The Downland Gridshell
Project History and Design Brief

Introduction

The Downland Gridshell is an innovative modern timber gridshell structure intended to become the national centre for the conservation and study of traditional timber-frame buildings.

The gridshell, which uses new techniques, researched especially for this project,  houses the Museum's collections in an open access store. The project has attracted considerable interest, and the process of construction has been accessible to visitors.

Four front-line firms of architects were interviewed to design the Museum's new Building Conservation Centre in 1996. They were required to propose an innovative building that would respond to the physical demands of the Museum and the sense of foresight held by its directorship. Rather than mirror the past, the new structure was to celebrate the special environment of the Weald and Downland region and serve as an exemplar structure for modern rural buildings.

Edward Cullinan Architects was appointed, working with Buro Happold engineers, Alex Sayer quantity surveyors and The Green Oak Carpentry Company Ltd. The Museum's Directors have actively participated in and supported the approach that led to this unusual and unprecedented project.

The Weald & Downland Open Air Museum, a leading international centre for historic buildings, attracts 140,000 visitors a year. For over 30 years it has rescued and preserved threatened 15th - 19thth century buildings and artefacts that record the history of traditional rural life in the surrounding region of Kent, Sussex, Surrey and eastern Hampshire.

The Museum's restoration and erection of original buildings provides visitors with a unique three-dimensional textbook of over 500 years of building materials and methods. The 50-acre open air site situated in the South Downs at Singleton, near Chichester, comprises more than 45 buildings and a major collection of artefacts and specialist library.

The new purpose-built Conservation Centre and Store will, for the first time, allow the Museum to make its research, conservation and restoration programme for houses, workshops, town and farm buildings accessible for viewing to the general public. The Museum's stored collection of artefacts, relating to buildings, building trades and materials and rural life, will be brought onto the site for the first time, giving the artefacts a greater level of security, environmental stability and visibility.

The new building is intended to be a national facility for the study and practice of building conservation, especially the timber-framing tradition of England. The Museum already has an enviable reputation as a centre of excellence in building conservation training and for its extensive programme of traditional rural trades, crafts and skills courses. The building incorporates three integrated workshops for carpentry, building, plumbing, roofing and wheelwrighting conservation and restoration, which will bring these key crafts into the public arena. A classroom will allow practical workshops for up to 12 people at a time to learn traditional crafts and construction skills no longer taught in the modern building industry, but desperately needed by the conservation sector.

The environmentally controlled artefacts store will ensure the long term care and safety of the Museum's collection, Designated by the Government for its international importance last year. Bringing the collection onto the Museum site will save at least £30,000 a year in off-site storage.

The new building has already generated new research and will continue to stimulate new ideas and solutions. The construction process of the timber gridshell was also a once-only opportunity for educational activity. Visitors were able to watch the gridshell take shape from a safe viewpoint.

The "Downland Gridshell" is a testament to architectural and building techniques of the early 21st century, as the Museum's existing buildings are to their own time, and at the same time complement and extend the collection and work of this leading Museum.

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Project Overview

The Open Access Building Conservation Workshop, Demonstration Area and Museum Store is be housed in an innovative greenwood structure designed by Edward Cullinan Architects with Buro Happold Engineers. Practical in operation and innovative in design, this unique building is both visually elegant and holistic in its environmental impact. The conservation centre reflects the theme of the craft of timber using traditional materials, in a design philosophy that is entirely consistent with the Museum's objectives.

Building the conservation centre for the Weald & Downland Open Air Museum has explored new techniques in greenwood timber construction. The undulating organic form of what has become affectionately known as the 'Downland Gridshell', seems to reflect the rolling South Downs countryside. The challenging site, in an elevated position within an area of outstanding natural beauty on the South Downs, called for a high quality structure, sensitive to the local environment. Given the contrasting uses of the building, the decision was made to employ a loosely clad clear-span timber gridshell workshop, set upon a sealed and sunken archive space of earth protected masonry.

The efficient and flexible structural form provided new potential for locally available materials and skills. The organic form is due primarily to the stiffness requirement for the shape of a gridshell, composed of a series of continuous curves. The complete form is a triple bulb hour glass, 12-15m wide, with 5m deep open ends to allow access to the building's interior. Although two storeys high, from the outside the 1800 m² building appears to be a single storey structure.

Internally the building is a single cell space enclosing two sealed workshop units. The gridshell is clad with a loose system of hanging plates of hardwood and glazing, while the difficult to drain upper pitches of the building vault is clad in a curving ribbon of watertight monolithic roof.

