Chimney effect 'created by unique Wang Fuk design'

An independent committee investigating the deadly Tai Po fire heard on Thursday that the unique architectural design of Wang Fuk Court created a dangerous "chimney effect” that significantly accelerated the blaze.

A fire engineering expert testified on the last day of the fifth round of evidentiary hearings that the building’s trapezoidal re-entrant spaces – recessed exterior areas open on only one narrow side – effectively functioned as vertical chimneys.

This geometry allowed hot, buoyant gases and smoke to rise rapidly through the vertical shafts.

Asif Sohail Usmani, chair professor of building sciences and fire safety engineering at the Polytechnic University, said the recessed spaces acted as more than just a conduit for smoke.

Unlike a fire in the open, a blaze within a re-entrant space benefits from intense thermal feedback.

"The walls absorb the heat and radiate it back to the fuel," Usmani testified.

This reflected radiation caused the materials to burn faster and hotter, creating the conditions for a rapid flashover – the transition from a developing fire to full room involvement.

The recess also intensified the "chimney effect", drawing flames and hot gases vertically upward.

This directly contributed to a "leap-frog effect", where flames bursting from lower-floor windows were forced to hug the exterior facade, subsequently igniting the rooms directly above.

Usmani also identified additional factors that contributed to the fire becoming rampant, including the accumulation of combustible materials, broken windows and flames spreading into the light wells of neighbouring blocks.

As an immediate interim measure, Usmani recommended that architectural features like re-entrant spaces be legally reclassified as special kinds of vertical shafts.

He urged authorities to ban combustible materials from any part of a building facade that could allow smoke to enter evacuation routes as well as the simultaneous cladding of multiple adjacent high-rises.

Citing the 2017 Grenfell Tower fire in London, which claimed 72 lives, Usmani noted that England and Wales have since adopted stringent facade requirements.

Usmani further emphasised that combustible materials should also be prohibited within re-entrant areas.

Where this is unavoidable, he recommended the mandatory installation of fire-retardant windows.

The independent committee will hear closing submissions from July 15 to 17.

Edited by Tony Sabine ...