Before works could begin on the new Central Plaza development on Dublin’s Dame Street, a range of complex demolition and engineering works were carried out by Hegarty Demolition. ROBBIE COUSINS reports on how these works were completed safely.

The redevelopment of the old Central Bank site on Dame Street into Central Plaza is one of the largest and most prestigious construction projects being carried out in Dublin city centre.

When completed, the Central Plaza development on the old Central Bank site is intended to be the landmark retail, restaurant and office space in Dublin’s south city centre.

Before the current development works could be carried out, a unique and comprehensive range of demolition and engineering works had to be completed in and around the listed building.

The original office building, designed by architect Sam Stephenson and built between 1974 and 1978, is a listed protected structure. It is a freestanding multi-bay seven-floor over basement structure, which is supported by the two lift cores with the floors ‘hanging’ from external steel hangers that run over the lift cores transferring the loads down through them.

In addition, there is an annexe and commercial building to the east of the plaza. These two buildings also form part of the redevelopment works at Central Plaza. The commercial building is a three-storey over single-basement construction, while the annexe building is a two-storey over double-basement construction.

The brief

Hegarty Demolition was contracted on behalf of the developer Hines to carry out enabling, alteration and extensive construction work. The main areas where works were carried out comprised:

• The tower building east and west core
• The roof of the tower building
• The plaza and basement
• The annexe and commercial buildings, and
• Open spaces around the buildings.

Works carried out by Hegarty Demolition and its subcontractors included:

• Structural demolition and alteration work
• Extensive temporary works design and installation
• Secant piling and capping beam
• Excavation and rock removal
• Mechanical and hydraulic flat-jacking to alleviate loads during alteration works
• Complex underpinning
• Structural steel and reinforced concrete (RC) construction
• Installation of 11 stairs, from large main access to internal stair core (both RC and structural steel (SS))
• Lift-shaft extension and construction
• Stair-core construction
• Installation of numerous RC slabs, including the use of lightweight concrete on the top level
• Installation of new foul and stormwater systems around the ground level perimeter
• All plaza civils works, including waterproofing, podium build up and services installation
• Construction of a double ESB substation housing unit and water-tight sprinkler tank in the basement using self-compacting concrete.

Tower building – east and west cores

The majority of works on the tower building were carried out within the two main RC access cores. The original design of the floor plates meant they were open plan and did not require much structural intervention. The works to the east and west cores consisted of structural demolition and alterations, remediation and repair of existing concrete, followed by new construction using structural steel and composite floors.

Mechanical and hydraulic jacking was used in many places within the two cores to permanently de-stress existing RC walls and allow for demolition and alteration.

“As a concept, this was relatively new to us and one which we wouldn’t have used in demolition projects too often,” comments Kieran McKeown, Director, Hegarty Demolition. “The use of jacking was a nice variation to traditional temporary and permanent works solutions. Our on-site team, who worked closely with our experienced subcontractors, developed invaluable knowledge and experience of various jacking techniques which can be applied to demolition, enabling and structural engineering projects in the future.

“While jacking isn’t required in most situations, it is particularly useful in large projects when making changes on structural sections carrying huge loads.”

Methodology

“Effectively we were working on two RC towers within the building,” Kieran McKeown continues. “Each one had a footprint of 100 sq metres and was 12 storeys high. When carrying out selective demolition, starting at roof level, 12 storeys was going to take some time to get through to ground level, only then constructing new load-bearing elements and working back up to roof level.

“Our temporary works engineers, Barrett Mahony Civil Engineering, devised innovative temporary works that enabled us to both demolish and construct top down. This allowed us to start at the top, and when demolition was complete, on say level 8 east core, the crew moved to the level 8 west core and commenced demolition there. This allowed the steel and concrete crew to follow and complete construction in the east, ensuring separation was maintained between the demolition and construction works.”

Hegarty Demolition was able to improve on programme significantly, by installing heavy crash decks in opes and shafts at level 2. This completely sealed everything in the cores below level 2 from everything above level 2 This allowed a demolition and construction team to work top down from roof to level 2 (seven storeys). It also allowed a second crew of demolition and construction operatives to work safely top down from level 1 to B-2 (five storeys).

The cores were confined areas, which suited small mechanical demolition using a remote control Brokk 100 and 180 demolition robot, fitted with muncher and breaker attachments. In other instances, the saw, cut and lift method was more suitable. Demolition rubble was transported to lift shafts, which were used as chutes to remove rubble and glass from the upper floors.

Larger concrete sections, which were saw cut, had to be transported across temporarily propped floor plates to external loading bays for lifting with a tower crane.

Tower roof

When completed, the top two floors are intended to house a restaurant. Construction for these new floors required the installation of a complex structural steel frame and lightweight composite slabs.

