Broadgate toast commercial success

The Broadgate Tower and 201 Bishopsgate was developed by British Land and designed by the award-winning Chicago office of Skidmore, Owings & Merrill (SOM). The two new buildings stand prominently in London’s renowned Broadgate business district, located above active rail lines on a 2.3-acre, air-rights site.

The site has been transformed into a dynamic development that not only includes the two commercial buildings but cafés, restaurants, bars, boutiques, and shops that enliven the galleria. This vibrant space serves as a public passageway that connects Liverpool street station to the adjacent emerging developments.

The quality of design, both in the structural approach to load distribution and architectural in terms of dramatic appearance, are the achievement of a project team who collaborated closely at every stage of the construction process, not only to create high quality buildings but also to embrace fully the issues on accessibility, sustainability and innovative design technology.

The 35-storey and adjoining 13 storey buildings are meticulously designed to meet the needs of financial and other professional occupiers. However, what complements the structure is the very striking external cladding system which is unusually made up of two visually identical unitised systems. Here the major external structural members are not integrated with window wall units as these would have resulted in horizontal joints at each floor level. To clad the diagonals of the A-Frame in the same stainless steel as the main façade was a clever move by the developer as it gives the galleria a major feature.

Entrance to the The Broadgate Tower is through an impressive five-storey lobby, and at 33 storeys, is the first developer-led tower in the City. With 13,000 sq ft column free floor plates and floor to floor glazing, the typical Tower office floor has generous daylight potential and exciting views to the City. The Broadgate Tower is the first installation of double deck lifts and destination hall control in the world and minimises journey time for office workers and visitors. According to Kone, the lift manufacturer, it is the first of its kind in the UK.

Meanwhile, the lower rising 201 Bishopsgate, at 12 storeys, provides 3800 m2 floor plates. With a central core arrangement, the result is generally column free tenant spaces. A generous lobby with entrances at both ground and lower ground level, along with the central interior atrium, provide a dramatic arrival for both the office worker and visitor alike.

In front of The Broadgate Tower there is a major new public space, and the glazed open walkway between the two buildings forms a tree-lined galleria, and a vibrant location for the shops, bars and cafes. Innovative methods of construction in compliance with the Building Regulations include:

Access for disabled people

Full access and provision has been provided including refuge and escape strategy. Two external glass enclosed lifts have been built especially on the north end of the development to compensate for the slope in pavement level. British Land wanted to ensu that the Galleria space could be accessed easily by all.

Structural engineering

The buildings are supported primarily on a previously constructed raft structure located over the tracks running to Liverpool Street Station. The raft also served as a working deck for the construction of the buildings and separates the building construction from the railway tracks below. The raft structure was designed to accommodate a range of potential buildings and has two structural decks forming an interstitial space which allows modifications of the raft structure and servicing of utilities without the need to access the railway space. This system in effect creates ‘land’ above the tracks which potentially allows the demolition of the buildings and construction of different buildings on the existing raft, which enhances the sustainability of the project.

The structural design for the Tower incorporated six massive five storey A-frames to distribute the load evenly to the highest load carrying portion of the raft and from there to the piled foundation. Stiffness is via the perimeter steel bracing and suspended lift cores.

Fire engineering
Structural fire protection requirements were analysed using the Time Equivalent method of Eurocode 1. Due to the nature of the fire load and building geometry, sprinkler and ventilation provision was accepted. The protection required for the external Aframe steelwork was analysed using the guidance in BS7974. The length of flame and flame temperature was analysed for either an internal or external fire. From this the method of exposure of the steel to heat and the actual temperature increase in the steel over the length of the fire duration was calculated.

The result was a fire protection requirement for 30 minutes to the A-frame steelwork. Other fire safety measures, such as the provision of fire compartments, fire fighting facilities and means of escape, broadly followed accepted guidance as did the protection provided to guard against the risk of external fire spread and the higher risk associated with plant and other areas such as the atrium.

Thermal and acoustics
The superstructure columns are separated from the substructure by vibration or acoustic isolation units to alleviate ground-borne noise and vibration from the adjacent railway. The vibration isolation units, comprising laminated rubber and steel plates, are designed to provide the structural supports for gravity loads and provide isolation from vibrations induced by trains. Lateral restraint is provided by separate rubber bearings.

Generally, the bearing assemblies are at ground level. On office floors, subject to the fit-out being in accordance with the Category A specification, the magnitude of floor vibrations induced by footfall will be limited to an R8 response factor in the Tower and R6 at 201 Bishopsgate.

Although the building was designed in 2005, thermal properties for the building envelope comply with and in some cases exceed Building Regulation requirements in the current 2006 Part L.

Sustainable construction
The complex is fabricated from around 20% recycled materials. The raft and foundations are reusable as they are designed to cater for varying loading patterns for future buildings without having to reinforce existing foundations.

Sustainable design features at 201 Bishopsgate building include both green and brown roof construction. This benefits the building by providing insulation, reducing the rainwater runoff, and promoting biodiversity through the creation of habitat to support local wildlife.

Both buildings are highly sustainable and as such have been awarded an ‘Excellent’ BREEAM rating. 201 Bishopsgate and The Broadgate Tower was completed in late summer 2008. The project also won the Best Commercial Project and Structural Engineering Innovation awards at the London District Surveyors Association Building Excellence Awards earlier this year.