The Courtyard Roof of the Museum of Hamburg History
Architect: Volkwin Marg
Engineer: Jörg Schlaich
Maximum Span: 17m
Yerab Ermias, Bar Shabtai and Peter Szerzo
|Fig. 1: View of the L-shaped courtyard with its new roof|
The courtyard roof covering the 50m long and 14-17m wide L-shaped courtyard of the Museum of Hamburg History emerged as a response to an ambitious architectural program with strict structural requirements. With its exceptional lightness and transparency as well as features of structural design innovations, the structure has become well-known amongst both architects and structural engineers. The roof has won several architectural awards and, as a pioneering gridshell structure, influenced and stimulated the development of several similar structures, such as the DZ-Bank in Berlin, later on .The design of this highly successful structure was largely influenced by several historical, social and economic constraints, which were dealt with through the skill and ingenuity of both the structural engineers and architects involved in the project.
Historical Context - Germany and Hamburg in the Late 1800's
The end of the 19th century saw several major changes in Germany. After defeating the French in the Franco-Prussian war of 1871, the German states were unified into the German Empire. Already a leader in higher education, the newly unified Empire experienced great economic growth, matching Britian's economy by 1900. Socially, there was a strong support for the social democratic party, despite Chancellor Otto Von Bismarck's efforts to repress this movement. Bismarck did, however, gain the support of the industry and working class through policies such as pensions, insurance, medical care and unemployment insurance. These improved conditions also assured the reduced number of outflowing immigrants to the United States, where wages were higher, but such benefits were nonexistent. In 1888, after the death of his father, Wilhelm II became the new Emperor and in 1890 forced Bismarck to resign, taking full control of the Empire. Wilhelm II vastly changed Germany's foreign policy, seeking a larger German influence in the world .
In Hamburg, the population greatly increased in the second half of the 19th century, quadrupling in size to 800,000 inhabitants, mainly due to the development of its port and trading on Atlantic routes . A former city of Hansa (a confederation of merchant guilds that dominated trade across the coast of Northeast Europe during the late middle ages and the early modern period), Hamburg's port remained influential over the years and became the 3rd largest in Europe by the end of the 19th century  By early 20th century, the world's largest shipping company, Hamburg-America Line, called Hamburg home, as did companies from South America, Africa, India and East Asia. As a result, Hamburg was a multicultural metropolis .
|Fig. 2: Facade of the Museum of Hamburg History - currently under renovation|
When the idea for the construction of the museum was conceived in 1909, Germany's economy was clearly very strong and growing. Nonetheless, due to the aggressive foreign policy instituted by Wilhelm II there was growing political pressure from the surrounding super-powers (France, Russia and Britain). These tensions eventually lead to the breakout of WWI in 1914 .
Conception and Construction of the Museum
In 1909, Chief Planning Director Fritz Shumacher began planning the construction of the Museum of Hamburg history based on exhibition plans from Otto Lauffers, the museum's first director. Construction began in 1914 and after a halt in 1916 due to World War I, was completed in 1922. The facade of the museum, currently under renovation, can be seen in Figure 2.
The museum is located on the grounds of the former Bastion Henricus. The cultural-historical museum provides an overview of the history of Hamburg from around 800AD onwards . The four story masonry building is L shaped in plan with an open courtyard in the middle. Typical masonry buildings have short spans and thus limited width. As a result, if large masonry buildings are constructed, the inner parts of the buildings are not exposed to day light. Therefore, to construct a large enough building which is fully exposed to natural lighting, the structure must surround a courtyard. Furthermore, the masonry system provided some restrictions on the design of the roof. Mainly, since a masonry structure is already very heavy, any courtyard roof had to be light in weight. This is discussed further below.
