiddler's rosin, finest iron borings and lamp black.

Bridge of the Silv'ry Tay 'So the train mov'd slowly along the Bridge of Tay, Until it was about mid way, Then the central girders with a crash gave way, And down went the train and passengers into the Tay!

The Storm Fiend did loudly bray, Because ninety lives had been taken away, On the last Sabbath day of 1879, Which will be remember'd for a very long time'.

William McGonagall

The Tay Bridge disaster 3prompted the first public inquiry in this country. The findings were inconclusive but the tensile reliability of cast iron had long been doubted by engineers.

Despite such major tragedies, the construction industry was generally successful, keeping pace with an unprecedented population boom - from nine million in 1800 to 45 million in 1930 - and despite using mainly semi-skilled labour and limited technical input.

But after the Second World War, the country was virtually bankrupt, with the largest debt of any nation in the world. Infrastructure was weak and of the 12m dwellings before the war, eight per cent were destroyed, and 28 per cent were damaged. Finding new ways to build quickly and cheaply was an urgent demand; stretching scarce materials further, and using a labour force whose skills had been depleted by the war.

Tried-and-tested buildings were abandoned in favour of new styles, materials, and techniques, such as the Scandinavian large panel system (LPS), first used in the UK in 1956.

Factory-made precast concrete, storey-height wall panels and floor plates were stacked and tied together on site, like a house of cards - but they went up quickly.

Dying for a cuppa And so it was that on 16 May 1968 at 5.45am, Ivy Hodge, a spinster of Canning Town, east London, awoke in her 18th-floor flat at Ronan Point 4and went to make a cup of tea. She struck a match and sparked off a gas explosion which changed forever the way engineers design tall buildings.

Hodge's flat was on the corner of the tower block. The explosion blew out the external wall, taking away support for the floor above. The corner collapsed progressively upwards to the top of the 24-storey building, and then the falling debris overloaded the 18th floor, which collapsed progressively down to the ground. Four people died and 17 were injured.

Miss Hodge survived to tell the tale to the public inquiry. Gas supplies were removed from similar tower blocks, and a programme of strengthening was commenced. Two years later, in 1970, the Building Regulations were changed to require new buildings over five storeys to be better tied together, and to have alternative means of support in the event of losing a load-bearing element.

Only five years on, the Building Regulations had to be changed again, to deal with another ill-conceived post-war innovation: High Alumina Cement (HAC) in precast concrete construction, to speed up the setting of the concrete. HAC had been manufactured in the UK by Lafarge since 1925 and used principally for its good resistance to chlorides and sulphates. But after the Second World War, it was more widely used for its rapid hardening qualities.

But, gradually, doubts began to rise about the reliability of HAC concrete. On 8 February 1974, a swimming-pool roof collapsed at Sir John Cass' School in Stepney 5, abuilding which had been constructed between 1965 and 1966 using prestressed HAC concrete roof beams, which spanned 10m between solid brick walls around the pool.

The beams were tested, and their failure was attributed primarily to chemical change of the HAC concrete with a substantial loss of strength. Then the weakened concrete had been attacked by sulphates from the plaster roof finishes, causing its disruption and collapse.

Caveat emptor In 1975, the 'deemed to satisfy' status of HAC was withdrawn from the Building Regulations, and guidance on the strength and durability of existing HAC structures was given by the Building Regulations Advisory Committee (BRAC) subcommittee.

Buildings were exempt from appraisal if they were residential, and not more than four storeys, and without persistent leakage or heavy condensation, and their beams did not exceed certain spans.

Many existing HAC structures have since performed satisfactorily, according to the Concrete Society's technical report No 46, 1997 'Calcium aluminate cements in construction - a reassessment'. HAC continues to be used for non-structural uses such as tidal barriers, roads, runways, screeds, sprayed concrete, and foundry floors.

