The design and engineering of the iconic spherical shell of the Los Angeles Film Academy Museum
The single-layer curved structural steel pipe is only 4 inches in diameter. The structure of the earth seems to blend into the background, with lightness and lightness. The construction of the steel glass structure was completed in the autumn of 2019. Knippers Helbig is responsible for the design and engineering of the structure and glass system of the spherical glass lattice shell structure from the concept stage to signing and sealing the drawings and calculations, and is responsible for the design and engineering of the bridge during the development.
The Film Academy Museum will celebrate the artistry and technology of film and become the world's first museum and event space dedicated to film. The total area of the project is 290,000 square feet (approximately 27.000 square meters) and consists of a six-story renovated building (formerly May Co. Department Store) and a new dome-shaped building with 1,000 seats Theater. Above the theater, the so-called Dolby Terrace is covered under a spectacular 150-foot (approximately 45 meters) wide steel and glass dome and the architectural core of the building, which will be used for events and special exhibitions.
The two buildings are connected by several partially suspended bridges. Together with design architect Renzo Piano Building Workshop (RPBW) and executive architect Gensler, Knippers Helbig (KH) developed the structure and glass system of spherical glass lattice shell structures in all design stages. The dome structure is a steel mesh shell with cable supports and flat tile-like glass panels on the second floor. KH also designed the concept for the four bridges between the dome and adjacent buildings throughout the DD phase.
The glass dome was developed in close cooperation with the design architect RPBW in Genoa, Italy. The canopy mainly needs to provide weather protection for the terrace, and was originally designed as a closed glass sphere. It was later transformed into a hemisphere, with wide openings to the south and north, allowing undisturbed views of the nearby Hollywood Hills.
When KH was awarded the contract to take over and carry out the dome design at all stages, the goal was to reduce the visual appearance of the main structure. KH proposes to switch from the previously designed space frame to a highly optimized single-layer lattice shell. From the initial sketch to the final structure, KH was able to achieve a circular high-speed steel section with a diameter of 4 inches (100 mm) for the main arch of the dome, which resulted in the seemingly weightless design of the dome.
In addition to the main structure, many other elements need to be coordinated, such as a layer of glass for weather protection, connecting brackets for the internal shading elements of the dome, sprinkler pipes and electronic pipes, heavy anchors to support future exhibition items, and maintenance stairs , Catwalks and tie anchors.
The overall geometry of the glass dome follows a precise 150-foot diameter sphere. The terrace is about 76 feet above the ground, and the apex of the glass dome is about 120 feet high. The opening in the south is about 11 feet high, and the opening in the north is about 22 feet high. The lower half of the sphere was cut off; however, it still partially overlaps with the reinforced concrete support structure of the Geffen Theater, forming the lower half of the sphere. In the overlap area, the steel/glass dome leaves a gap with the exposed prefabricated panels, and several pins are used to stabilize and provide support for the EW arch. The screen box and projection box of the cinema are visible from the outside because they protrude from the sphere.
The main structure of the steel mesh shell itself consists of a circular high-speed steel arch with a diameter of 4 inches (100 mm) in the east-west direction. The arches are parallel to each other in plan, with a spacing of 4 feet OC. In the north-south direction, the radial arch (NS arch) is made of custom solid rectangular cross-sections and intersects the east-west arches perpendicular to each node. The resulting quadrilateral grid structure is further supported ("cross-supported") to provide in-plane stiffness. Double cables with a diameter of 10 mm extend diagonally across the entire dome and are clamped at each node of the grid.
The secondary T-shaped profile runs approximately. Located 10 inches above the main east-west structure, it is used as a support for the glass panel. The secondary structure is supported on the columns at each intersection between the EW arch and the NS arch. The column and the sphere are radial and used as the support of the cable clamp. The following figure illustrates this structure construction. The glass window consists of two flat laminated glass made of 12 mm glass plates. Due to the change in the size of the quadrilateral grid, the size of all panels are different. The iconic shingle appearance of the glass window is created by stepping on the top of the T-shaped section. The glass panels overlap slightly at each step.