The Courtyard Gridshell
During the original planning of the Museum in the beginning of the 20th century, a glass roof over the L shaped courtyard was considered, but its construction was deferred . By the 1980's, however, the Museum desperately needed more exhibition space. Instead of building a whole new structure, the management opted to construct a roof to cover the courtyard, as proposed in the original plans . The roof also fulfilled an additional need: to protect the statues already exhibited in the courtyard from increasing levels of atmospheric pollution and toxicity .
In 1989, the institute in charge of the Museum chose Professor Volkwin Marg and his firm, Von Gerkan, Marg and Partners, as architects for the project .
The completion of the roof (Figure 3) in 1989 coincided with the 800th anniversary of Hamburg's harbor, as well as the opening of the new Hansa exhibition, which presented Hamburg as a part of the Hanseatic League . The enclosed space now serves as an exhibition space, as well as an area to hold concerts and other large events .
|Fig. 3: Innovative barrels, domes, cross-braces and cable diaphragms comprise the complicated yet still transparent gridshell - result of the strict requirements on lightness, transparency and budget|
Design Brief and Specifications
There were a number of design specifications the roof had to follow.
- The roof had to transmit as little load as possible to the existing building (i.e. the structure must be light in weight) 
- Structural changes to the original museum building had to be kept to a minimum 
- Funds were limited, as they were provided by a private donor 
- The roof had to let in as much natural light into the courtyard as possible 
- The design and construction of the roof had to be completed as quickly as possible, in order to:
- Limit the damage done to the statues in the courtyard by the atmosphere 
- Minimize the time during which the Museum would not be able to host exhibits and events in the courtyard 
Given these strict requirements, the original plans for a glass roof drawn up by Shumacher in the early 20th century were outdated at this point. New materials (such as stronger steel) and technology (such as computer modeling) made it possible to design new, more complex systems that suited these requirements better. The steel-glass gridshell (Figure 3), comprised of a steel grid with installed glass panels, was one of the systems that became available due to these advances in technology, and was ultimately adopted for this project.
Choosing the Design
Marg needed an engineer who was able to design a roof that was light, transparent and flowing, which overlapped the edges of the existing roof while satisfying the imposed restrictions . These constraints favored the steel-glass gridshell structural system, which was innovative at the time. Jörg Schlaich, already working on a similar structure design for the Aquatoll swimming pool in Neckarsulm, was ultimately contacted by Marg for help on this project . The architect described his vision of a light and flowing roof to the engineer in detail; however, he did not impose any restriction on the geometry and structural system that would be used .
This project came at a convenient time for Schlaich. The design of the Aquatoll brought slight disappointment to the engineer, who believed the roof did not interact well with the rest of the building, describing the roof and substructure as "strange bedfellows" . Schlaich therefore saw this project as the perfect opportunity to prove that the system used in the Aquatoll roof can adapt to a geometric form . Furthermore, Schlaich had experience in shell design of other materials such as fiberglass and concrete. For example, he designed a shell roof for the Garden Exhibition in Stuttgart in 1977, which made out of concrete reinforced with glass fibers. This experience proved to be valuable for the completion of the roof over the museum courtyard . Given this expertise, it only took the engineering firm 3 months to complete the structural design.
Structural Analysis and Technicalities
The 50m long roof is L-shaped in plan and consists of two singly curved barrel-shaped shells 14-17m in span and one doubly curved spherical dome at their intersection (Figures 4 and 5). Barrels and domes, predominantly build out of masonry and concrete in the past, were now discretized in quadrangular grid elements and built out of steel. A number of innovative features were necessary to ensure transparency, efficiency and good structural behavior, requiring strong technical background and expertise .
|Fig. 4: Wireframe view showing main structural elements (cross-braces are applied
at each node and are omitted on the rest of the structure for clarity; the subdivision
is likewise simplified for clarity)
|Fig. 5: Top view of the L-shaped courtyard with dimensions; the height of the barrels range between 3m and 5m, whereas the dome reaches a height of nearly 8m|
Since this roof structure was one of the first modern steel-glass gridshells, form-finding and development was still at an early stage. Jörg Schlaich just recently started developing the principle of the gridshell based on the common kitchen sieve implemented for the Aquatoll swimming complex in Neckarsulm, the project that lead to the courtyard roof . However, important questions such as creating a dome mesh with planar quadrangular elements were not resolved at that time . These issues were resolved at later projects based on the experience gained through this project .