The introduction of the structural steel frame and slabs to facilitate the new two-level rooftop structure meant that additional weight was being added to the main structure. To keep this additional weight to a minimum, the new slabs were constructed using lightweight concrete.

“On the tenth floor of the building, we poured lightweight concrete to create the new floor slabs,” explains Kieran McKeown. “A relatively new concept, lightweight concrete is designed to flow easily and create structural concrete sections, while keeping weight at a minimum.

“The use of this concrete provided a new challenge for our on-site team as it was the first time they had experienced using it on this scale. Lightweight concrete has to be absolutely right when pumping at high pressure over a long distance. The main pour on the roof required pumping vertically 50 metres, followed by 50 metres of horizontal lines across the roof. Our experienced on-site team, with the help of the technical guys in Kilsaran, got it right on the day.”

While there wasn’t much demolition work carried out on the roof of the tower, Hegarty Demolition carried out the majority of structural works. These included:

• Fabrication and erection of structural steel members
• Installation of structural concrete floor slabs
• Extension of RC lift shafts
• Installation of steel stair over multiple levels, and
• Precision-welding to strengthen supernodes.

“As part of the works on the roof, we also carried out extensive temporary works for other contractors. The roofing contractor required floor slabs to be back-propped and robust loading decks to land roof members and glass stillages” Kieran McKeown continues. “The scaffolding contractor required pairs of cantilever steel needles overhanging the perimeter roof nodes by four metres. This enabled them to suspend multilevel hanging scaffolds around the supernodes. The needles were anchored off the new structure on the roof.

“When the scaffolds were in place, we carried out investigatory works to the supernodes, and we also carried out precision-welding to strengthen them. This is an example of the many non-demolition works we carried out.”

Plaza and basement reconfiguration

The plaza area will be a focal point of the new Central Plaza development and probably the most eye-catching section of the new development. To create this space, extensive demolition and reconstruction work was carried out. This involved demolition of upper precast plaza slab, lower RC podium slab and existing tree-pit structures. Many new supporting columns and beams, both RC and steel were installed to create the structure for the new sunken and upper plazas.

“When completed, the sunken plaza will be at below ground level and be occupied by a range of retail and restaurants,” Kieran McKeown adds. “We carried out the installation of RC and steel stairs from ground level to minus-one level, and another at level -2 to provide access to these levels.”

To the west of the new plaza, a new double-level stair and lift access were constructed outside the existing basement. To enable its construction, the area was secant piled, to provide earthwork support along Fownes Street and to seal against water ingress. A capping beam was constructed over the piles, which had to be raker-propped back to the existing basement.

“There were significant temporary works involved,” explains Kieran McKeown, “to enable an 80-ton piling rig to set up on the podium slab of an existing double basement and carry out piling works.”

In addition to the demolition and construction works for the plaza, Hegarty Demolition also managed and carried out the following other works:

• Screeds to create falls on existing podium slab
• Waterproofing podium slab and drainage board
• Insulation layer and void former buildup
• RC cover slab to receive new paving
• Construction of new sprinkler tank in the basement, and
• Construction of a new three-storey double car lift.

The construction of an RC sprinkler tank in the basement required the use of self-compacting concrete, as the tank was being constructed between an existing floor and ceiling.

“The shape of the tank was shuttered, and the self-compacting concrete poured in sections. Once set, a waterproofing layer was applied to the interior of the tank to ensure water tightness. The water fittings to be used to take water from the tank were fitted before the concrete pour,” Kieran McKeown adds.

Annexe and commercial buildings

As with the main tower building, both the annexe and commercial buildings required extensive demolition and construction works. Demolition was mainly done with Brokk 100s and small hand-held breakers.

“As part of the new layout for the commercial building, a new six-storey RC lift shaft and stair-core were constructed between the annexe and commercial building,” Kieran McKeown says. “This glass structure gives the building a contemporary look, and it works well against the original granite stone façade.”

Other works carried out by Hegarty Demolition in the annexe and commercial buildings included:

• Structural alterations
• Screeds
• Structural steel and metal decking
• RC slabs and walls
• In-situ concrete stairs
• Basement internal waterproofing
• Roof plant structures to annexe and commercial buildings
• New fourth floor to commercial building.

Drainage and other services

New primary foul and stormwater perimeter drainage lines and manholes were installed. Basement drainage was upgraded, including the installation of two new pump chambers and one petrol interceptor. In addition, exiting gullies and manhole covers were changed for modern high-spec products. Above podium level, Hegarty Demolition installed all ducting and chambers for new services to the plaza. To the north along Cope Street, a new double ESB substation on the ground floor of the tower building completed the service works.