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Collapse triggers urgent jack arch safety review

THOUSANDS OF historic jack arch structures suffering from chronic dampness could be at risk from sudden catastrophic collapse, claims a report sent to the Standing Committee on Structural Safety (SCOSS) this week.

Consultant Cameron Taylor Bedford (CTB) submitted the report after its three month investigation into the failure of a cast iron beam beneath a roof terrace behind a row of Grade II listed houses in Hyde Park Gardens in west London (NCE 8 August).

The beam supported the jack arch terrace structure, and CTB's study concluded that chronic water penetration over the years had destroyed the bond between the beam and surrounding lime concrete.

'This composite action had managed to stop what was always an overstressed beam failing even though successive layers of terrace surfacing were added over the years, ' explained CTB technical director Clive Richardson. 'We believe that similar structures dating from between 1790-1850 are in similar danger of collapse if they too have suffered from long term water penetration.'

A spokesman for SCOSS, which is a joint committee of the IStructE, ICE and Health & Safety Executive, said this week that it would consider the report before making recommendations. These could include investigation of the thousands of jack arch structures in the UK.

Remedial action could include installing carbon fibre plate reinforcement on the bottom flange of the cast iron beams.

CTB's calculations at Hyde Park Gardens showed that even when first installed in 1836 the beam was undergoing more than three times what is now accepted as maximum permissible tensile stress - 141N/mm 2asagainst 46N/mm 2- if composite action is disregarded. Over the years as new surfaces were laid on the terrace above, this stress built up to well in excess of 200N/mm 2.Research into beams of this period has shown brittle failure occurring from 116-232N/mm 2.CTB therefore concluded that composite action between the beam and the low strength lime concrete infill between the jack arches and the original terrace surface had been essential to the structure's integrity.

'But when we looked at the lime concrete it was saturated and very friable', Richardson said. 'The maximum compressive strength found was less than 3N/mm 2, which is very weak, and some of the lime was still uncarbonated.

'This indicates the concrete had been deprived of air for a long time, probably by damp.'

In this low oxygen environment graphitic corrosion of the beam web also contributed to the weakening of the beam/concrete bond. No one was hurt when the beam suddenly failed, but the implications for other similar structures could be serious.

'Roof terraces are particularly prone to a build-up of dead load, and waterproofing membrane failures are common, ' said independent structural engineer Mike Bussell, who has written a guide on appraising existing iron and steel structures for the Steel Construction Institute.

'And cast iron beams give little warning of impending failure, unlike steel or concrete, so there is cause for concern, ' he added.

In his report, Richardson differentiates between 'industrial' structures of the period, such as warehouses, factories and railway infrastructure, and domestic applications. 'Industrial structures would usually have had some engineering design input, ' he explained. 'On domestic structures, however, it was usually a case of the builder sizing up a beam by eye or a questionable rule of thumb.'

Dave Parker INFOPLUS www. scoss. org. uk

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