John Hope Gateway Biodiversity Centre, Edinburgh

Introduction

The Royal Botanic Garden, established in 1670 as a physic garden, is now a world-renowned centre for plant science, research and education. To improve facilities for visitors and to encourage their interest in the Garden’s work and the relevance of plants, a new building has been designed by Cullinan Studio to house exhibitions, a media studio, indoor and outdoor education spaces, offices, a shop, restaurant and a new biodiversity garden. The Gateway acts as a threshold to one of the world’s most important botanical institutions and aims to capture the spirit and enthusiasm of that organisation. As part of the brief, it is designed to be an outstanding example of green construction and sustainability.

 

Building description

The Gateway is set on an important crossing of routes and gives wheelchair access to the central part of the Garden. Most visitors enter through the double-height entrance foyer, a dramatic space supported by a diagrid of tapering glulam beams and framing a view of the Garden beyond. The foyer leads into a large exhibition space with a fully glazed east wall, 60 metres long, which forms a gentle curve to frame views of the new biodiversity garden. At the centre is a helical timber staircase set in a rooflit atrium where the diagrid roof is again revealed, oversailing the open-plan first floor restaurant and extending beyond to shelter the outdoor terrace of the restaurant.

Rather than a traditional ‘front and back’ layout, the building can be approached and entered from several directions and from different levels, through the glazed and ‘permeable’ façades. In contrast, the service elements of the building are enclosed in solid external walls of broken-edged, stacked Caithness slate slabs.

Appropriately, given the botanical nature of the building, timber was chosen as the material to unify and bind the building together; it is used as structure - roof, floors and staircase - as cladding to parts of the external walls and as furniture.

 

Structural design of timber floor and roof

The roof, a single horizontal plane about 100 x 50 metres overall, is supported by a series of deep, tapered glulam beams on which rest cross-laminated spruce planks covered with insulation, membrane and a sedum blanket. The glulam beams are laid on the diagonal and the resulting coffered soffit shapes give an individual identity to the open plan spaces below. The beams are supported on pencil-thin steel columns formed of four steel angles – a deliberate strategy, as the architect explains; ‘We wanted this visually weighty timber roof to appear as if it is floating’.

The first floor structure of 226mm thick cross-laminated spruce planks rests on the lower set of glulam beams, 210 x 815mm deep and set in pairs at 6 metre grid centres. To achieve a visually discrete connection, the beams are bolted to steel flitch plates welded between the angles of the cruciform steel columns. This provides continuity past the columns to help control deflection of the roof structure. The connections are carefully tailored to suit their position in the building; for instance the use of paired glulam beams allows them to be reinforced with additional steel flitch plates for special situations, including cantilever ends and in one location, load transfer of a column that supports the roof but does not extend to the ground (to create a column-free space for the educational studio area).

The roof structure of 146mm thick cross-laminated spruce planks rests on the upper set of tapered glulam beams, 210mm wide and tapering from 1035mm deep to 500mm.

At the top of the columns, steel rods receive the vertical load from the roof. In architectural terms, this rod is the opposite of the classical capital; rather than expressing and celebrating the connection between column and beam, the junction is visually diminished. Being on a diagonal grid, the beams meet at the centre of the rectangular grid. The steel flitch plates which are bolted to the beams are welded to a steel bar which provides moment continuity in the structure and creates a strong visual location to the centre of the coffered slab. While at first floor the bolts to the column flitch plates are arranged in rectangular groups, at roof level, for visual reasons and for structural efficiency, the bolts are arranged in circular groups, providing a strong visual contrast. The arrangement also helps visitors understand the structure; a circular arrangement indicates a rotational force or movement while a vertical arrangement indicates a vertical force or shear.

Particular features of the roof structure are as follows:

  • Flitch plates allow moment continuity across the column head detail. Large hogging moments are resisted over the supports and low sagging moments occur at centre span locations.
  • The low stiffness of the column leads to small moment transfer from the beam into the column. By reducing the bending moment, the use of a slender steel rod is permitted at the top of the column.
  • The countersunk bolt detail leads to a loss of section, which affects the local stress in the timber. Great care was taken to ensure that edge distances complied with the minimum spacing requirements of Eurocode 5.

