The building is organised on three main levels; basement, ground floor and first floor. Its principal interior galleries are relatively simple, box-like volumes constructed in concrete. Around these are situated smaller-scale side galleries, whose forms are more playful and sculpted. These spaces are connected by a circulation zone, which acts as a central element of the experience, offering multiple opportunities to pause, with views out to the surrounding woods. The journey through the museum is completed by an external circulation route leading to a series of terraces on three levels, stepping up from east to west. The entire building is wrapped in a series of glass sails, or “verrieres”, which serve as a permeable enclosure for the terraces and exterior circulation zones, defining their volumes and sheltering users from the elements. The resulting spaces are unique, while the nature of the glazed surfaces themselves are ethereal, blurring the sense of enclosure and breaking down the distinction between the building and the sky.
There are twelve verrieres – each one an independent structure – supporting a total of around 13,400 m² of glazing. They vary in size from the largest, at over 3,000 m² and nearly 500 tonnes, to the smallest at 500 m² and 60 tonnes, and range in orientation from horizontal to vertical. They are a central element in the external form and identity of the building, taking their inspiration from the sails of racing yachts. The photo below of the America’s Cup J class yacht Susanne, from 1911, served as a focal image for the project.
The form of each verriere is part of a global composition, developed by the architect largely through the medium of scale models. The project took shape through multiple iterations, spanning a number of years; from simple massing models up to models at 1:50 of the whole building and even larger scale detail models. While each verriere is unique, ten of the twelve have a very particular form, generated by two intersecting, broadly developable surfaces. At the head of the vertical verrieres the intersection is marked by a sharp fold that flattens out, moving down the verriere, to a central point. Beyond that the intersection changes direction and approaches a perfect continuity or tangency between the two surfaces. Each component of the system plays its part in the composition of the verrieres. Structure is laid out to enhance the perception of movement and tension in the surfaces, and create the sense of space and volume below. Surfaces are given a strong directionality by emphasizing the principal tertiary support elements, or mullions, evoking the seams of traditional sails. A distinct hierarchy is maintained between systems (glazing, tertiary supports, and principal structure), with each one clearly identifiable and differentiated in terms of its scale and materials.
The vertical verrieres are hung up to 20 m out from their supports on the building and the largest horizontal verriere cantilevers 25 m from the building. Given this scale, one of the principal challenges of the project was to evoke the lightness and tension of a sail. Furthermore, this structural feat had to be accomplished while threading the supporting structure of each verriere through the circulation and terrace spaces below to support points whose locations were severely restricted by the capacity of the primary building structure. The goal was to create a coherent family of structures, with similar detailing and scale of components, but the reality was that each verriere was so unique in its support configuration that its structural behavior bore little or no resemblance to that of its neighbors. A second major challenge for the design of the structures was the classification of the building as a monumental structure with a 100-year design life. This issue was particularly delicate for the timber elements of the structure, whose durability depends principally on their degree of exposure to humidity.
The mix of timber and steel also generates issues of differential movements, either under thermal loads, where the steel expands and contracts at more than 2.5 times the rate of timber, or due to changes in the moisture content of the timber. Timber has a tendency to creep and slip at connections under long-term loads, changing the balance of forces in various elements of the structure, while also displaying time-dependent strength characteristics.
Creating structures of this scale and geometrical complexity in timber posed multiple challenges, particularly in the development of structural connections. These introduce a degree of fragility into the system, due to their non-ductile failure characteristics, which must be compensated by an increased level of global robustness in the structure. The building authorities also required that each verriere have an enhanced performance under fire conditions, above and beyond classic code criteria, such that they are stable and remain in place under any realistic fire scenario. Finally, the geometry of the verrieres created complex wind flows, which had to be carefully assessed to ensure that they did not provoke a resonant dynamic response in the structures.
The verrieres comprise of four key sub systems, described below:
• Glazed skin
The glazing panels are typically rectangular, hot-bent cylinders, roughly 3 m × 1.5 m in size. They are supported only their long sides, by an integrated stainless-steel support stiffener.
• Tertiary system
The panels are carried by a high-strength Duplex stainless steel grid. The principal elements of this grid run in the direction of the panels’ long sides and are typically fabricated sections formed from an 80 mm diameter tube and a 120 mm deep flat plate stiffener. A simple 70 mm diameter tube forms the grid in the other direction. The system is completed by complex fabricated tube and plate sections running along the edges, and intersections of the two surfaces. These integrate a gutter and downpipe, forming the principal drainage elements of each surface.
