Sunday 21st September 2008

Safedem successfully carried out the demolition of a complex, pre-stressed and post tensioned structure by the controlled use of explosives in Edinburgh’s Sighthill.
At just 12 stories many would have thought that Broomview House would simply be demolished by super long reach demolition rigs, however the structures complex configuration was highlighted by client City of Edinburgh Council who commissioned a detailed structural investigation by technicians from Heriott Watt University and structural engineers Grontmij.
The long narrow tower block had been designed to provide large clear undercroft parking areas at ground floor with the thin concrete floors above being suspended from a folded plate diaphram roof beam by a series of prestressed concrete hangers. The roof plate spanned continuously over a series of lateral shear walls at approximately 10m centres and at roof level a series of horizontal and deflected prestressing tendons tied the various elements together at the head of the wall.
The structural investigations also revealed a number of variations from the construction drawings which included additional post tensioned, un-bonded concrete members at roof level, which the reports concluded had been installed retrospectively in response to movement detected in the structure.
The consultant engineering experts concluded mechanical demolition would potentially lead to an uncontrolled collapse and dynamic energy release from the contracting tendons within the concrete members.
The vertical columns which effectively carried the suspended floor slabs were only 200mm x 500mm on plan, each with two vertical tendons housed within 100mm metal sheaths. This configuration prohibited standard drilling of the columns for the placement of explosive charges and so Safedem developed an innovative technique for the placement of the light charge weights required to disrupt the column and effectively release the energy within the tendons at predetermined locations.
The number of blast floors was selected in order that the hangers and tendons would be cut into small sections by explosives during the collapse releasing the prestressing forces in a controlled manner. The roof level prestressing tendons were also severed by specialist cutting charges triggered immediately prior to the initiation of the main collapse. This ensured that the protection measures put in place to contain the release of energy from these tendons were intact when the tendons were cut.
Prior to the blast over 300 households were evacuated from an exclusion zone by Safedem’s community liaison team. With the evacuation completed, the final firing and security checks were carried out and the charges initiated at 11:15 on Sunday morning.
Following the demolition the debris pile was inspected where it was clearly visible that the tendons had been successfully cut and the structure had dropped well in the centre allowing the gables to be pulled into the centre of the site away from nearby low rise structures and roads.