The two levels are separated by a built up floor structure of laminated beams on a central row of glu-laminated columns supporting a sealed and insulated industrial grade timber plank deck.

In contrast to the airy, loose construction of the upper floor, the lower level is sunk into the surrounding chalk site. Mostly buried archival areas are sealed and atmospherically controlled to maintain an overall 18-25C temperature to allow for long term processing, storage and display of the Museum's archival stock. Within the main shell smaller areas provide their own macro environments for more sensitive materials. To reduce loads on water and power supplies the building and site systems take advantage of natural features such as earth mass, found by digging into the ground, and rainwater collection.

The building makes use of materially and energy efficient systems.

Structural and mechanical systems used get the most out of relatively small amounts of resources. An example of this is that the gridshell covering the entire workshop is constructed from timber. The material has been tested for strength and quality and applied in a way which makes effective use of computer supported engineering techniques.

Environmental issues

The building will run on minimal supplies of energy. Heat and power will be used as necessary and only in rooms where required. Where people will be exerting themselves physically, the building will not provide unnecessary heat. Controlled natural light will make artificial lighting during daylight hours redundant. Direct solar collection will pre-heat water, to be pumped through an underfloor heating system in the lower level. This floor heating strategy enables a direct thermal connection between the floor slab and the ground which will act as a heat sink, keeping the archival spaces thermally stable throughout the year. The only insulation materials  used are at the upper zones of the external walls and the encasing ground.

Carpentry Specialists

This building presented exciting challenges to The Green Oak Carpentry Company team. In many ways this is a 'carpenter's building' incorporating as it does a wide range of carpentry disciplines and structural techniques, such as solid oak framing like that on display at the Museum, large curved 'glulam' beams made up from thin planks, bent and glued together, and the gridshell itself, which might be likened to a giant 'wooden basket' moulded into a triple domed structure.

Also unique is the green jointing of the gridshell laths from freshly sawn oak. This means that the timber is more supple and therefore more easily formed. Glue technology has moved forward to a stage where this is now possible on a structure of this type.

Of interest is the use of locally grown Western Red Cedar boarding fixed vertically in tiers up the building, and which resemble the articulation of medieval armour.

The construction of the 'ribbon roof' that undulates over the entire length of the structure, is perhaps more akin to boat building than conventional roof construction as we know it.

The design process for the building has also been unusual in that from an early stage, the carpenters sat around the table with the architects and engineers to design the detail of the building. Putting all this together we have a building that is unique and innovative, both in its conception, engineering and its carpentry. It is our belief that the results speak for themselves.

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Gridshells and the Construction Process

Steve Johnson of Edward Cullinan Architects wrote this article in 2000 to explain the process of the raising of the Downland Gridshell, the first true gridshell building to be raised in Britain.  The article is in its original form, no changes have been made to the tenses or syntax.

What is a gridshell?

A shell is a natural, extremely strong structure. A gridshell is essentially a shell with holes, but with its structure concentrated into strips.

Timber gridshells have two lives. In their built incarnations, they are formful, resilient, yet strong objects. In their genesis stages they are perhaps more mysterious as, while being made up of a multitude of relatively stiff woven or overlapped linear elements, they behave more like stiff rubber than loose cloth. The particular properties of timber allow it to be deformed into a shape, and then locked. Steel and concrete gridshells need to be fabricated into similar shape.

There are very few true timber gridshell buildings built in the world and ours will be the first in Britain. Perhaps the two most valid reasons for their rarity are found in the fact that force flows within shell structures are difficult to picture and therefore difficult to develop strategies to counteract the greatest forces to be dealt with such as wind or snow. Extensive analysis is needed to find the shape that can be formed given the original flat pattern.

Raising the Downland Gridshell

The Workshop at the Weald & Downland Open Air Museum will be a structure born of straight lines crisscrossed to form a grid plane. Rotating joints are formed at each crossing point allowing the sheet to concertina, or more magically, to warp forming any variety or combination of three-dimensional curvatures - so long as the radius of curvature does not exceed the bending capacity of the base material. With a bit of cleverness, the curves can be designed in such a way as, once pinned down around its base and locked at its joints, the gridmat becomes a self-supporting shell of continuous curvature that can enclose spaces ranging from small to colossal.

Secondly, there is great difficulty found in transforming a gridmat into a gridshell. Computer software has now been developed to allow scientific analysis of the structure to occur and, with this project, we hope to revolutionise the gridshell construction method.