• Secondary structure
The main load bearing system of the verrieres is named the secondary structure (as opposed to the primary structure of the building). Timber is adopted for the principal elements, framing the external perimeter of each verriere and typically tracing the intersection of the two surfaces. Infill secondary beams in steel are configured to enhance the sense of movement and tension in the sails. The system is completed by lightweight bracing, made up from painted steel tubes and stainless tension rods.
• Tripods
The secondary structure is supported by a series of struts and ties, called the tripods, arranged individually or in groups of two or three. Their configuration determines how each verriere functions structurally, but the location of each tripod also had to be carefully chosen to suit the use and sense of space below each verriere.
One of the key principles in developing the structure was to follow as closely as possible the visual hierarchies in the architecture. Thus the tertiary system supporting the glazing was isolated from secondary structure to ensure that the structural demands imposed on it were minimized. A significant jump in scale between the two systems was maintained through this logic, enhancing the sense of lightness of the glazing system in keeping with the image of a lightweight sail.
The desired architectural image is of a billowing sail – a flexible tension form generated by lateral wind pressure. The glazed, panelized skin of the verrieres is not particularly coherent with this image, as it requires a relatively rigid support. This inconsistency is further exacerbated by the fact that the principal load on the verrieres is in reality gravity, which is around 3 to 5 times greater than the lateral wind loads. To mitigate this, the inherent rigidity of the sculptural form of each verriere is exploited by bracing its surfaces to form a rigid shell.
This is achieved by the addition of lightweight, high-capacity tension-rod bracing (of stainless steel) in the planes of the secondary structure. This bracing has the effect of increasing global stiffness by a factor of around 5 to 10. More importantly, the approach radically reduces the bending demand on the principal structural elements, which in turn has a significant impact on the size of these elements. In the case of the timber elements, without this bracing, beam would have to be around three times deeper – not only architecturally unacceptable but quite simply outside the range of feasibility.
But this approach brings with it a certain degree of complexity, particularly due to the highly variable nature of the geometry of each verriere. Stiffness depends on the degree of curvature in the surfaces, and each verriere has its own particular characteristics. Not only does this create differences between verrieres, but also significant variation in stiffness over the area of each one. The verrieres typically have a very rigid zone at their heads, where the fold geometry is most acute, and this decreases down towards their bases, where curvature is softer.
With the concept for the secondary structure fixed, the challenge posed was to find a configuration of tripod supports adapted to the particular characteristics of each verriere. At the scale and stiffness of the verrieres there is a risk of a significant structural interaction with the primary structure. This is ideally avoided and at minimum must be controlled.
Typically this is achieved for a rigid body by developing an isostatic support solution – one in which the number of supports is limited to the theoretical minimum required to prevent the structure becoming a mechanism. In such a case the system is unaffected by differential movements of its supports, and is free to expand or contract under temperature loads, without generating internal stresses.
In practice the variable curvature and rigidity of each verriere meant that perfectly isostatic solutions were not typically feasible. Moreover, the potential support points were often ill-adapted to an isostatic solution due to the numerous constraints imposed on them by the building and primary structure.
The only process possible to develop and test solutions was a painstaking case-by-case effort, involving the entire design team. Each verriere presented its own particular set of problems, and what worked for one was rarely applicable to the next. The resulting structures were typically hyperstatic, which leads to issues of internal stresses due to differential temperature and moisture movements, and a more complex structural interaction with the primary structure.
While this created its own set of technical concerns that had to be resolved, due to the increased robustness it provides, it did offer one significant advantage over an isostatic solution. This robustness gives a structure a capacity to absorb accidental damage without collapse, as it is able to develop alternative load paths to compensate for the loss of components. Clearly this is a significant advantage in terms of the security of the verrieres.
Notwithstanding the evident architectural benefits of adopting curved glazing, it was clear from the earliest stages of the project that the fabrication of the panels would pose a major challenge. The classic fabrication technique of heating and softening glass, then forming it onto steel molds, is adapted to small-scale, repetitive geometries, but would be prohibitively expensive if applied to a surface area of more than 13,000 m².
