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Glass Structures & Engineering

, Volume 4, Issue 1, pp 3–16 | Cite as

Interlocking glass spiral as building structure of the watch museum “La Maison des Fondateurs”

  • Josua VilligerEmail author
  • Philippe Willareth
  • Florian Doebbel
  • Viviana Nardini
SI: Challenging Glass paper
  • 178 Downloads

Abstract

La Maison des Fondateurs in Le Brassus Switzerland is going to represent the watch making manufacture Audemars Piguet. Audemars Piguet stands for the finest quality, precision as well as innovation based on traditional watchmaking expertise and craftsmanship. La Maison des Fondateurs has been developed from a competition proposal to the execution project by the architects BIG—Bjarke Ingels Group. The created interlocking all glass spiral evolves the building volume out of the rough, natural landscape providing a perfect linear museum path for the building program. Despite the severe loading and weather conditions, inspired by the high values of Audemars Piguet the project team designed the building to the technical limits. The curved and very large insulated façade glass units as well as the curved glass partition walls of the interlocking glass spiral are forming the load bearing structure. All vertical and horizontal loads are transferred by these glass components making solid columns and shear wall obsolete. The article reflects the close collaboration of the designers, engineers, specialist contractor and industry. The global structural concept, transfer of the concentrated point loading into the curved structural glass element, testing procedures as well as the structural bonding beyond standards and the context of energy efficiency are discussed and elaborated.

Keywords

Structural glass Curved glass Glass façade Structural bonding 

1 Introduction

1.1 Context

During 2013 Audemars Piguet launched an invited competition for a watch museum in Le Brassus, Switzerland. The competition team headed by BIG—Bjarke Ingels Group developed the successful project proposal in an intense, interacting design process. The design team focused on creating a building for Audemars Piguet, which is reflecting the company’s values, refereeing to the past and to the future as well as to strengthen the brand with an iconic, sculptural like building close to an architectural emblem. Many options and solutions have been investigated. Finally, the interlocking spiral has been created. The one-story tall spiral is located at the north side of the historical founders building and is directly linked to the new founder’s hall, which is positioned between the existing buildings (Fig. 1).
Fig. 1

Rendering of the spiral, founders hall and historic buildings (courtesy of BIG)

The geometrically very strong spiral is providing a perfect linear museum path along the company’s history as well as watches, and shows a clear analogy to the mechanic of a mechanical watch. Workshops and workplaces of the company’s best watchmakers are positioned along this museum path.

The materials and structural elements are reduced to the essentials. Glass, brass and cast stone are the predominantly, visually used materials.

Structurally, the lightweight roof is supported only by glass elements for the vertical and horizontal loading. This dematerialized glass structure ensures full transparency throughout the building and façade (Fig. 2).
Fig. 2

Rendering of structural glass elements (courtesy of BIG)

The development from the competition project throughout to the execution phase has been subject to many challenges, such as structural robustness, enhanced durability as well as energy efficiency and sustainability.

1.2 Project fundamentals and design process

The essential and most important parameter of the project is the shared vision to create the aspired load bearing all glass structure. This vision has been the base of the successful, interdisciplinary design process. A process that has been exposed to conflicting parameters and focuses. But exactly the constant work on these parameters is the base of the current, successful execution. It is the execution of a carefully developed and enhanced competition project, preserving the original DNA of the design intend.

Due to the location in the Vallée de Joux with the particular, harsh micro climatic condition, the building is exposed to high snow loads above 5 kN/m\(^{2}\), even doubling locally, and very cold temperatures well below \(-\,20\,{^{\circ }}\hbox {C}\).

Sustainability and energy efficiency are imperative design values. Audemars Piguet enhanced these values with a strong commitment to satisfy the Minergie label as minimum. Minergie is a very common Swiss label rating the energy efficiency but paying attention to the user comfort additionally.

These main design drivers illustrate the design challenges. The building structure, formed by the glass elements, have to carry severe loading, and be highly insulated glass walls with premium optical performance. Furthermore, solar control devices have to be implemented to prevent overheating during the warm seasons.

