A technological investigation into The Savill Garden gridshell.


Group Investigation.

Statement of investigation — How are the various loads distributed through the roofing system and quadruped legs? The Savill Building is a double layer timber gridshell structure and is a great representation of the many possibilities that gridshell structures can achieve. In what we identified during our group investigation, the building is structurally composed of 4 main components: the timber gridshell roof, the steel CHS (circular hollow section) perimeter ring, the steel quadruped/twin legs and the foundation that they all rest upon. The roof system itself is composed of various elements: 4 layers of individual laths, shear blocks, plywood, aluminium layer and oak. As a group, our line of enquiry was geared towards how the structure is able to deal with applied loads, both lateral and vertical.

To examine the proposed enquiry, we required a physical model of the structure in its entirety to be able to study the effects of the applied loads as accurate as possible, this was done at the scale of 1:50. In the case of vertical loads, we determined that snow loads would have the most effect on the structure and in order to replicate this, we used multiple bags of sugar at 500g each which were then placed on the roof of the structure. Comparably, the lateral loads were tested through attaching the bags of sugar to a string that was then wrapped around the roof structure along its x - axis and y - axis respectively. Our findings suggested that the structure was able to effectively withstand both loading variants. The sinusoidal form of the roof enabled it to ‘adapt’ and successfully distribute the applied vertical load, we were able to see refractions in the acetate strips (used to replicate plywood) which indicated a slight change in the form and informed us of the characteristic that the gridshell roof possesses. However, with exerted force during the lateral testing, the roof structure including the perimeter ring detached itself from the steel legs. Although this is most likely due to the model itself and not in representation of the parameters that are present in the actual structure.

Team Members: Emraan Mayow, Beth Mamicha, Alex Marangoci, Grace Mulligan and Kok Dong Eng.

Individual Report.

