And the winner is.... the 2009 Demolition Awards Contract of the Year

The statistics covering Keltbray's demolition of the various structures on 20 Fenchurch Street are impressive - a £22 million (US$36 million contract), in excess of 500,000 working hours, the installation of 500 tonnes of temporary steel, 3,500 tonnes of asbestos removed, completion ahead of schedule and a string of awards.

The work involved the demolition of a 26 storey tower block that included 20 floors that were suspended from a heavily reinforced, post-tensioned concrete ‘umbrella' roof structure. According to Keltbray, understanding exactly how the roof structure supported the building was fundamental to determining the most effective and safest method of demolition. While work on this structure was underway, Keltbray had to concurrently demolish two 8 storey office blocks on the site.

Keltbray's in-house temporary works engineering department developed a scheme that allowed the tower to be resupported off the original raft foundation, making the considerable roof structure redundant and allowing the structure to be demolished conventionally on a floor by floor basis.

Central to the temporary works scheme was a truss that connected with the underside of the third floor. This was connected to the tower's central core vie large steel rakers and purpose-built reinforced concrete columns dowled into the core and raft foundation. Over 1,300 props were provided to support the upper floors off the truss as the perimeter steel hangers were not able to carry the weight of the structure if they were used as a support.

The 250 tonne truss was bolted together at first floor level over an eight week period and finally lifted in place using strand jacks. At is closest position to the core walls, the truss was less than 25 mm from the face of the core walls when lifted. Following the successful lifting of the truss into place, steel raking columns were installed to transfer the load back to four 2 m square reinforced concrete columns. The placement of the truss into its final position was a major achievement that took place one Saturday morning.

The truss also supported the perimeter scaffold that provided protection during the demolition.

The installation of the floor support props were coordinated with concurrent asbestos removal, demolition of the 8 storey office blocks, the podium - a 200 tonne space frame structure surrounding the base of the tower - decommissioning of the services to the entire site containing a live network substation and construction of a new temporary network substation and temporary relocation of HV supplies.

Asbestos was removed from all the steel hangers and the soffits on the hung floors. In addition, it was found trapped between the holorib decking and the thin concrete slab on each of the hung floors. As a result, all of the concrete floor slabs had to be removed as asbestos waste. Brokk demolition robots were used to minimise the manual labour involved in the work.

The complex arrangement of the roof structure required extensive research using construction photographs, as built drawings, ground penetrating radar and low-frequency ultra-sonic scans and structural investigations. The information gathered was used to create a 3D model that was animated to reflect the demolition sequence.

The results of the low frequency ultrasonic scans proved that the ducts containing the post-tensioned steel strands (tendons) were fully grouted and therefore breaking into the beams would not result in an explosive release of energy. Trial removals of a horn and raking member were undertaken using a wire saw allowing the members to be monitored throughout the cut. The wire saw cut demonstrated that there was no response from the tendons.

Following the installation and inspection of the temporary works system, the steel hangers on the top-most hung floors were severed. The hangers were cut in a predefined sequence to progressively transfer the load from the roof, through the props to the truss.

One innovation was the 3D modelling. Before beginning work on the roof, Keltbray presented the results of its research to the client and the client's team, and the local authority. The presentation was also used as part of the method statement briefing given to all site supervisors and operatives involved in the roof demolition. It was also used in the tenant liaison meeting prior to demolition.

One key to the success of the scheme was the way the Keltbray team achieved the segregation of the work force from traffic and other concurrent work activities. The ground floor was divided to allow separate operational route for the asbestos removal and craneage/delivery access and egress. The first floor was designated as a steel storage and assembly area while the lower ground floor was given over to archeological investigations and foundation works.

Minimising the impact of the demolition on the surrounding area and its inhabitants is key to delivering a successful project. Methodologies are assessed with regard to noise, dust and vibration minimisation and each are monitored routinely. However, demolition is still an inherently noisy operation.

In addition to the usual super silenced machines and secondary acoustic quilting, 20 Fenchurch Street pioneered noise masking on a construction site to further reduce the perceived nuisance. The technique involved analysing the noise signal generated by the demolition and that of the general background. A sound track was then created that had the effect of evening out the peek nuisance noise produced by the demolition.

Work began on this challenging and innovative project in November 2006, with the 111 week work programme being completed in December 2008 - ahead of schedule.