All these objectives are literally wrapped up by an absolute transparent, crisp all glass façade. Triple glazed units (IGUs) are forming the weather skin, providing the water tightness, low air permeability and the thermal insulation. The inner leaves of these triple glazed units are activated as load bearing elements, proving the vertical and horizontal structural capacity. As an additional element, a brass curtain shades the façade passively but never interfering with eye vision and therefore maintaining the full visual transparency. This brass curtain, a three-dimensional net of individually shaped weaves, is hung above eye level in front of the upper area of the glazing, providing just enough solar protection for comfort and required energy efficiency (Figs. 3, 4).
Fig. 3

Rendering of sun shading (courtesy of BIG)

Fig. 4

Mock up of brass sun shading

The outlined design shows very well how the best solution was developed on these most important building parts and components, satisfying all major parameters. Iterative design steps, constant testing and improving and a dialogue of the designers and engineers based on expertise have been the key factors.

1.3 Material and surface qualities

The materialization, respectively the dematerialization is an important part of the project specific design language. The materialization aims for real, raw materials. Materials, that enhance in beauty by aging and building up a natural patina. Untreated brass has been chosen for the visual components such as sun shades and cladding parts, for example cover of roof edge, slabs etc. At the opening, these components will be shiny but darken in different grades due to the natural patina. This patina is a great material characteristic, reducing cleaning demands but also creating esthetic qualities.

The dematerialization depends very much on the glass surface qualities. Surface flaws, such as surface anisotropies and visual distortions, eliminate the aspired dematerialization and transparency. Instead of the visually desired “nothing”, visual errors “kill” the transparency and lead to a surface spiked with visual errors. Technically, these surface flaws or errors belong to the characteristic of processed glass and are mostly subject to thermally introduced surface stresses. Furthermore, the typical total reflection of glass at a particular viewing angle and mirror effect of the applied low e-coating work against the transparency.

Therefore, the dematerialization and transparency are partially opposed by physical laws. Taking on the challenge to get as close as possible to the wanted appearance, the following parameters have been subject to a challenging development. Firstly, the g-value had to be optimized in accordance to the passive shading devices described above. A g-value of 23% proved to be the upper limit in order to avoid the overheating of the building. Esthetically, the low g-values has been a not negotiable value and had to be accepted, as the passive shading devices should not have been extended in the area. Despite this low value, the samples and the first manufactured units show a nice appearance.

The more challenging issue has been to tackle the surface errors. From an engineering point of view, the thermally introduced stresses are used to strengthen the glass. Fully toughened or heat strengthened glass provides a higher load bearing capacity. Furthermore, the risk of failure due to thermally introduced stresses during service is obsolete.

Optimizing the glass towards the annealed state (float condition) with the best possible surface condition requires a careful consideration of the load bearing capacity under long term loading condition. Additionally, the risk of thermal breakage during service has been assessed carefully, taking into account the interface details, climatic exposure, the applied low e-coating, etc.

The decision to use annealed glass on all layers in the triple glazing influenced the bending method fundamentally. Hot (roller) bending and lamination bending had to be eliminated as a processing option due to the introduced and, in case of the lamination bending, also required thermal stresses, respectively strengthening. Slump or gravity bending is the used bending method, ensuring the best possible annealed surface condition. The first assembled and inspected glass units prove to have an excellent appearance and are achieving the very much aspired and promised transparency.

1.4 Key figures

Some key figures of the project are listed in the Table 1 below in order to give a better overview and understanding of the project characteristics.
Table 1

La Maison des Fondateurs—key figures

Description

Characteristic

Spiral diameter

approx. 30 m

Maximum building height

approx. 7 m

Energy standard

Swiss Minergie standard (Ug-value =0.5 W/(m\(^{2}\,\)K))

Number of individual glass units

approx. 140 pieces

Smallest and largest bending radius

\(4.7 {\vert } 18.8~\hbox {m}\)

