A maintenance contractor's inappropriate undertaking to remove an illegal structure caused a key supporting column to fail, and eventually caused the collapse of the tenement at 45J Ma Tau Wai Road, an expert from the Buildings Department said on Thursday.

Senior Structural Engineer Robinson Chung Kam-yin testified as an expert witness at the inquest into the deaths of four tenants who died when the building collapsed on Jan 29, 2010.

The wreckage showed three key support columns collapse, two on one side of the building against the adjacent tenement, and an independent column in the centre of the building.

Chung said he believes the key column in the middle failed first. The huge destructive force of the rupture then broke the other two columns, and caused the building to fall.

Chung said he suspected the key column was much more fragile than the other two at the side. The precarious state of the column prior to the collapse did not come to light until the day Chu Wai-wing, the private contractor, removed decorative boards that were covering it. Chung made his assumption based on Chu's words, who described the column as seriously broken.

The steel fracture of the column also indicated the abnormal situation from the other two columns.

Chung said he suspected the fracture of the column was caused by external forces, "such as renovation works".

But the chemist from the Government Laboratory Wong Tai-wai said the broken bits of steel were covered by thick rust.

He cannot tell the causes of the steel fractures, but he said he believes the fractures were at least six months old.

Chung's theory was that the collapse of the key column was triggered by Chu's renovation work.

Chu removed a two-story illegal structure using an electronic saw on that morning, right before the collapse. But the illegal structure was connected with a beam on the top of the key column. Without proper reinforcement, the strong vibration may have caused the fragile column to fail.

Chung also backed up his colleagues' assessment of the safety situation of the tenement before the tragedy.

He said he saw no "structural cracking" at that time.

He noted that rusty steel was not a concern since concrete supported most of the load in old buildings such as 45J, as was required by old standards.

Chung also claimed the building load was almost at the point of overloading, considering the extra weight caused by dividing flats and other illegal structures, aging contractual materials, and poor maintenance.

Then, taking into account the broken key column, it would be likely to drop below the safety line, Chung said.

Judge Michael Chan Pik-kiu questioned Chung's report noting there was much ambiguity in the way conclusions were worded.

guojiaxue@chinadailyhk.com

China Daily

(HK Edition 08/12/2011 page1)