FEARS OVER the safety of cast iron beams supporting brick jack arch roofs in 19th century town houses were raised following collapse of a roof at a property in London's Bayswater last week.
Engineers believe the collapse of the 1836 vaulted structure at 1-24 Hyde Park Gardens was caused by the failure of a cast iron beam supporting two shallow roof arches.
Investigations will continue on the other properties in Hyde Park Gardens. There is particular concern about nine similar roof structures in the five storey terrace. This includes five each on the east and west wings, and the Sri Lankan High Commission which forms the central part of the terrace.
'If we can't justify a margin of safety then we will have to do something about them. The consequences are too great if they fall, ' said Clive Richardson, technical director at consultant Cameron Taylor Bedford, who is investigating the collapse on behalf of the Church Commissioners, freeholder for some properties on the terrace.
The beam is thought to have failed after being weakened by increasing loads from decaying lime concrete fill which had been placed in the void between the arches and the terrace garden above.
The 6.6m span cast iron beam snapped at its centre point with a brittle fracture. The two brick arches and concrete infill deck above collapsed leaving only the paving stones on the terrace above intact.
The property was unoccupied at the time of the failure which went unnoticed until discovered by the owner's chauffeur - just one day before a party was to be held on the weakened terrace.
Richardson said his initial inspections indicated that the collapse had been caused by a loss of strength in the decaying lime concrete over 150 years which had gradually overloaded the beam.
This loss of strength seemed, he said, to have reduced the concrete's ability to carry overarching loads above the beam, and increased compression forces on the brick arches. Over time the additional load on the cast iron beam would have caused it to snap.
Further calculation and laboratory analysis would be needed to confirm this theory, he added.
Richardson said it was the first case of a cast iron beam failure in a residential property that he had found in 25 years. He is sending the results of lab tests on the concrete and beam to the Standing Committee on Structural Safety (SCOSS).
'Jack arch construction with cast iron beams was very popular in houses, factories and warehouses between 1800 and 1840. I can't believe there aren't a lot of similar designs elsewhere in the UK, ' he said.
Work began last week to install props in all basement rooms of the ground floor flats in the terrace. Letters had been sent out but not all residents had responded as NCE went to press.
Long term solutions could include the addition of permanent central props in the affected rooms, introducing a new roof structure underneath the cast iron beam or completely replacing the roof with a modern structure.