Minimum glass dimensions

\(2.46 \times 0.50\) m

Maximum glass dimensions

\(2.46 \times 5.80\) m

Total glass weight

approx. 110 t

Highest concentrated downforce

approx. 400 kN (design load)

Highest concentrated uplift

approx. 25 kN (design load)

2 Structural engineering

2.1 Overall structural concept

The transparent design of the spiral did not allow placing columns or even walls inside of the building. Therefore, it was decided to find a solution with load bearing glass. Since the architectural concept of the interlocking spiral required separation walls in the interior, these walls were simply being used structural. The roof span could thus be reduced to 4.7 m and only the center of the spiral required a heavy steel construction in the roof to overcome the approximately 13 m span.

The façade and the interior separation walls transfer the vertical loads (dead load of the roof construction, snow load) and also the horizontal loads (wind load, seismic load). The horizontal loads are acting globally on the building but need to be transferred to the edge of the spiral to be introduced as in-plane forces into the glass. The roof construction itself must therefore have a high shear stiffness. This concept works very well on the spiral because there are always multiple individual glass units which can transfer the load for every possible wind direction (Fig. 5).
Fig. 5

Global model for structural FE-analysis

The structural analysis model for the global calculation was set up in a finite element software using beam and shell elements. Most of the steel construction parts could be represented by linear beam elements while the glass elements were being represented by shells. It is not necessary to model the glass elements with the exact buildup in the global model. Instead, the forces acting on the individual glass element have be extracted and applied in the detail model of the glass pane.

2.2 Concept for individual glass pane

The concept for the individual glass panes was designed as simple as possible. At the outer façade, where the IGU is carrying the loads, the innermost layer of the IGU acts as the load bearing layer. The load bearing glass consists of a three times laminated 12 mm annealed glass with a SGP interlayer. The steel edging transfers horizontal in-plane loads from the roof construction to the vertical edge of the glass. The pressure resistant connection between glass and steel is accomplished with Hilti Hit hybrid mortar. From there the load goes diagonally through the glass to the bottom support. The vertical loads are being transferred from the roof bracket directly through the steel shoe into the glass. The gap between glass and steel shoe is again filled with Hilti Hit. The length of the support depends on the actual load which has to be transferred. The connection between the bracket on the roof and the shoe is only pinned. This ensures that bending moments are not being transferred from the roof into the glass pane and the glass can deflect freely in out-of-pane direction. This pinned connection can be accomplished with steel bolts that have a rounded end and are resting in blind hole within the steel shoe. The outer layers of the IGUs are only being supported at the bottom. These supports only take the dead load and reduce the permanent stress on the edge sealing, which now only has to carry the wind loads and climatic loads.
Fig. 6

Principle for load path in a single glass element

The bearing concept of the inner glass units is exactly the same. Due to the larger forces in the center of the spiral, the glass buildup increases to a maximum of five times 12 mm annealed glass depending on the actual load (Fig. 6).

2.3 Glass design

The whole project was depending on the load capacity of the glass. It was crucial to define the different glass buildup’s as early and as precise as possible because the number of glass layers has a huge impact on the construction costs as well as the transparency of the whole building.

The IGU had to be triple insulating in order to achieve the low Ug-value which was required. It was quickly obvious, that the innermost layer had to be the load bearing one. Like this, the connection details between the steel shoe and the roof construction was structurally simple to achieve and there were no special issues concerning the insulation. The outermost layer of the IGU was therefore only responsible for the wind suction and pressure loads and could be designed accordingly. The dead load of this laminated annealed glass is supported at the bottom which eliminates permanent stress on the edge sealing. The middle layer is a single annealed glass without lamination. The innermost layer for the IGU consists of a three times laminated 12 mm annealed glass with SGP interlayer. The SGP interlayer is on one hand favorable for the stiffness of the glass and it is, on the other hand, more transparent than PVB (Figs. 7, 8).
Fig. 7