The circular HSS arch is oriented strictly in the east-west direction and is the main load-bearing element. They transfer the load of the dome to the embedded connection of the concrete dome. The north-south cross members connect the arches together. Although the outer diameter of the continuous east-west arch remains the same, the wall thicknesses are adjusted to the north and south openings to account for higher internal forces. According to the control load combination, the dimensions of the north and south pillars are the same throughout the structure.
The key component of the structural integrity of the outer shell is the double cable diagonally passing through the dome to avoid any in-plane diamond deformation of the dome. The structural design is developed, analyzed and optimized in an iterative manner, using various cable prestress values to ensure that the structure is perfectly adjusted. One of the challenges of pre-stressing is to have sufficient pre-stress in all cables after all construction phases are completed, because if the initial tensioning of the cables is performed while the structure is still on the scaffolding.
Although the project is located in California, the seismic acceleration is less important due to the basic isolation structure of the entire dome sphere. The structural design of lightweight steel reticulated shells is usually the most sensitive to the effects of wind. Therefore, the shell was tested in the wind laboratory and the results were used for structural analysis. In terms of dead load, the analysis of individual masses shows that the weight of glass is significantly higher than that of steel structures. This is mainly due to the thick glass plate. The design of the glass panel must be strong enough to support maintenance personnel.
When using the third-order theoretical analysis design in the structural analysis software SOFiSTiK, Knippers Helbig engineers found that the shell was very stiff overall, but the large openings facing the north and south of the shell produced the greatest deflection and stress. When introduced to Russia When the engineer Vladimir Shukhov (1853-1939) named the diagonal support system internally, the deflection and stress at the two openings can be fully controlled.
When connected to a reinforced concrete dome, the construction tolerance and long-term deflection of the concrete sphere must be compensated. The concrete dome was designed by a different engineering team and executed by another contractor. Therefore, KH and Gartner, a specialist glass dome contractor, developed a connection detail that can be adjusted on all axes. At the site, as-built investigations revealed that the reinforced concrete dome moved much farther than the foundation construction engineer initially expected. Therefore, KH and Josef Gartner engineers adjusted the global position of the glass dome to follow the movement of the reinforced concrete dome.
In terms of force, the embedded details must be strong enough to transmit the compressive and tensile forces of the supporting pillar. Due to the strict east-west orientation of the pillars, the geometrical layout of the dome produces many different angles, some of the inserts facing the north and south of the dome, where the spherical geometry produces acute angles, a large amount of shear force must also be fixed by strong shear bolts On the dome. In addition, the diagonal support compression struts and lateral support rods create this tangential force.
Except for the connection with the dome, the rest of the structure almost does not need to consider tolerance adjustment. The typical connection of the continuous arch to the connecting pillar is precisely manufactured, so it can be perfectly installed on site. The two bolts connecting the strut to the arch mainly transmit axial force and out-of-plane bending moment, because the diagonal cross bracing prevents any rhombus distortion, thus minimizing the in-plane rotation and bending of the strut connection.
When connected to the arch, the two fin plates embrace the circular HSS and act as stiffeners. Not only is the force from the two pillars introduced into the circular HSS, but the considerable force from the pillar not only supports the T-shaped substructure of the glass, but also acts as an anchor for the cross-supporting cable through the cable clamp.
The column is designed to have a certain degree of freedom, so it limits the force transmitted from the glass substructure to the main structure (the purpose is that the secondary structure does not contribute to the overall structural system), and it is also adjustable (up and down) to be on the plane Precise positioning of the glass holder in the outer direction.
The architectural goal is to design a transparent and light-weight glass system on top of the steel structure layer. The shingled flat glass panel is supported by a 60 mm wide steel T-section curved in the east-west direction; this means that the glass is basically supported on two sides.
