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.