Glass buildup for IGU

Fig. 8

Glass buildup for interior load bearing glass

The IGU has been analyzed using a finite element software. The center pane of the IGU with its geometry could be extracted from the Rhino model with its exact dimensions and curvature and imported in the FE software. First, the layers according to the buildup shown above had to be created and the material properties had to be applied. As listed in the table below, these material properties vary depending on load duration and temperature and need to be defined in accordance to the respective load case. The climatic loads acting on the glass can be determined depending on the geometry and the dimensions of the glass. Additionally, the exterior and interior pressure has to be applied and combined with the climatic loads according to the standards (Figs. 9, 10).
Fig. 9

IGU with applied loads

Fig. 10

Layer buildup

For the determination of the structural strength of the annealed glass, the more detailed determination of the k\(_{mod}\) factor according to prEN 16612 in comparison to the DIN 18008-1 have been taken into account. The extent of gap between DIN and prEN depends on the load duration as Fig. 11 below shows. If the decisive load duration is, for example, 1–2 days, the k\(_{mod}\) factor is approximately 10% higher according to prEN than according to DIN.
Fig. 11

Comparison for kmod factor between DIN 18008-1 and prEN 16612

The calculations were made considering large deflections. This analysis method allows the evaluation of stresses and stability in one. However, it is necessary to apply a reasonable imperfection in the numerical model in order to consider the negative effects of geometrical imperfections properly. Therefore, the first few eigenmodes had to be calculated and applied as initial imperfection of the glass. The deflection amplitude was determined in accordance with the glass supplier and defined as span/500.

The calculation results as shown below could then be assessed as usual. The deformation was usually not problematic since too large deflections led to structural instability anyway. The stresses on every layer could be made visible and assessed separately thanks to the detailed analysis model (Figs. 12, 13, 14, 15).
Fig. 12

Initial imperfection

Fig. 13

Global deformation

Fig. 14

Stresses in innermost layer of load bearing LSG

Fig. 15

Stresses in outermost layer of exterior LSG

2.4 Load application and structural sealing

The support and fixation of the glass was one of the key factors of the project. On one hand, the components had to be as unobtrusive and hidden as possible, on the other hand they were the most loaded building parts. The glass is bordered at the top and bottom edge with a steel shoe. It is designed to transfer vertical loads—mainly downforce, but on rare occasions also uplift—as well as horizontal loads in and out-of-pane.

The vertical down forces are being transferred through a pressure-resistant grout between the steel and the glass edge. This system is also used for the horizontal in-plane forces. The steel shoe therefore has a cap on its ends, the glass is notched and the gap between the two is filled with grouting material (Figs. 16, 17).
Fig. 16

Notch in load bearing LSG

Fig. 17

Hot dip galvanized steel shoe

The uplifting forces are considerably less intense than the down forces. They can be transferred by the bonding Structural Sealant Glazing (SSG) joints made by Sikasil SG-500 on the surface area of the inner and outer glass layer within the steel shoe just above the support point. The steel shoe also stiffens the glass edge against out-of-pane deformation. The glass is being separated from touching the steel shoe by the bonded areas mentioned above and by spacer tapes.

A 50-year life expectancy for the designed SSG system was required. This is beyond provisions given by EOTA ETAG 002, which regulates SSG performances based on a design life expectancy of 25 years.

As a part of the concept set by EOTA ETAG 002 for a 25-year life expectancy, it requires to test adhesion on original project substrates exposing them to artificial aging of 7 days at \(23\,{^{\circ }}\hbox {C}\)/50% r.h. and 42 days of water immersion at \(45\,{^{\circ }}\hbox {C}\). For the project target of 50-year life expectancy, adhesion tests on the original project substrates were performed based on prolonged artificial aging consisting of 7 days at \(23\,{^{\circ }}\hbox {C}\)/50% r.h. and 84 days of water immersion at \(45\,{^{\circ }}\hbox {C}\). Test results confirmed the excellent adhesion of Sikasil SG-500 on the substrates.

