The dislodging of a ventilation duct from the ceiling of a cinema hall at Nex shopping mall on Sunday was possibly a result of weakening duct supports, such as loose nuts and bolts or spalling concrete slabs, said industrial experts here.

These ducts carry cool air from a central air-conditioning system to different parts of a building.

Associate Professor Daniel Wong from the department of building at the National University of Singapore's School of Design and Environment said that the duct is usually bolted to metal rods that are secured to a concrete ceiling slab.

"For the ventilation duct to fall, there are three main possibilities: a damaged rod, nuts and bolts which are loose or faulty, or that the concrete slab is beginning to rust or break," explained Prof Wong.

"However, based on the photos of the dislodged duct, it seems that the metal rod is still intact and remains connected to the duct, so there is a higher possibility for the other two scenarios," he said.

Prof Wong, who has been in the projects and facilities management industry for over 20 years, said he has seen only three or four cases of ventilation ducts becoming dislodged.

In those cases, the rod was accidentally hit during cleaning, or nuts and bolts that had been removed during maintenance had not been securely tightened.

Prof Wong said that to prevent such accidents, these parts have to be regularly replaced once their product life cycle has been reached, and carrying out regular inspections can detect wear and tear early.

Project manager Zaw Myo Lwin, who works at a local air-conditioning maintenance company, noted that the metal rods attached to the ducts are typically able to carry a weight of between 50kg and 100kg.

However, based on the photos of the dislodged duct at Shaw Theatres Nex, the likely weight for its size would be around 20kg, he said. Hence he suspects that it was a drop-in anchor, a type of fastener which secures the duct to the concrete, that became loose.

In response to queries from The Straits Times, a spokesman for the Building and Construction Authority said that a professional engineer has been appointed to investigate the incident, along with other ducts in the cinema.

Rectification measures will be recommended depending on the findings, and these will be carried out by the building owner, she added.