The vertical pin between the structural steel layer and the glass frame is designed to accommodate tolerances perpendicular to the glass surface, with internal threads. On the glass plane, the glass system is designed for zero tolerance. This is achieved by prefabricating the steel shell and glass system in the trapezoidal frame and checking the geometry with a template during the manufacturing and installation process.
The glass system is designed with a 15 mm frame occlusion, which requires careful investigation and restriction of all types of structural movement, especially the diamond-shaped deformation effect. The glass composition consists of two 12 mm low-iron glass plates and Saflex DG41 interlayer. Only one of the two glass plates is statically supported, which means that the outer glass plate is only supported by the interlayer. All laminated glass panels are stepped on the lower edge to provide hidden static load support.
Since buildings require highly transparent glass windows, the use of solar control coatings must be avoided. In order to achieve high comfort (temperature) on the terrace under the glass window, roller blinds and operable vents are integrated into the system.
The requirements for glass maintenance, such as window cleaning, require a special solution so that workers can access the entire surface of the dome. Most of the inner surface of the dome can be easily cleaned using traditional maintenance platforms and manual lifts, while the portion of the inner dome that overlaps with the concrete domes on the east and west sides and the entire outer surface of the glass requires an alternative method.
The solution was developed in close cooperation with the design team and maintenance consultants. It is an internal walkway running between the glass and precast concrete of the dome, and the outside is a maintenance staircase leading to the apex of the dome. The staircase has become one of the main design features of the dome. The staff can use the designated staff rated anchor bolts to fasten from the top platform of the stairs. With the help of additional functions, workers can reach every corner of the glass surface.
The 14 mm frame occlusion and the dead-load support system of the glass plate must usually be confirmed by some shelf testing. Structural analysis predicts that in the worst case, the distortion in the north-south direction is 27.1 mm, and the distortion in the east-west direction is 8.4 mm.
A model composed of 4 glass panels, the actual details are mounted on a flexible sub-frame and twisted in two directions with hydraulic jacks. The distortion is increased to 70 mm in 5 mm steps in both directions. As a result, no faults of any kind were found within the maximum range. The distortion is 2.5 times larger than predicted in the structural analysis.
Since the glass plate is basically only supported on both sides, and the glass must be designed to be easy to maintain and clean, a design standard similar to that outlined in ASTM E 2751 was chosen. Glass analysis shows that some load conditions are close to the limit; so another small model has been established to simulate the worst maintenance load conditions.
A representative glass panel consisting of 2*12 mm HS glass and 1.52 mm DG41 sandwich is installed on the sub-frame with actual project details; the temperature rises to 50°C and remains at this level during the entire test. Apply the load at the most critical position of the 4 square inch surface area; increase the load from 50 pounds to 300 pounds and hold for 10 minutes.
When no failures of any kind are found, repeat the test a) upper glass plate breakage and b) upper glass plate breakage and lower glass plate breakage. Therefore, the cracks of the glass basically have no obvious effect on the performance of the glass; no large deflection etc. are found. When all tests passed, the load was further increased to 825 pounds-when the heavy object fell over and severely damaged the glass. However, it was found that no heavy objects fell from the glass and no harmful glass parts fell out of the laminate.
The diagonal double cables of the dome structure are clamped each time they pass through the nodes of the main dome structure. Each part of the cable will be uniformly pre-tensioned during installation, but once the scaffolding is loosened, especially in wind conditions, part of the dome geometry will deform into a diamond shape, as described in the previous section.
The diagonal twin cables that counteract this rhombic twist will bear greater axial tension in the dome area where the structural deformation is greatest. This creates a constantly changing cable force from one part to another. The differential value of the tension is carried by the cable clamp. Therefore, the fixture is an important part of the structural system.