The intuitive understanding of the behavior of individual components, as well as the interaction between them was essential to create such innovative structure, especially since analysis and design software packages still did not allow for convenient ways to fully analyze the structure . In the following, the main structural principles of the roof are explained element by element.
The Barrel and the Dome
Both the barrels and the spherical dome are comprised of interconnected arches. The arch as a structural element relies on horizontal support or thrust to decrease bending moments compared to those occurring in a simple straight beam. The lightweight arches of these systems are so light that the self-weight stresses are small compared to asymmetric loading such as snow on one side or wind suction and pressure loading. It is these asymmetric load cases for which special features were required.
|Fig. 6: Schematic of a point-loaded, simply supported barrel shell - loads are distributed among neighboring arches by longitudinal members|
In particular, barrel shells work as a series of arches interconnected by straight longitudinal beams, each of the same 60 mm by 40 mm cross-section (Figure 6). The longitudinal beams act as load distributors among several neighboring arches. The loaded point (a) deflects the most, while neighboring nodes b and c deflect less (87% and 68% of the maximum deflection respectively).
Deflections of even farther nodes diminish quickly to the point that they can be ignored. If the longitudinal elements are stiff (strong interconnection), the attenuation of the deflection is small, whereas it is larger for weak longitudinal members (weak interconnection - no transfer). The level of interconnection necessary is determined from strength and serviceability criteria, including the strength of the material as well as the deflection limit that the attached glass can take .
The Cable Diaphragm
At specified locations along the barrel-shaped shells and at the merging points of the barrels with the dome, the structure is stiffened by diaphragms consisting of a fan cable arrangement connected to a central hub (Figure 4). This system works analogously to the spokes of the common bicycle wheel, which would be too weak without its stiffening (Figure 7). The cable diaphragms installed at specified locations (Figures 3, 8 and 9) stiffen arch segments by a redistribution of axial forces in the cables, greatly reducing bending and deflections in the arch.
|Fig. 7: Spokes stiffening the bicycle wheel|
The diagrams on Figure 8 illustrate the results of a simplified finite element analysis of a 14m arch segment with cross-sections true to the real structure (60mm by 40mm rectangles), both in the stiffened and unstiffened cases. According to this analysis, the maximum deflections of the stiffened arch are 67 times smaller, using just 20 mm thick cables.
|Fig. 8: Diaphragm stiffening an arch - both bending moments and displacements are reduced by an order of magnitude|
The combined steel-glass self weight bending moment diagram of a barrel segment indicates the behavior of a barrel stiffened by these diaphragms (Figure 9). The arches in the middle experience the highest bending moments, which 'attenuate' towards the diaphragms at the ends. This attenuation is present for all load cases acting on the structure.
|Fig. 9: Self-weight bending moment diagram with color scale - barrel with diaphragm (kNm)|
Connection of the Glass Panels to the Steel Grid - the Planarity Problem
Subdividing or meshing a structural surface into constructable quadrangular elements posed a significant challenge for this project. As curved or warped glass panels were very expensive to fabricate, the panels had to planar to the extent possible; yet no such direct meshing method was available at the time .
The barrels could relatively easily be meshed into planar grid elements, as the longitudinal members are straight and parallel to each other (parallel lines are always in one plane). On the other hand, achieving planarity is not so trivial for the dome, and therefore curved glass panels had to be used for a number of quadrangular elements . At a number of locations at the corners of the dome, the glass panels were cut into triangles and display a kink along the diagonal (Figure 10).
|Fig. 10: Problematic area for meshing the dome - a number of quadrangular panels here are composed of two triangular panels meeting at a visible extension joint and display a visible glass joint|
Even if the meshing was not perfect for this project, it fostered the development of better meshing methods, leading to the method of translational surfaces . This method uses the principle that parallel vectors are always in the same plane (for which the barrels in this project were a trivial case), and involve translating generator curve (called generatrix) along an arbitrary directional curve (called directrix) . This method was used in many gridshell projects later on such as the House of Hippopotamus in Berlin.
