A capital blast

Published 2/3 at 13:51

Vacant for more than two decades, a 15 storey former hotel and later residence for the South African Police Service, was demolished by implosion last October. With other properties only half a metre away, the undertaking was not without its complexities. Contractor Jet Demolition pulled it off in 2.5s on a Sunday morning prior to the site being cleared for another development. PDi’s Kevin Mayhew reports.

The HG de Witt building in central Pretoria, South Africa’s national capital, also referred to as Tshwane, comprised steel reinforced concrete columns and walls with lintel and block floors and clay brick internal walls. It had one basement level and 14 floors above ground.

Implosion was decided on in preference to top-down demolition of the main structure because of time, cost and safety considerations. Mechanical removal of the eastern 1-3 bays to provide fall space for the implosion rubble would have compromised the structural stability and would have required installation of extensive additional bracing on each floor of the building, also undesirable because of time, cost and safety concerns.

“The site was extremely tight for an implosion,” said Jet Demolition managing director Joe Brinkmann. “The east wall of the building was less than half a metre from the parking garage of the adjacent building, while the west side of the building formed the edge of the pedestrian pavement. Low-rise structures connected to the north and south of the main building were demolished mechanically prior to the implosion.”

To avoid flying debris damage, up to five different layers of redundant blast protection were placed over charged columns and walls, fixed as secondary fragment catch curtains on the exterior of blast floors, and positioned over windows of nearby buildings.

Specifically to avoid damage to the delicate façade and large stained glass windows of a nearby church, a 12m high curtain hung midway between it and the imploded building.  This protected it from small particles contained within the implosion wind generated by displaced air as the building collapsed.

It was decided to design a precision centre drop implosion of the building with an east mobilisation bias. High-strength, high-elongation steel tendons were incorporated across the building from east to west to bolster the effective tensile strength and stretch of each floor. The installation of two steel columns extending three storeys upwards from the basement served to strengthen the eastern spine of the building and to preclude any kickback action to the east. These design features achieved the necessary collapse behaviour and the formation of a suitably shaped debris pile that did not affect the adjacent properties. This was a huge challenge in view of only one basement level to accept the collapsing rubble. 

The structural design and layout of the building was such that very limited pre-weakening could be done, requiring extensive sequential blasting on six floors to achieve the desired collapse behaviour. An electronic initiation system was used to obtain the desired sequencing and time delays when detonating the 528 drill holes up to 4m long over a time of 2.5s. In all, 82kg of Watergel and detonating cord-based explosives were used to break the concrete structural elements that were 150mm to 620mm thick.

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