The cable clamp works by the mechanical principle of friction. The pressure on the cable clamp is generated by two pre-tensioned M12 bolts. Bolt pre-tightening force and friction coefficient are the main factors to achieve high-performance fixtures. Because the friction value cannot be accurately determined through theoretical analysis, tests must be conducted to confirm the friction value in each specific application situation. The friction coefficient of the cable clamp can be improved by considering certain rules in the design and manufacture of the clamp.
In addition, design factors must be considered to maintain the structural integrity of the cable when pressure is applied. The notches on both sides of the clip allow the cable to be guided during assembly and use. Notched steel is the most important part and requires special attention because the pressure should be applied to the cable in a distributed manner. This avoids damage to the cable structure due to high local pressure.
The diameter of the notched steel must be accurate to 0.1 mm. In addition, the start and end points of the notch steel have a so-called "flare", and the notch diameter is widened in a spiral shape to allow a smooth transition from the clamping (thus reducing the cable diameter) to the non-clamping area of the cable. The clamp is passed High-strength steel parts formed by drop forging process.
After manufacturing, the jig receives a zinc coating on the notched part of the steel and a typical coating on the remaining surface. On the one hand, zinc produces a rough surface. On the other hand, the cable is protected by a relatively soft zinc layer and will leave visible traces in the zinc coating when prestressed.
During the test, the cable is fixed at both ends of the test equipment and kept in place. The clamp is connected to the double cable and, once pre-tensioned, will be pulled by the hydraulic cylinder. The test is carried out in a force-controlled procedure, applying force in an iterative manner, including waiting time, until the sudden sliding of the cable clamp can be monitored.
Permasteelisa North America / Josef Gartner (JG) is the professional curtain wall contractor for this project. The design of Renzo Piano Building Workshop and Knippers Helbig was further optimized and adjusted in cooperation with JG. Through close coordination, the team was able to avoid the initial estimated cost increase with minimal changes, while maintaining the architectural intent and improving the technical concept of the structure.
One of the main goals in the manufacturing process is to maintain the strict tolerance requirements of the global structure. The structure is assembled on a template that represents part of the dome in the store. This ensures that the assembly accurately creates the final geometry when assembled on site. These parts are manufactured using a variety of techniques, mainly through traditional welding of flat steel plates, some are manufactured by CNC milling, and some parts are manufactured using drop forging, such as cable clamps or cable end fitting details.
At the site, the steel parts arrive in a so-called trapezoidal frame, which means that the two parallel arches have been pre-assembled, including connecting the north and south pillars and the secondary steel structure. This simplifies transportation, handling and installation. After placing the main structure (and the additional secondary frame) on the scaffolding, the various parts are connected using the internal pre-stressed connection in the arch and the typical bolt connection of the north and south pillars.
The remaining elements of the structural system, the diagonal twin cables, have been installed and fully pre-tensioned. After interconnecting the ladder frame and connecting the structure to the connection of the installed embedded parts, and finally checking the cable pre-tightening force, carefully remove the support column of the scaffold to allow the structure to span freely.
The shingle glass is installed in a specific order, carefully balancing the extra weight and avoiding uneven loading. The weight of the glass panels adds the most weight to the structure, which is much heavier than the supporting steel structure itself.
Upon completion in the summer of 2019, the spherical steel and glass dome of the Academy Film Museum will be the core of the highly anticipated museum on Wilshire Boulevard and one of the most spectacular examples of light buildings in the United States.
The author would like to thank the key people who contributed to this article: Renzo Piano Building Workshop: Mark Carrol, Luigi Priano, Daniel Hammerman, Gensler: David Pakshong, Richard Stoner Knippers Helbig: Thorsten Helbig, Thiemo Fildhuth, Steven Ball Josef Gartner: Stefan Zimmermann, Felix Schmitt, Tobias Born, Rob Sanders Transsolar: Matthias Schuler Paratus Group: Andy Klemmer, Antonio Domingues, Ryan Kelly
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