Furthermore, the technique results in annealed glass, with a low characteristic strength and a susceptibility to rupture provoked by thermal shock under differential solar loads. The search for a fabrication process that could produce curved, high-strength tempered glass was of fundamental importance to the project in order to minimize the thickness of glass required. This not only presents an economy of material, but also increases the flexibility of the panel, making it more tolerant to movements of the structure. This, in turn, allows for a softer structure, leading to further economies, and, critically, to a reduction in the visual bulk of its elements.
Theoretically this could have been achieved by cold-bending tempered glass to the required form: by first fabricating flat panels and then forcing them onto a curved supporting frame on site. However, many of the panels had a very tight curvature, which would have generated excessive stress in the glazing to take up the desired form, leaving insufficient residual capacity for resisting wind loads. The solution to this delicate problem was found in the latest generation of industrial bending and tempering ovens that can produce high-quality tempered-glass cylindrical panels (see illustration).
At the core of this technology is a quenching module whose complex mechanism is able to bend hot glass to a chosen radius, then rapidly blow cool air on its surface, while rocking the panel back and forth to ensure that the cooling of its surface is as homogeneous as possible. It is computer controlled and thus enables each panel to be bent to its own unique radius without significant added cost. Furthermore, each panel may be introduced into the machine at a specific angle, so that its axis of curvature is not parallel to its sides.
The decision was taken to engage as early as possible with a glass transformer, and Sunglass of Italy was chosen on account of its extensive experience with glass-bending machines. This proved invaluable to the development of the technology, as not only was the team able to test solutions and feasibility at an early stage, but Sunglass was also able to upgrade its ovens to produce larger panels.
While the machine opens up the possibility for each panel to have a unique radius and angle to its axis of curvature, it still has a significant geometrical limitation in that it can only produce cylindrical forms. As none of the project’s panels are cylindrical, this limitation had to be overcome by a geometrical approximation where the best-fitting cylinder was found for each panel.
The approximation inevitably leads to theoretical steps between panels, whose magnitude depends not just on the geometry of the panel itself, but also on that of its neighbors. Thus, in order to control and minimize steps it was necessary to find an appropriate set of cylinders for the whole of each surface. This was achieved using an iterative computational technique that ran until the steps in all panels were limited to a visually acceptable tolerance.
This kind of cylindrical discretization provided very good results for the great majority of the surfaces. However, for some extreme cases – typically where the design surface was doubly curved or strongly conical – the steps between panels were significant and constituted an important constraint for the design of the glazing system. Along the long edge of panels, where the joint was covered by a decorative cap, the maximum step was limited to 10 mm. In the other direction, where the joint is a simple silicone seal, the step was limited to 5 mm, which was not visually perceptible. The nature and composition of the panels was the subject of extensive trials and research, completed in close collaboration with Gehry Partners and Studios Architecture, the project architect of record. The architect was searching for a subtle degree of translucence to give a presence to the glazing and ensure that it captured light and shadows. Additionally, the glass had to provide solar protection to people on the terraces. Extra-white, low-iron glass was adopted in a two-ply laminated buildup of 6 mm plus 8 mm to ensure that the panels would remain in place if accidently broken. A white frit with a 50 % coverage of 2 mm diameter dots was incorporated, combined with a 24 % reflective coating, creating a panel whose transparency changes radically in different light conditions. In all, more than 3,600 panels, each one unique, were produced over a period of around one year to cover all the surfaces of the 12 sails.
The design of the glazing system is founded on the principle that the panel supports should be as close as possible to isostatic. This not only ensures that each panel is isolated from interaction with the supporting structure, but also that the system adapts to a wide range of geometrical conditions. The technical implementation of this theoretical principle was achieved by silicone-bonding stainless-steel stiffener plates to the long sides of each glass panel, enabling it to be supported on only 4 fixing points. These incorporate sliding spherical bearings, ensuring that each only provides a restraint normal to the panel surface. The panel is then held in its plane by 3 adjustable bearing fixings. The relative sophistication and complexity of the system represented a significant investment on the part of the contractor, ECM, given that the detail is repeated around 14,000 times. However, it more than paid this back due to its universal applicability and the resulting ease of glass installation.
The principal element of the tertiary system is a fabricated stainless-steel profile composed of an 80 mm tube and a 120 mm deep plate stiffener. The laser-cut plate offers geometric control of fabrication, while the tube readily absorbed the variation in twist between the various components. A 70 mm tube completes the structural grid in the other direction. The verrieres required about 12 km of these structural profiles. To minimise their dimensions and ensure the highest level of corrosion resistance and durability they were fabricated in Duplex, a high-strength stainless steel.