Due to the column heads being rather flexible in the horizontal direction, it was important to provide a stiff diaphragm action to transmit lateral loads to the various concrete walls and cores, which carry lateral loads down to the foundations. The cross-laminated panels are screwed into the glulam beams and to adjacent panels to form these stability diaphragms at first floor and roof levels.

 

The use of timber

As befits a building in the Botanic Garden, the Gateway is made out of natural materials with low embodied energy, including a predominantly timber structure. Wherever possible the use of Scottish, then British, then European materials were specified to minimise transport distances to site.

As the building is designed for long life it was important to use durable and stable materials. Three types of engineered timber were used in the building’s construction:

  • Glulam timber, used for the primary and secondary beams to the first floor and roof, is European whitewood from Sweden, formed into glulam beams by Cosylva in France, using 45mm thick laminations.
  • First floor and roof decks are of cross-laminated spruce panels manufactured by KLH in Austria. Exposed partitions are also made of these panels.
  • Douglas fir structural veneered lumber (SVL) from Germany, supplied by Woodtrade, was used for the mullions and transoms of the timber-framed glazing system. To maintain a consistent palette of materials, SVL was also used to construct the helical staircase and major items of furniture such as the reception desk and bar. SVL is made of thin veneers of timber (approx 2mm wide), glued together to form large sheets.
  • Wherever possible the timber has been exposed, with cross-laminated timber panels forming the finished surface of the ceilings and exposed walls to public areas.
  • Vertically lapped, untreated Scottish larch boards fixed on battens act as a rainscreen cladding system that was designed in consultation with TRADA Technology to ensure it needs minimal maintenance and easy replacement of the boards. The lap runs in different directions on three overlapping layers, creating different shadows that draw the eye along the façade.
  • The helical staircase continues the horizontally layered emphasis of the building’s design. It is constructed from SVL sheet, cut and bonded together to form solid treads and curved balustrade, and reinforced with vertical steel bars. The staircase drawing was fed into a CNC machine which laser-cut the SVL sheet into precise pieces, including the holes for the bars and the handrail (the handrail itself was hand-cut).
  • Tables in the restaurant were cut from seasoned logs felled in the Garden itself; they join together to form large ‘composite’ tables in the evenings.

 

Sustainability

A sustainable, low-energy, minimum-waste approach to the building’s design was part of the message the Garden wished to convey to its visitors. The Gateway has many demonstrable environmental solutions including the extensive use of timber; they include a biomass boiler, a green roof, rainwater harvesting, a wind turbine, photovoltaics, solar collectors for hot water, natural ventilation and passive night-time cooling. The sedum roof reduces heat gain to the building in summer, slows down rainwater run-off and provides an extra blanket of insulation. The design of all these elements is explained in the permanent exhibition on the ground floor of the Gateway and this engagement with the public is an important contribution to the project.

Completion Date:

August 2009

Year Published:

November 2010

Building Type:

Visitor centre

Location:

Royal Botanic Garden, Edinburgh

Client:

Royal Botanic Garden Edinburgh

Architect:

Cullinan Studio

Structural, Civil, Ground, Fire and Façade Engineer:

Buro Happold

Building Services Engineer:

Max Fordham

Contractor:

Xircon

Project Manager:

EC Harris

Timber Subcontractor:

Donaldson & McConnell

Joinery:

Elmwood Furniture: Ben Dawson

Timber Supplier:

KLH (cross-laminated spruce), Woodtrade (SVL), Russwood (larch)

Timber Element(s):

Glulam beam structure. Cross-laminated plank floor, roof and partitions. External cladding. Upper level floor and deck. Helical staircase, reception desk and bar, timber curtain walling – SVL (structural veneered lumber). Tables.

Timber Specie(s):

European whitewood, Spruce, Larch, Douglas fir, Ash, Elm, Sycamore

Award(s):

Building Design Public Building Architect of the Year IStructE Best Arts & Entertainment Building Award

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