Since the heating air-conditioning requirements of the courtyard are much less strict than in fully enclosed spaces, 12mm thick glass panels could be used, which could bend and thus adhere to the curvature of the non-planar steel grid elements when clamped directly to the lats . Electric heating has to be provided to avoid condensation when the temperature difference between the two sides of the shell is large .
Connection of the Grid Members - Nodes, Bolts and Cross-Braces
Since solid sections were chosen, a seemingly complicated yet relatively simply installed bolted connection could be used to join the grid members. Near the joint, the members were cut to half the depth (30 mm) at a short distance, which, overlapping with the corresponding lat of the nodal connection piece (where 4 members meet), were bolted to the rest of the structure (Figure 11).
Though the connection model was fairly simple, it did make the section at the connection area considerably weaker in counteracting bending and shear forces, since the shear connection between the connection piece and the half-depth section is realized only locally through the two bolts, and therefore the section was weakened to half the section depth at certain locations (Figure 11) .
|Fig. 11: Main connection detail; 4 grid members meet at the cylinder-shaped connection piece - the bracing in two directions is clamped to this piece via grooved steel plates|
The cable diaphragm played a significant role in reducing these forces and allowing for both slender members and an easily fabricated and installed connection .
The grid resulting from longitudinal and transversal members is quadrangular, which is much weaker than triangular ones due to the close to 90 degree angles. To compensate for this weakness, the grid is cross-braced by double cables that provide the necessary rigidity that comes from axial forces in the cables rather than bending at the grid joints.
The schematic image displayed on Figure 12 illustrates the difference in rigidity of an unbraced and a braced rectangular frame. A finite element analysis has been conducted to quantify the reduction effect in deflections and bending moments. Realistic cross-sections of 60 x 40mm solid rectangle were used, and the behavior due to a load of 3 kN consistent with previous calculations from a global model in the direction indicated on Figure 12 was applied. Cables were pre-stressed to 2.5kN.
|Fig. 12: Effect of bracing on system rigidity - shear forces occurring in a quadrangular grid element are resisted by axial force in the cable rather than bending in the members|
It was found that deflections and moments are reduced by a factor of 20, indicating the dramatic improvement of these cross-bracing cables in improving local rigidity and ensuring good structural behavior. Braces are well visible on Figures 10 and 16, but hardly noticeable when looking at the structure as a whole (Figures 1 and 3).
|Fig. 13: Braced Model - bending moment diagram (kNm)||Fig. 14: Figure 14 Braced Model - bracing cable force (kN)|
The boundary elements are I-beams with plates welded to them at the joint with the arch member (Figures 16 and 17). The bolt passes through these plates as well as through the arch element to realize a close-to-pin (bending-free) connection. Underneath the I-beams at these joints, steel members joined the edge I-beam to the building, at locations where individual tiles were locally removed from the roof (Figure 16).
|Fig. 15: Unbraced Model - bending moment diagram (kNm)|
An important goal of the structural design was not to transfer significant loads to the existing buildings or alter them in any way [6, 10]. Lightness and transparency was therefore also a structural requirement, and not merely an architectural one. The thinness of the boundary beam and connecting elements are visual signs that this was indeed achieved.
|Fig. 16: Effect of bracing on system rigidity - shear forces occurring in the shell are resisted by axial force in the cable rather than bending in the members||Fig. 17: Connection of the edge grid member to an edge beam supported on the roof of the building. The edge beam is connected to the building locally at these joints (details not documented here).|
Construction Process and Costs
Selecting a contractor to build the gridshell also posed a challenge to the project. As this structural system was very new at the time and unknown to construction companies, general contractors were not eager to give price suggestion. Eventually, Schlaich found a contractor who was willing to undertake the unusual construction of the structure.