The use of timber as a component of the secondary structure was integrated into the project relatively early both to introduce a warm contrast to the steel and glass, and because it was perfectly adapted to the complex curved geometries of the verriere. It was laid out to trace the principal lines of each surface, requiring a total of around 800 m³ of timber. This introduced a series of challenges into the project, primarily due to its non-uniform material characteristics. It has a marked directionality, being much stronger in the direction of the grain, the natural fibers running through wood, than perpendicular to it. It expands and contracts with changes in its moisture content, particularly in the direction perpendicular to the grain. This can cause the timber to split on drying, or, with curved profiles, to warp or straighten. Its strength depends on the duration of load, and it tends to creep and slip at connections over time. Timber is naturally highly durable, but can degrade over time due to fungal attack, depending critically on its exposure to moisture. Although exterior, the majority of the timber is protected from direct exposure to rain, placing it in a Service Class 2 Environment, as defined by Eurocode.
The architects wanted to use timber from sustainable European sources, precluding the use of tropical or exotic species, which tend to have the best durability. Consequently, it was decided to use larch, a resinous softwood well adapted to Service Class 2 environments. Where parts of the beams were exposed directly to rain, closer to a Service Class 3 environment, local protection was provided by covering the area with a stainless-steel cladding. In more extreme cases, the timber was substituted with steel. The timber also needed protection from termite attack, as well as a lazure to protect the surface from UV degradation and limit its permeability to moisture.
The structure’s scale necessitated glulam technology. This offered numerous advantages in fabrication, where the specific curvature required for each beam was readily reproduced by bending the planks on computer-controlled jigs during gluing. Beam sections were typically 400 mm wide and ranged in depth from 600 mm to 1200 mm.
The majority of the elements are only curved in one plane, but at the intersection between surfaces, a more complex twisting, doubly curved geometry is generated. These sections are fabricated by producing a curved planar beam, then cutting it into 25 mm thick slices and re-gluing these together on a jig with the required curvature in the other axis. The section is then finished using a computer-controlled five-axis milling machine. The glulam approach can also generate higher-strength sections, as each individual plank can be graded and specifically chosen, creating a more uniform section, less prone to natural defects in the wood. Relatively high-strength GL28 timber was produced in this way, sourcing the planks from high-altitude forests in Austria, and only employing planks with a high density ranging from 500 to 700 kg/m³, and an average density of 585 kg/m³.
To reduce the risk of fissuring and splitting of the timber over time the moisture content of the timber was also carefully controlled. Over a period of years the moisture content of external, sheltered timber in the Paris climate will settle down to around 14 %. For the project, all of the planks were pre-dried to a controlled moisture content of 9 % to 14 %. This ensures that the timber’s moisture content will tend to increase over its lifespan, reducing the risk of fissuring due to drying.
The fabrication process adopted for the double-curved beams also has the advantage of introducing glue planes close to the surface in both directions. This tends to slow down changes in moisture content of the section, making it less sensitive to cyclical changes in the external environment, both reducing the risk of fissuring, and limiting crack depth to the outer layer of timber. In the end, this form of double lamination was adopted even for the planar curved elements to enhance their durability and resistance to fissuring.
In timber design the connections are typically the system’s most critical elements, and this was certainly true for this project. The nature of the structure meant that connections had to be continuous, transferring principal loads in the strong direction for timber (axial and bending forces in the direction of the grain). However, due to the structure’s geometrical complexity, eccentricities were inevitable, and these tended to introduce additional secondary torsional and bending forces. The classic connection detail involves slotting a vertical steel plate into the timber section and linking the two with a grid of 16 mm diameter transversal pins. This approach had to be adapted to incorporate two plates in order to generate the push-pull necessary to transfer the secondary torsion and bending forces. This technique required high-precision fabrication so that the pre-drilled holes for the pins in the timber aligned perfectly with those in the twin plates. Stainless steel was used in all of the pins and plates to ensure maximum durability of the assembly. The theoretical strength of the connections was then justified by full-scale testing.
Both the construction system selected and the detailing and execution of the verrieres for the Fondation Louis Vuitton support the architects’s intention to create a rich and varied experience combining contemporary art display with a strong connection to the surrounding woods and the Jardin d’Acclimatation.