This paper investigates the structural properties gained from timber gridshells, specifically looking into the characteristics/purpose of the geometry used in The Savill Building and how it affects the overall stability of the structure when responding to applied loads. Research methodologies include a site visit to the building, a model of The Savill Building at 1:50 and relevant journals, articles and books. // // Shell structures are a type of construction known for their inherent strength and their ability to withstand an extensive amount of applied loads. They are essentially outer membranes that behave in a fashion that allow for the interior of a structure to be free from load - bearing elements. Likewise, gridshell structures follow the exact same principle. However, rather than the three - dimensional form of the shell being made up of a continuous surface, it is instead composed of long strips of material running along a grid that forms the ‘skeletal’ plane of the structure. In a gridshell structure, also known as a lattice structure, the amount of material required is significantly reduced due to the surplus material that is needed in a shell structure being condensed into the strips (or laths when referring to a timber gridshell). What we find as a result of complex parameters is enormous strength and rigidity in a somewhat simple form. Although they possess high structural qualities, not many long - span timber gridshells have been constructed. Most recently the Japan Pavilion (2000), the Weald and Downland Gridshell (2002) and the Savill Garden Gridshell (2006). Also worthy of mention is the notable Mannheim Multihalle (1974) by Frei Otto. // In recent architectural engineering we can identify two constituting types of construction, these are either analytical or free - form. In free - form shell structures, the architectural design has the ability to have a dominant influence over the freedom of structural components. Free - form structures are usually organic forms produced through implementing a code or a series of scripts to modify a shape, thus resulting in an uninhibited design with parameters set by the architect or engineer themselves, this is known as parametricism. Contrastingly, analytical shell structures are the product of mathematical/calculated reasoning where the form is adjusted follow the structural restrictions, and in turn the parameters are influenced by both the formal and structural requirements. // Based on Adriaenssens et al. (2014), the authors establish dynamic relaxation as a methodology for form finding the structural limitations of both strained and unstrained gridshells. In accordance to each design, the basic principle of a strained timber gridshell is that it is “made from initially straight elements” (Adriaenssens et al., p.89) consisting of short timber spans jointed together to form a series of continuous laths, unlike unstrained gridshells that consist of prefabricated or already curved parts. In order to achieve the desired architectural form, the construction of such gridshells require an assembly that starts as a flat surface before being manipulated into their final form. The continuous laths are arrayed in a linear sequence and are then layered with a second set of laths perpendicular to the existing set, thus forming a latticed grid. Each intersecting point/node between the two sets of laths are bolted together resulting in a configuration that enables the lattice to obtain maximum elasticity. It may seem that attaining smooth curvature within such structures is unlikely, and whilst there are limitations as to the scope of curvature that can be achieved, timber however possesses ”small torsional stiffness” (Harris et al., 2008, p.28) which in turn allows for it to bend to a certain extent. It is only once the third dimension, i.e. the z axis, is triggered, that the lattice can be considered as a gridshell structure. // A common process of form finding in early gridshell structures would involve the use of a hanging chain model, a prime example being Otto’s Mannheim gridshell. They are mirrored representations that serve the purpose of establishing geometry based on the self weight of a structure solely through gravity and allow the architect/engineer to envision the radii of curvature needed for the shell to be able to sustain itself in tension. Comparatively, technological advancements have created a shift towards the use of computer generated calculations and algorithms to inform structural systems, as seen in the Savill gridshell roof. It is described by Harris et al. (2008, p.29), that the form of the roof is generated “by means of a program written to define the shape as z = f(x, y)” resulting in a form that resembles an oscillating sine wave. Through the process of form finding, what we end up with is a symmetrical structure of double curvature along its xy - axis and z - axis, with two small domes siting on either sides of a larger central dome. // Load Movement: Gridshell Roof -- Perimeter Ring Beam -- Twin/Quadruped Legs -- Foundations. As mentioned earlier, the roof system is composed of multiple components that work as a collective, with each part having a fundamental purpose so that it is able to function as a double layer gridshell. The lattice component on its own is not sufficient enough to transfer loads effectively and for that reason plywood is introduced to help transfer the loads acting upon the roof directly to the perimeter ring beam. Running along a diagonal path between the laths, the plywood becomes a bracing system that also provides structural stability (unlike previous gridshells that relied on tension cables), thus the basis of a shell is somewhat being re-implemented as it encloses the entire roof. Another component to the gridshell system is the shear blocks which were also present in the Mannheim gridshell, they establish what is described as “in plane shear strength” (Harris et al., p.28) and are able to reduce any movement, i.e. horizontal shear, between the two separate sets of laths, which if without, would create a roof that is less rigid and could subsequently result in an increase in the amount of tension/compression forces in the laths. The combination of the form and roof allow for the loads to be constantly distributed in multiple directions within the gridshell. The undulating shape causes the roof to act in both compression and in tension when applied with vertical loads; the three domes acting in compression in contrast to the valleys that act in tension (Based on Harris et al., p.29). It is also important to mention the significance of the perimeter ring beam, as any load that is acting upon the roof is directly transferred to it via lumber stiffening fingers connected to steel plates welded along the perimeter. So how does it work? There is logical reasoning as to why the ring beam (as well as the roof) features a double curve, meaning that it is curved in both planes. Curvature in Plane 1 is for the aesthetic. However, the curvature in Plane 2 follows an oscillating path creating three main high points and four low points along the east and west elevations respectively. The high points align to the centre of each dome whereas the low points are aligned to the valleys, the reason behind this is due to the behaviour of loads as they are distributed through the structure. When a vertical load such as snow for instance is carried from the gridshell, the perimeter ring is then able to redirect the flow of the forces along its curvature before they are transfered to the twin/quadruped legs. This then correlates to the placement of the steel legs as they are allocated according to these points. // The steel legs themselves have different lengths and thicknesses; twin sets on the front (east) elevation and quadruped on the rear (west) elevation. From an initial impression, one might suggest that the reason for this could be due to the fact that there is no supporting structure on the rear end of the building, and therefore requiring four sets of quadruped legs to support the dead load as well as any other loads. On the other hand, it is also possible to say that the eight sets of twin legs on the front elevation are much shorter in length and arranged the way they are due to the existing concrete structure that they sit on, therefore reducing the amount of load being concentrated in one place. If we look into the finer detail, it is mentioned that “the loads from the east elevation legs are similar to those on the west elevation, however, as they are shorter and stiffer the tangential forces are more evenly distributed between them” (Harris et al., p.33). In the case of wind loads the tension and compression forces within the gridshell are reversed, which is mainly due to wind entering the building and causing the roof to lift through the torque bars that are in place. // Through my findings, I can conclude that the structural properties of timber gridshell systems are quite remarkable, they have the ability to function as an enclosure whilst also being able to deal with various types of loads in an effective manner. My understanding is that the laths are able to function and respond to loads due to the ‘mesh’ that is created as a result of condensing the material into a grid pattern. It is as though the gridshell is in fact a continuous shell, but to achieve the desired geometry/form, it is broken up in separate parts and formed as components that in the end consitute to complete shell. The elasticity of the lattice system is also a great tool that enables forces to travel through the structure whilst maintaining its aesthetical value. As a result of geometry and material properties, we end up with a lightweight structure.