To adhere to the stringent time constraints, only bolted connections were used as opposed to welding. Extensive formwork was nonetheless needed, as the shell only became self-supporting once it was fully built and the thrust from the edge beams was activated .
The concept of the main connection detail shown on Figure 11 also guided the construction sequence of the structure. First, the entire steel assembly was laid out on top of scaffolding. A grid of cables was laid on top of grooves in circular metal plates, which were then bolted to the cylinder-shaped nodal connection piece. The grid members were then bolted to this node. After that, a neoprene gasket was put between the glass covering and the glazed steel members of the structure, sealing the roof against water leakage. Finally, once all nodes were securely bolted together, the structure was tensioned upwards and thus brought to its final form .
Despite the careful planning, once the gridshell was brought to its final form, it was found that the movement experienced at the edges of the dome was too large because of the double curvature of the surface. To solve this problem, the diaphragms were added .
The roof cost can be estimated to $2.31million in 1989, which amounts to about $840 per square meter in 1989, equivalent to $1,500 per square meter in 2011 . While this was more expensive than the conventional roof designs of the time such as a simple truss roof, this design achieved transparency and lightness. At present, gridshells still cost about $1500 per square meter in Germany .
Conclusion - Evaluation and Implications
Marg and Schlaich were able to design and build a roof within all of the given design constraints and restrictions. This was an achievement given the technology that was available at the time. For this reason, as well as for the final aesthetic value of the structure, the roof of the Museum of Hamburg Museum won respect and acclaim worldwide, as well as engineering awards in Germany [1,5].
In an interview, project engineer Hans Schober of Schlaich Bergermann and Partner mentioned that the design of the roof was a "fight" that made future gridshell projects much simpler. The courtyard roof led to the development of translational surfaces, a technique through which planar faceted gridshells could be developed using the property that translated vectors stay in a single plane . This technique led to a series of Schlaich Bergermann and Partner gridshells in the future, such as the House of Hippopotamus in Berlin.
1. Schober, Hans. Personal interview. 29 Oct. 20012.
2. N.a. History of Germany. Wikipedia, Web. 8 Nov. 2012.
3. N.a. History of Hamburg. Wikipedia, Web. 8 Nov. 2012
4. N.a. Hanseatic League. Wikipedia, Web. 8 Nov. 2012.
5. N.a. hamburgmuseum, Museum. Hamburgmuseum, Web. 10 Nov. 2012
6. Holgate, Alan. "Glass Grid Roofs." The Art of Structural Engineering: The Work of Jörg Schlaich and His Team. Stuttgart: Ed. Axel Menges, 1997. N. pag. Print.
7. Inst. Munchen, ed. Glass Roofing of a Museum Courtyard in Hamburg. Detail. 1991, Print.
8. Brookes, Alan. Museum of Local History, Hamburg. P 72-75. New York: Whitney Library of Design, 1994. Print.
9. Schlaich, Jörg, and Rudolf Bergermann. High Energy. Berlin: Akademie Der Künste, 2010. Print.
10. Bögle, Annette. Personal interview. 2 Nov. 2012.
11. Glymph, J., and H. Schober. "A Parametric Strategy for Free-form Glass Structures Using Quadrilateral Planar Facets." Automation in Construction 13.2 (2004): 187-202. Print.
We would like to thank Professor Adriaenssens, Professor Glisic and Alex Jordan for organizing the class field trip to Germany. Seeing the courtyard roof in Hamburg in person provided the opportunity to experience and study the structure in its real-life scale.
We would also like to thanks Hans Schober and Annette Bögle for their time interviewing with us. Their knowledge and experience with this project helped solidify our knowledge of the roof.