10 December 02 

Sheffield Heart of the City Project

Winter Garden
The Winter Garden together with the Millennium Galleries forms part of the much bigger redevelopment of Sheffield city centre - the Heart of the City Project.  Pringle Richards Sharratt were appointed architects for the Millennium Galleries and Winter Garden following a competitive interview in July 1996. Buro Happold were appointed as structural and services consultants and engineers. Weddle Landscape Design were appointed as Landscape Architects.

Background
The Heart of the City project comprises a master plan for the whole city centre, covering 2.5 hectares in area, including the demolition of the existing 1970s Town Hall Offices - 'the Egg Box' - and construction of new Council offices, as well as commercial development (offices, retail space and a hotel). 

The site is in the centre of the city and stretches from the Town Hall through to Arundel Gate, a busy inner city ring road that has been downgraded as part of the project. It lies at the heart of Sheffield's cultural quarter, including the Crucible Theatre, the Lyceum Theatre, Graves Art Gallery and Central Library, and the City Hall.  It is also close to Sheffield Hallam University's campus, facing Arundel Gate, and the city's main public squares: the Peace Gardens (in front of the Town Hall) and Tudor Square.  The demolition of the existing town hall offices frees up the site for the new Winter Garden and the Millennium Galleries in the middle of the Heart of the City development. 

The larger project also includes public realm works, with new landscaping for the Peace Gardens, Town Hall Square and Hallam Square, together with a realignment of Arundel Gate.  Both the Winter Garden and the Galleries are designed to be highly energy efficient and provide a model for sustainable development in a city centre site.

The Heart of the City project has been funded by the Millennium Commission, Sheffield City Council, English Partnerships, The European Union and Yorkshire Forward.
The Overall Design Concept
The Winter Garden and Millennium Galleries were conceived and designed as a fully integrated part of the larger urban configuration. The key urban elements are the covered arcades - the Winter Garden and the 'Avenue' in the Galleries which together form a cultural route through to the city centre - a route surrounded by a variety of stimulating and attractive spaces and activities.

The Winter Garden
The Winter Garden is a spectacular glazed public space in the heart of the city centre. Extensively landscaped with exotic trees and plants it is an indoor park providing an important amenity for the public in the city centre and a counterpoint to the open air 'summer' Peace Gardens nearby.

In urban terms the building was conceived as a covered galleria linking Tudor Square, Hallam Square and the Peace Gardens and forming an integral part of the network of pedestrian streets in the area capable of being occupied all the year round, day or night.

The Winter Garden is a grand urban space where people can walk in and through - a covered park in the City Centre. Together with the Millennium Galleries the Winter Garden forms a pedestrian hub linking the Civic, Arts and University quarters of Sheffield - a cultural route through to the city centre.

The form of the Winter Garden steps down at either end to relate to the height of existing buildings. On Surrey Street it completes the enclosure of Tudor Square. It rises toward the centre to provide height for the taller plants, a sense of drama to the interior and to define and express a central court or gathering space opposite the Millennium Galleries.

The structure of the Winter Garden is clear and legible, the forms and materials natural and familiar. The scale is spectacular and awesome providing a memorable experience for all visitors - young and old.  The building nearly 70 metres long and 22 metres wide and rises from either end in a series of steps to  a lofty 22 metres over the three central bays. Here, in a central court, the Millennium Galleries seamlessly connect into the building, and the tallest growing plants - the Norfolk Island Pines -are located.

The single glazed building envelope is supported by a composite structure of laminated timber and stainless steel connectors in the form of primary arches and purlins. The arches and purlins are made from Larch - a durable timber that requires minimal maintenance and which will, over time, turn a light silvery grey colour.

Along the length of the building three garden areas have been formed and gently ramped paths rise through the planted beds to provide an alternative 'wild' route through the building.  The building is paved throughout with local gritstone, this also provides the supports for the benches distributed along the routes.

The majority of the trees and plants are from the southern hemisphere, where they have evolved quite differently to our own native flora, and are strikingly exotic and unusual in their forms, textures and colours. The plant beds contain over 2,000 plants from a wide diversity of 150 different species. The largest trees are over 6 metres tall with the potential to grow taller than the building.

Construction and Environment
The Winter Garden structure is formed by a series of timber arches formed from glue laminated (Glulam) European Larch. Glulam is made by forming and gluing 50mm strips of timber to generate large structural members in shapes that can suit both their structural duty and architectural function.  The largest component brought to site was 24m long and 900mm deep, far larger than any Larch tree. The material is not new, but in some instances the design was beyond the scope of UK design guidance, resulting in the use of European and German technical guides.

The form of the arches is developed into an inverted catenary (the shape formed by a hanging chain). This shape is very efficient as it matches the force profile in the arch, thus reducing the size of the structural elements compared with traditional arch profiles.

Matthew Lovell, the structure’s designer from Buro Happold comments "The concept of inverted catenery profile arches was developed to achieve predominantly axial forces with low bending moments.  This gives an extremely efficient structural form, reduces the material used and results in a structure that is ideally suited to engineered wood."

The requirement to maximize the internal space without internal supports led to the development of glued laminated timber parabolic arches at 3.75m. This is not only a very efficient structural form but also provides a large enclosed volume allowing flexibility in the planting height and layout. The arches are supported by a concrete substructure, which also includes a basement car park and loading bays to the adjacent Millennium Galleries.

Buro Happold Structural engineer Richard Hennessy, comments "Glue laminated timber was the perfect material for forming the geometry of the arches. The material can be curved to the required shape without the need for heavy plant, as required for bending steel.  And unlike the difficulties in casting smooth concrete, an excellent surface finish can be obtained in timber by simply sanding the finished elements after fabrication."