In addition, substrates approved by EOTA ETAG 002 for structural bonding are stainless steel, anodized aluminum, coated aluminum and glass but the specific project design required that the support frames to retain the IG units were composed by galvanized steel plates, with the only exception of stainless steel outer plate used at the bottom connection. This material selection was implemented to balance project costs and structural needs, while minimizing corrosion risks. As a matter of fact, stainless steel was used where contact to ground and outdoor environmental conditions occur, drainage is more critical and risk of corrosion exists; galvanized steel was used where exposure to outdoor environmental conditions is excluded and uniform indoor temperatures apply.

In addition to the technical considerations above, galvanized steel could be approved for structural bonding under confirmation that (a) the substrate corrosion protection is adequate to the service life of the bonding joints and (b) the coating performances are adequate to the mechanical resistance of the silicone adhesive in the long-term, e.g. the coating must consist in a layer able to transfer loads from the joint to the core metallic substrate.

Design loads required SSG joints approx. 200 mm wide on both sides of the bracket, what challenged the adhesive application. Indeed, the curing process of 2-component silicones generates by-products that must be released from the joint into air, in order to ensure a proper adhesion buildup. Experimental tests on Sikasil SG-500 show that maximum joint depth must be limited to 50 mm to allow a complete by-products elimination, when access to air is limited to one side only of the joint. Thus, such a limitation was critical for the adhesive application in the project brackets.

To overcome this issue, an alternative adhesive application method was recommended: unlike typical SSG application by gap filling from open side, adhesive viscosity of Sikasil SG-500 allowed for injection through holes, regularly patterned along the metallic substrate. Brackets could be manufactured with holes drilled at a max. distance of 100 mm throughout their height and length and bonded by injection on factory according to specific procedure (Fig. 18).
Fig. 18

Steel shoe attached on glass and ready for bonding

2.5 Testing

Many design relevant details and structures with structural glass are not standardized. The few reference projects with similar structures served as an indication during the design phases. Nonetheless, additional testing was unavoidable. The load transfer from the shoe into the glass had already been tested in the design phase. This gave a certain security for the planners to continue with the all-glass design. Further tests, including tests of the IGU and the five times laminated glass, have been carried out by the contractor. The load tests have been elaborated and carried out by the Lucerne University of Applied Sciences (Figs. 19, 20).
Fig. 19

Load application tests during tender phase

Fig. 20

a Buildup for full size glass tests, b artificial damage over the entire width of the glass, c completely broken glass layers

The defined test regime for the full size glass units includes the testing of a reduced steady load as it will be acting on the building for seven days, after that a load increase in steps according to the load levels in the various load cases which has been terminated at the ultimate limit state load. After that, one of the glass layers had been cracked on a height of approximately 1 m from the floor on its entire width. This crack should represent the extraordinary damage which could occur when a medium sized object hits the load bearing glass. The glass has then been tested again with different load level according to the applicable load cases. Finally, the glass layer on the opposite side had been damaged too. This meant for the three times laminated glass, that only one intact layer remained. After testing the different load levels again with this setup the load could finally be increased above the ultimate limit state level. The glass showed an enormous residual carrying capacity; it surpassed the required value roughly by factor 4. These results gave all involved parties the certainty that the calculations have been correct, that the building as designed can withstand the actions and last but not least, that even a damaged glass has sufficient reserves to carry the loads until a replacement unit is available.

3 Design engineering

3.1 Geometry

The main part of the building consists of two spirals rotating in opposite direction that interlock in the center. The way it was designed, only four different glass radii exist. This reduces the expenses of the formwork for the slump bent glass significantly. The shape of the glass is, however, always unique. The upper edge of the façade follows the design of the roof edge. The roof edge was designed in consideration of a consistent overall geometry as well as proper drainage paths.

3.2 Durability

Since the façade is the thermal envelope as well as a structural element in one, the durability of it became more relevant than usual. It is very important that as little moisture as possible passes through the edge seal in order to keeping the glass units transparent and the insulating value low. Additionally, the climatic loads in the cavity of a bended IGU cause higher stresses on the edge seal due to the higher stiffness of the glass. Typical IGU’s are being tested for its durability with artificial aging procedures.

