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Repeat performance

Structures Moor House

It may not be square, oval or gherkin shaped, but Moor House will be the next exciting addition to London's skyline. Ruby Khan reports.

Sprouting up in the heart of the City, the 19 storey Moor House office building will soon give its famous big brother, Swiss Re, a run for its money as London's new landmark.

Seeking the same success, the trio that worked on the Swiss Re building are behind the $135M project: Skanska as design and build contractor, Arup as engineer and Foster & Partners the architect. With the reinforced concrete core complete, the first sections of its 'bandolier' - a curved tubular steel structural element running diagonally across the building from level 1 to the top floor - are now being erected.

But although the building contains a few twists and turns, the structure is the least of Skanska's problems. Situated at the junction of two main arteries into the City and straddling the security ring of steel, Moor House is cursed with all the factors which make building in the area a logistical nightmare: zero storage space, an island site adjacent to an underground and train station and next to residential, retail and office areas.

'Moor House is not a straight up and down building, ' says Arup lead structural engineer Nigel Annereau. Its asymmetric plan follows the straight lines of its north and west faces and the curve linking London Wall and Moorgate to the south east. The taller flat facades mirror the height and rectilinear lines of the Barbican and blocks to the west.

The building is curved horizontally along the London Wall facade, but also fans out from the first floor in the north east corner to the western edge of the roofline.

Incorporating this vertical curve ensured that the height of the south east side was in proportion with other buildings on London Wall, such as the Grade 1 listed Armourers Hall. It also protects sight lines to St Paul's Cathedral. Without it, planning permission for Moor House was unlikely.

The building comprises two levels of reinforced concrete basement, an 18 storey slipformed core and 17 floors of composite floor of C30 normal density concrete with steel beams and columns.

'It's the concrete core which makes the building stand up. It deals with all the wind forces and in plane forces from the curved geometry of the building, ' says Annereau. 'The floor plate is relatively simple since it is a grillage of beams supported by columns.'

The bandolier forms the boundary between the two curved facades. Nodes where vertical ribs, column and bandolier meet at levels 6, 10 and 14 are fully welded connections to transfer forces and moments.

'The challenge arises from the bandolier since it acts like a beam at the top of the building when it is virtually horizontal, but more like a column as it approaches vertical, lower down the building, ' says Annereau.

The bandolier is made up of straight lengths of circular hollow sections (CHS) up to the 17th floor, which share the common outer diameter of 762mm.

'Originally, the whole bandolier was curved, but it was cheaper to build it in straight sections. Above the 17th floor the sections are precurved as they are exposed. Section thickness increases down the building as it carries a greater proportion of the load, ' says Skanska's senior project engineer, Matt Carver.

Sections of the bandolier arrives on site in lengths determined by transport constraints.

These are site welded because there is limited space behind the glazing line for bolted connections. However, at level 1, where the bandolier is connected to the main slab, a bolted connection between elements is cast in.

A stainless steel member will trace along its length on the outside to define it architecturally, similar to the diagrid at Swiss Re.

To model the steel structure, 3D software package X-Steel was used. 'Without some form of 3D modelling, it would have been extremely difficult to set out the geometry of each member, especially in the case of the bandolier which gives the impression of being curved in two planes, ' says Annereau.

This level of accuracy had positive knock-on effects elsewhere, Carver explains.

'Another advantage of having the computer model is that contractors can prefabricate oval cut outs in ceiling panels and false floors to accommodate rib and bandolier sections.'

All members of the project team also use another piece of software, Navis Works that can superimpose drawings to check for clashes and determine how plant will fit into allocated areas.

'Co-ordinating the roof steelwork has been the biggest issue since all the plant, including two moveable 60t cleaning machines, will have to be negotiated around fixed beams, ' says Annereau.

Steelwork subcontractor William Hare spent about three months at Arup's offices to iron out other co-ordinating issues.

'Each section of bandolier is unique and has its own lifting plan because of the close proximity to pedestrians and traffic.

All steel arrives within an hour of when it is required because of the problems of storage on site, ' says Carver.

Moor House has both top down and bottom up construction, enabling the building to rise without waiting for the basement.

This worked relatively easily on the western half of the site where bottom up construction allowed the basement slab and wall to be built prior to steel erection. The most heavily reinforced basement walls contained six layers of 40mm diameter bars linked by 40mm diameter lacers, the largest that are currently available.

'To limit congestion of reinforcement, couplers were used instead of splicing the bars, ' says Carver.

However on the northern edge the future Crossrail project made its presence felt. Moor House will contain the ticket hall for Moorgate station but a 3m exclusion zone was imposed around the tunnels which run along the northern edge. This meant the 45m long piled foundations had to be offset 2.75m from their original position.

Consequently the basement slab and wall act as a transfer structure between the piles and columns at ground floor (see diagram).

In addition, a 40m deep draught relief shaft in its lower basement is linked to the tunnels. For this 'Crossrail Box', bottom up construction over a two-storey basement had been programmed to isolate the shaft for safety reasons.

This threatened to delay work on the steelwork. Elsewhere on the site, plunge columns had been cast into the piles to start the main frame.

The solution in the box area was to construct temporary piles to the north of the building footprint and extend the ground floor in the north east corner to span on to it. The steel column spanning from the ground to first floor at grid line A5 could then be constructed on top of the beam, allowing main steelwork erection to begin as basement construction continues.

To distribute the building load between the temporary pile and adjacent wall, the column at A5 was tied into a 'hanging truss'.

The truss and temporary beam and pile were designed to take the maximum load of seven floors of steelwork and two levels of concrete.

Once the basement wall is built, load would be transferred to the permanent construction.

This week, a 5t steel column will be slid under this temporary beam, directly under the column at A5 and 2,000kN of load from the hanging frame will be jacked into it and down to the permanent foundations.

When the basement walls and slab have been constructed in the Crossrail Box, the redundant members of the hanging frame will be burnt off.

Protection policy

The steel columns between ground and first floor are encased in concrete and designed for vehicle impact.

The whole structure is fire protected for two hours using intumescent paint on some steel members and enclosing others in protective boarding.

The paint expands in the heat of a fire, introducing air pockets to insulate the steel.

The concrete core is fully enclosed and staircases are pressurised to keep the escape routes clear of smoke in the event of a fire.

Who's who

Structural mechanical and electrical engineer: Arup

Architect: Foster & Partners

Client: Moorhouse Property Developments

Steel frame: William Hare

Steel decking: Richard Lees

Concrete: PC Harrington

Concrete core: Slipform International

Cladding: Permasteelisa

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