Another vexatious post-war innovation, still in use today, is the trussed-rafter roof, although when they were first introduced from America in 1962, they were involved in a spate of collapses, such as the Sports Hall at Rock Ferry School, Birkenhead in 1976 7.The trussed rafters were unbraced, buckled sideways, pushed over one gable end, and collapsed. After so many failures, perhaps Rock Ferry School was the final straw. The Building Regulations were amended in 1976 to make rafter bracing mandatory.

How had we come to this position? Before the advent of trussed rafters, most timber-pitched roofs were a collection of rafters variously supported by purlins and struts off internal walls. The rafters were principally expected to resist only bending forces. However, trussed rafters enabled clear spans between external walls, so now the rafter members were put in compression as well as bending - a treacherous combination of loads for any slender structural member.

This subtle, but fatal, change in the way rafters were being used escaped many designers and builders. Bracing was often not provided, and sometimes failure resulted. There are still many unbraced roofs in existence today, just waiting for a strong gust of wind.

Parking laid to falls LPS buildings, HAC concrete, and unbraced trussed rafters were but three of many immediate post-war problems. By the 1980s, most of them had manifested themselves, and we had time to reflect upon the shortcomings of those desperate post-war years.

We began to enjoy a new era of good construction until, on 21 March 1997, dozens of multi-storey car parks throughout the UK faced long-term closure and eventual demolition after the sudden collapse of part of a 1965 car park in Wolverhampton 8.No one was hurt when part of the top floor of NCP's four-storey Pipers Row car park crashed down to the floor below. There were no cars on the top two floors; the columns simply punched through the upper floor.

The car park had been built using the 1950s American-developed lift slab technique, whereby slabs are cast on the ground, lifted up the columns, and locked into place. The regularly inspected car park had not been showing signs of deterioration, and the sudden failure of an unloaded structure without signs of distress, was particularly disconcerting.

Within weeks it was revealed that all flat slab structures without dropheads, not just lift slabs, built in the 1960s and 1970s, were at risk from catastrophic punching shear failure.

The structural design code at that time was CP 114; 1957. It gave inadequate guidance on flat slab design. A new code, CP 110, remedied the situation, when it was published in 1972; enhanced by BS 8110 in 1985.

After the Pipers Row collapse, it was feared that ageing flat slabs designed to the old code CP 114 could pose a widespread problem 9.The Standing Committee on Structural Safety (SCOSS) reiterated its 1994 recommendations that owners and operators of existing multistorey car parks should commission structural appraisals periodically, and such appraisals should include deterioration due to road salt, progressive collapse, and edge barriers.

That advice is still relevant today.

Don't panic

We should remember that the vast majority of buildings in this country outperform the expectations of their designers and builders, lasting longer, and often with far less maintenance than originally intended. In conclusion, trial and error has always been with us and will remain so if design and construction is to continue developing and improving.

While error is not desirable, we cannot eliminate the risk of failure unless we simply repeat what has succeeded before. Designs would then stagnate - we would not have, for example, the recent trend in modern bridges. Foster's Millennium Bridge in London may have wobbled, but its trial exposed the error which has now been rectified.

Because of it, we are able to delight in its design for years to come. Perhaps society at large can repay such delight by greater forbearance when failure occurs.

Clive Richardson is a structural engineer and technical director of Cameron Bedford Consulting. Contact clive. richardson@camerontaylor. co. uk

References

1.Heyman, J, Beauvais Cathedral, Transactions of the Newcomen Society, London,1967-68.

2.Petroski, H, Design Paradigms, Cambridge University Press,1994.

3.Prebble, J, The High Girders, Pan Books, 1968.

4.Report of the Inquiry into the Collapse of Flats at Ronan Point, Canning Town, HMSO,1968.

5.BRE CP 58/74 (Sir John Cass).

6.Concrete Society Technical Report No 46,1997 Calcium aluminate cements in construction - a re-assessment.

7.BRE CP 69/76 (Rock Ferry).

8.New Civil Engineer 27.3/3.4.97, pages 3-4 and 10.4.97, page 3.

9.BCA Conference 29.9.97 Concrete car parks - design and maintenance issues.

10.13th Report of SCOSS, Structural Safety 2000-01, Institution of Structural Engineers, May 2001.

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