The larch, derived from sustainable forests, requires no preservatives or coatings.  This is a double benefit as it reduces the use of solvents and also avoids the use of chemicals that could kill the plants.
A research study, carried out by Buro Happold, examined the environmental impact of constructing the building. This showed the timber had an eco-rating of less than 5% of that for steel or concrete alternatives - a 95% reduction in energy used in the construction.

In addition to this advantage, weight could be saved in the foundations for the Glulam alternative, as their weight is significantly less than steel or concrete alternatives - about 65% of the weight of a steel alternative and about 15% of a concrete alternative. This represented a cost saving and further reduction in environmental impact with less material needed in the foundations.

The building is single glazed with over 1,400 framed roof panels of which 128 open for ventilation. The ventilation panels are all the same size, as are over 80% of the fixed panels.

The environmental strategy has been linked closely to the diverse species of plants and the need to maintain frost-free conditions. These results helped to inform the envelope design including glazing specification, shading devices and ventilation openings. The Winter Garden is a frost protected temperate environment. In winter it will provide protection against rain and wind and conserve heat gains from the surrounding buildings and the sun. High level fans prevent stratification and induce air movement within the building helping to equalise the temperature and minimise the risk of fungal growth, which would adversely affect the plants. For cold winter nights under floor heating prevents frost within the gardens allowing a wider variety of vegetation. The primary heating is provided from the sustainable Sheffield City centre district heating main. 

In summer, it will benefit from solar shading from surrounding buildings and the high level operable windows together with opening glass louvres at either end will maximise the natural stack effect ventilation and induce evaporative cooling from the plants. Small water features help modify local humidity levels. The louvres also provide smoke ventilation in the event of fire.
Fire Strategy
The innovative design solutions and consequent design quality achieved for the Winter Garden would not have been possible except for the fire engineering. Application of traditional (standard) fire safety guidance would have resulted in poor value and constrained the building functionality, building impact, and build standard.

Smoke control systems enable the Galleries and the Winter Gardens to be open without the need for compartmentation. A natural ventilation system was used for smoke clearance in the Winter Gardens, taking advantage of the natural system provided for the environmental strategy and accounting for adverse wind effects.

A risk assessment demonstrated that that fire-resisting glazing to the Winter Garden was unnecessary and that the toughened glazing achieved the required compartmentation performance; enabling a good value interface with the surrounding buildings.

The means of escape strategy was developed to account for the needs of all, including disabled persons.

Rules for use of the Winter Garden were established to enable the desired flexibility of use (for exhibitions and similar) and efficient fire safety management.
 
A Building Management System monitors the external and internal environment and automatically operates the heating, vents, fans and lighting as required.  This ensures that energy is consumed only as and when it is required.
The Plants
The plant beds rise by up to 600mm above finished floor level with a further 500mm provided by a 12 metre wide down-stand 'trough' which extends along the full length of the Winter Garden.  Together this produces a bed depth of up to1100m - large enough for the rootballs of the taller trees that are up to one metre in diameter.

In horticultural terms the building is a cool temperate house providing conditions suitable for a wide collection of plants from the Mediterranean climate zones. The majority of the plants are from of the southern hemisphere, where they have evolved quite differently to our own native
flora, and are strikingly exotic and unusual in their forms, textures and colours.

The plants have been grouped in five beds to broadly reflect the different character that has evolved in response to the environments they grow in. Spiky cactus-like plants from dry arid areas have evolved in a completely different way to the tree ferns of the moist rain forests.

Apart from the sheer diversity of the plant material one of the most impressive features are the ten Trachycarpus fortunei palm trees, which by their size and form provide a strong sense of the exotic character of the habitat of the majority of the plants.

In many instances the plants selected are native to threatened environments where the moderate climate is most suitable for urban development consequently this is resulting in the increasingly rapid decline of the natural vegetation. These plants provide an important reminder of their economic, ecological, medicinal and educational value to mankind – and underline the importance of a careful and balanced use of our planets precious resources.

Information panels in each bed provide further details of the plants, their natural habitat and the uses they have been put to over the years. The plants have been supplied by specialist nurseries in Europe.
Buro Happold is a multi-disciplinary international practice of consulting engineers established in 1976 offering civil and structural engineering, mechanical and electrical engineering, quantity surveying, building services and environmental engineering, infrastructure and traffic engineering, geotechnical engineering, façade engineering, fire engineering, Computational Fluid Dynamics analysis, access consultancy, project management, urban design and a range of specialist CAD services.

Further information and images of the Winter Garden please contact:

Danielle Dickinson
Tel:0113 204 2200
Email Danielle.dickinson@burohappold.commailto:Danielle.dickinson@burohappold.com">Danielle.dickinson@burohappold.com>
Clients: Sheffield City Council
Architects: Pringle Richards Sharratt Architects John Pringle, Penny Richards, Ian Sharratt, Douglas Oyugi, Basil Kalaitzis, Adam Blacker,
Project Manager: Sheffield Design & Property
Quantity Surveyors:  Sheffield Design & Property Quantity Surveyors 
Structural Engineers:  Buro Happold, Matthew Lovell and Richard Hennessy
Services Engineers:  Buro Happold, David Hull, Neil Arundel and Gary Coates
Fire Consultants: Buro Happold FEDRA, Nigel Hiorns
Access Consultants:  Buro Happold 
Lighting Consultants:  Bartenbach LichtLabor/ Lichttechnik Martin Klingler
Winter Garden Landscape Consultant: Weddle Landscape Design
Planning Supervisor: Sheffield Design & Property
Management Contractor:  Interserve Project Services Ltd.

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