Although a gridshell is one of the lightest and most efficient structural forms yet devised, once assembled, it represents the building's entire structure. Regardless of the weight of individual structural members, gridshells must be erected as a heavy and cumbersome whole and, because they must contain curvature to maintain structural integrity, they can achieve great heights. With all previous gridshell erection projects, the strategy has been to press the mat into the air either by poking up from underneath using poles or towers or plucking up from above with cranes. In both cases, the process is potentially violent as gravity is being defied on the back of thin structural members risking critical and expensive breakages.

The larger the mat, the greater the weight to lift and height to be gained thus making more problematic the goal of lifting an entire building's structure into the air at once. With the Downland Gridshell, we will be attempting to make a major breakthrough in the forming of gridshells by reversing the accepted process. We intend to summon the genie from the bottle and turn the universal enemy of buildings, gravity, into our friend and servant.

After it is erected, our gridshell will be suspended upon a timber topped masonry box mostly buried into the side of a wooded hill. The gridmat will have to be assembled after the oak laths have been milled and jointed into long lengths. The flat form of the mat will be a rectangle measuring 3OX52 metres (approx. 1/4 of a football pitch). The first perceived strategy was to press the gridshell into position from the timber workshop floor. In addition to the known difficulties associated with this method, we would have to find a way to clear the hill to the south of the building in order to keep the mat flat. The decision was made to erect a base scaffold platform 2.5 metres above the workshop floor attempt to clear the hill. Through meetings with various innovative scaffolding contractors, it became clear that the working height of the platform would not have an enormous impact on the overall costs of the scaffolding.

At this point, the decision was made to erect scaffolding to place the platform at 7.5 metres above the workshop floor. This would mean that rather than fighting gravity by pressing the mat into the air, we will use gravity to release the gridshell into its planned form. With new scaffolding technology, the carpenters will have complete control over the dismantling of the scaffold to enable a gentle and controlled programme of gridshell forming.

The carpenters take 2 weeks to assemble the gridshell 10 metres above the lowest point on the site - literally at the tops of the adjacent trees. Once the mat is assembled from hundreds of 35X50mm oak laths and thousands of stainless steel bolts, the fun will begin.

In well-calculated fashion, the scaffolding will be struck section by section from the four corners and edges of the mat. Allowing gravity to take its course, the mat will slowly take on the form of a barrel vault. The aim will be to strike scaffolding until its final form will resemble an extruded step pyramid with three peaks. Once at this point, the gridmat will be draped over the scaffold structure, taking on its intended form. The carpenters will then become sculptors as the mat will be gently pushed and pulled until it is artificially suspended by the scaffold structure. A series of well-placed ropes and clamps will subject the shell to some of the combinations of tension and compression that it will have to resist through its life.

A series of datum marks will be set upon the receiving timber deck and the gridshell to help locate the curvilinear form in space. Once aligned, the gridshell will be progressively fixed to the workshop floor's edge. Simultaneously, heavy timber frames will be erected at the building's east and west ends to form the only openings within the workshop shell. These large arches will form and support the end edges of the gridshell but, will in effect, be supported laterally by the gridshell which will be well braced and tied to form a sound composite structure. At this time, the gridshell will take on some of the characteristics of a true gridshell - but not all. While this warped gridmat will support its own weight, it will not possess the required stiffness to achieve the rare state of shellness.

The carpenter-sculptors will then become carpenter-sculptor-acrobats, as they will make use of the self-supporting gridshell as its own scaffold. The feat will be to put in place the extra layers of oak laths that will triangulate the entire gridshell system thus giving it its great strength to weight ratio. These rib-laths will run the length of the building over its lower two-thirds and across the structure over its upper third. Only when all of the rib-laths are in place and securely bolted down will the shell be set. Not to waste materials, the rib-laths will also serve as cladding mounts for the building's western red cedar boarding and polycarbonate clerestory panels.

With the shell complete, the ziggurat scaffold will stay in place to give access to all parts of the new structure while the building is clad, its intricate ribbon roof is installed, and the mechanical and interior fit-outs are completed. After the building has been sealed, the scaffold will be completely struck and excreted through the workshop's loading bay doors.

Some might argue that gridshells are complicated to assess structurally and difficult to erect. With the construction of this project, milestones will have been achieved regarding both perceived problems. Specific to this project, the prime goal will have been achieved in that a large working space will have been created using a collection of materials that were previously thought unusable as a structural material due to their fineness of size. Through the application of materials that are common to this area to make a shell we will have created a large building from a mesh of relatively thin filaments.

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