3.3 Thermal concept

The thermal concept of the façade and glazing is predominantly defined by the Minergie criteria. The described exterior passive sun shades and the low g-value of 0.23 of the glazing is satisfying the energy demands in summer case and prevent overheating. The low Ug-value of 0.5 W/(m\(^{2}\) K) in conjunction with heaters below the glazing ensures the user comfort, avoiding any down drift during the cold seasons. The energy demands in the winter case are within the set limits. The large, glazed areas are compensated by the highly insulated green roofs. Additionally, photovoltaics on the adjacent existing building are contributing to the strict overall energy assessments and good rating.

3.4 Redundancy

The brittle material behavior of the glass calls for a certain redundancy in the load bearing structure. Although the use of annealed glass in the laminated glass does provide quite a high residual carrying capacity, different load cases with damaged glass plies have been defined for the analysis. Glass panes with one or two damaged glass plies are being calculated as well as the failure of an entire element.

4 Construction

4.1 Current status

The first works on site on the sub construction for the glass units started in July 2017, when the inserts had to be placed in the formwork of the concrete. Since there were other building parts which had a higher priority than the spiral there was little progress until October. Then it continued with the steel construction of the roof. Since the spiral does not have any solid walls, the roof had to be installed entirely on temporary studs. This was very demanding given the fact that the roof lines are not horizontal and the roof surface is inclined in two directions.

Once most of the steel parts were in place, the glass units could be brought in. Most of the units had to be lifted with a vacuum suction unit from the mounting frame and rotated by 90 degrees because they could not be transported standing upright. They would then be picked up with textile slings and moved into position, one by one, up to 20 pieces a day.

Up until today (January 2018), all glass elements have been produced and about 90% are installed on site. The work on site is going to continue until approximately July 2018, when all of the façade contractors’ works are expected to be finished (Figs. 21, 22).
Fig. 21

Roof construction cinsisting of element and a massive steel construction in the center

Fig. 22

Glass installation with the tower crane and a view from within the spiral

4.2 Difficulties

Many foreseeable difficulties have been treated beforehand in order to keep the construction works as smooth as possible. Nonetheless, some aspects could not be avoided or simplified more and had to be dealt with on site at the given time.

The geometry of the building is one of that. Almost every construction piece is unique and if one of them was inaccurate there was no similar piece which could be used instead. This also applies for the glass. If one glass gets damaged during transport or on site, it needs to be remanufactured. It is not possible to have a backup unit for every single unique piece of glass.

Another challenge was the weather. Since the beginning of the project it was clear that the weather conditions in Le Brassus were harsh. Very low temperatures during winter and heavy snowfalls would affect the workers on site and slow down the progress. Additionally, low temperatures would make sealing works impossible for a very long period of time and therefore delay the construction schedule.

5 Conclusion

It has truly been a very challenging and interesting time to work on that project. It all started with that idea of a full glass building in the competition. Back then, nobody of the project team knew exactly what this would mean exactly. There were of course some reference projects with load bearing glass already built but probably none of them had to deal with so many different requirements. However, the current situation with the passed tests, the progress on site and the upcoming completion of the project proves that the proposal was neither too bold nor too square; it was just about what was possible within the given boundary conditions.

Notes

Acknowledgements

Client: Audemars Piguet, Le Brassus/Architecture: BIG - Bjarke Ingels Group, New York/Local Architect: CCHE, Lausanne/Structural Engineering: Lüchinger \(+\) Meyer, Zürich/Façade Engineering: Lüchinger \(+\) Meyer, Zürich/Lüchinger \(+\) Meyer \(+\) Hermansen, Copenhagen/Façade Contractor: Frener & Reifer, Brixen/Execution Design & Calculations: Ingenieurbüro Dr. Ing. Alois Neulichedl/Glass Processor: SFL Glastechnik, Stallhofen/Full Scale Testing: Façade and Metal Engineering Center, Lucerne University, Lucerne.

Compliance with ethical standards

Conflicts of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Dr. Lüchinger + Meyer Bauingenieure AGZurichSwitzerland
  2. 2.Sika Services AGZurichSwitzerland

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