Winter wonderland

Past a forest of bamboo, banana trees bear their fruit and palm trees soar skywards into the temperate air. The balmy atmosphere is filled with the fresh smells of vegetation, a sense of calmness – and Saturday afternoon shoppers. For this is no tropical location, this is Sheffield, South Yorkshire, which is now home to one of the largest temperate glasshouses to be built in the past century.

The 21-m high structure houses 150 species of plant, forming the city's Winter Garden, a project which so impressed Sheffield-born Michael Palin that he agreed to be its patron. The building also forms the focal point of Sheffield's £120 million Remaking the Heart of the City regeneration scheme.

The Winter Garden opened to the public on 12 December to great acclaim. Located on the former site of a town hall extension, it provides a covered walkway between the adjacent Peace Gardens and the Millennium Galleries, which – designed in a combined project by architect Pringle Richards Sharratt – connect directly into the Winter Garden. The building is as wide as it is tall, at 22 m, and stretches to a length of 70 m. The glazed structure is formed of 18 pairs of glue laminated (glulam) timber arches, which rise in steps to the 21-m peak. The parabolic shape resulted from the need to maximise the internal space without using internal supports. Glulam was chosen over more traditional materials such as concrete and steel for its surface finish, light weight and ability to be shaped without the need for heavy plant. This is one of the largest glulam structures to be built in the UK.

The vegetation in the Winter Garden is as striking as the building itself. In order to preserve and cultivate the plants, special care had to be taken to ensure that conditions within the structure will fall within strict parameters. This task went to Powerminster, which was contracted to supply and fit the m&e services as designed by consultant Buro Happold.

Natural ventilation plays a large part in the services design, with heating and mechanical ventilation provided mainly for frost protection and to allow increased plant diversity.

The mechanical ventilation system is linked to that of the Millennium Galleries, reducing the energy consumption and long-term running costs of the buildings. It comprises ten fans suspended at high level from the arches and 128 actuator-controlled, ventable windows arranged over six zones. Nathan Briddock, mechanical engineer with Powerminster, explains: "The fans are mainly for air movement and are set at low level movement until a temperature of 20°C is reached, at which point they will switch to high speed." An intelligent bms links control of the fans to that of the ventable windows; windows open automatically, starting with the zones at either end of the building and moving inwards, when a setpoint temperature is reached. External rain and wind sensors can override the vents opening to prevent damage to the plants and building, turning on the fans instead.

A water-based underfloor heating system was installed for frost protection of the plants. This is connected to the bms and controlled by four manifolds in the basement plantroom via internal and external temperature sensors to ensure that the temperature is maintained at 10°C throughout. "Originally this setpoint was to be 8° higher than the external temperature at all times, but advice from horticultural experts meant that this was changed to a constant temperature," explains Briddock.

To ensure that visitors' eyes are not distracted from the building and plant displays, all of the m&e services had to be hidden from sight. Large items are held in a basement plantroom, but distribution required more careful thought. A three-to-four feet wide gutter running around the building conceals pipework, controls boxes and actuator connections to the windows. Rainwater downpipes provide further hiding places – electrical trunking and mechanical services take the place of rainwater and pour down four, six inch, galvanised steel, dummy downpipes spread around the building.

With a tight programme to follow, logistics were a vital part of the project. Close co-ordination with the construction team simplified the m&e installation. First-fix work at high level was carried out from the scaffolding erected to carry out construction. Briddock elaborates: "When we started putting the services in, the wooden arches were up and we were following the glazers along the scaffolding. It was a rolling programme, once the glazers finished a section we carried out the work that needed to be done there, moving along the building with the scaffolding."

At the second-fix stage a more unusual method of reaching the height was needed. The scaffolding had been removed by this stage and weight restrictions on the floor slab meant that equipment such as cherry pickers could not be used, instead "spider lifters" were employed. "These lifts are on wheels on four legs. When they are in position, the legs are set down on the floor. This spreads the weight over each foot and over a wider area. A module then lifts the operatives up the middle to the working area," explains Briddock. Specialist training was needed to operate the equipment and one person from each of the mechanical and electrical sides was nominated to attend a suitable course, one of whom was then always present when the lift was in operation. The project was completed on schedule and open in time to give Christmas shoppers a respite from the winter weather. But the Winter Garden is set to be a year-round attraction.