Mesh shell structures: future and innovation

When it comes to mesh and shell structure, first of all, let’s look at the definition. “Netshell structure is a space structure system formed by arranging the rods along a certain curved surface in a regular manner, and its force characteristics are similar to those of thin-shell structure, with the role of “thin film” as the main force characteristics, i.e., most of the load is borne by the axial force of the net-shell rods”.

The Future of Mesh Shell Structures

When we talk about mesh shell structures, we have to start with thin concrete shells. In the middle of the 20th century, thin concrete shells flourished, and Eduardo Torroja, Felix Candela, Nicolas Esquillan, Heinz Isler and others practiced thin concrete shells all over the world. Their thin and bright shells are now considered as fine examples of architectural and structural integration.

　Though highly efficient and aesthetically pleasing, thin concrete shells are largely obsolete today. The reason for this is that before building a concrete thin shell, shaped, complex curved surfaces are built out of formwork, and the formwork can only be used once and cannot be reused.

　　This resulted in construction that required costly formwork and labor, which in the mid-20th century was offset by savings in material costs. However, as we enter the 21st century, labor costs have increased dramatically, and as a result, the economics of thin concrete shell structures have declined dramatically.

So people began to use more efficient single-layer mesh shells to replace the thin concrete shells. But the same net shell production, installation is also more difficult than the same price, so the net shell development direction in which?

　　1) Lightweight and standardization of the rod.

　　The force transmission efficiency of the net frame is high, so it can use the least material to complete the cover.

　　At the same time, if all the rods are of the same length and the nodes are standardized, then the manufacturing process of the net frame will be greatly saved. Schleich has done a lot of exploration in this regard.

　　2) Utilizing the toughness of the material to convert complex curved shell construction into a simple planar mesh for fabrication.

　　This is the idea that Otto applied to the Mannheim multipurpose hall in the 1960s.

　　3)The development of digital construction.

　　In the future, if robots are involved in construction, the surface will be nothing more than a combination of node coordinates for them. At that point, the cost of construction will drop dramatically.

　　Exploration of lightweight mesh shells

This is an old cast iron mesh shell, built in the late 1800s. As you can see, this mesh shell has no diagonal bars; it uses the bending capacity of the bars to resist torsion in the plane.

　　A direct way to increase in-plane stiffness is to use a triangular mesh. Fuller invented the short-line presentation dome and applied it to the US Pavilion at the 1967 Montreal International Exposition – a 3/4 spherical building with a diameter of 76m.

　The triangle mesh is certainly stable, but it’s not translucent enough or light enough. And Schleich wanted to make quadrilateral meshes. The first opportunity came at the Neckarsulm swimming pool. The architects wanted the swimming pool’s roof to be part of a sphere.

 

Schleich used a spherical shell divided by a quadrilateral mesh. The mesh bars bear the axial forces and additional tie ropes tie the diagonal nodes to ensure the stability of the quadrilateral mesh.

For ease of transportation and installation, all rods were designed in standard lengths of 1m and bolted together at the nodes.

　　In order to ensure a smooth spherical surface, the bars must not be too rigid and need to be able to bend and twist slightly. However, they should not be too flexible either, otherwise they would not be able to withstand the loads. Ultimately, the cross section of the bar was determined to be 6cmx4cm.

 

The tension cables are arranged to be tensioned after the bars form the mesh shell. After tests and calculations, it could be seen that the deformation of the structure was greatly reduced by the addition of the tension cables.

　　Schleich was not very satisfied with the previous project and felt that the combination of building and structure was very rigid. He felt that the mesh shell system could be adapted to any shape. His next opportunity soon came.

 

　　A roof was to be added to the atrium of the Hamburg Museum. The atrium was L-shaped, with a width of 14 meters at one end and 17 meters at the other.

　　The project had two main claims. Firstly, the project was funded by private donations and had a limited budget; secondly, the museum wanted the roof to have as little impact on the original building as possible. Therefore, the roof needed to be as light as possible, using as little material as possible, and at the same time looking light.

 

The roof structure utilizes a single curved cylindrical shell in the two parts of the L-shape and a double curved shell at the intersection. The shell is divided into quadrilateral meshes, which can be viewed as an arrangement of bays and arches connected by longitudinal bars.

 

　　It is conceivable that there is little ability for the bays to act together. As shown in the figure below, the deformation is very large at point a, where the concentrated force is applied, but the deformation rapidly decreases at the neighboring points b and c. The deformation at point a is very large, but the deformation at point c is very small.

　For this reason, Schleich employs tie-down spacers at regular intervals to reinforce the cylinder shell.

 

The bars are bolted to the intermediate cylindrical nodes and orthogonal ties connect the diagonal nodes of the mesh to increase the mesh stiffness. The ties not only increase the stiffness but also reduce the bending moments that the bars are subjected to.

Steel beams are placed at the bottom of the mesh shell to transfer the loads as evenly as possible to the original building below.

 

　Development of a special class of mesh shells

　　Roof for the Multihalle (multi-purpose hall), Mannheim, Germany/1970-1975

　The shape of the Mannheim multifunctional was found by means of the counter-hanging method. The maximum span of the structure is approximately 60 m x 80 m. If the total amount of timber in the structure is spread equally over the area of the shell, the height does not exceed 4 cm. scaled to the span, this thickness is thinner than an egg shell.

 

　　The construction was carried out as follows: the timber strips were first laid out in a horizontal square grid, the nodes of which were not tightly connected at the nodes by means of pins through adjustable holes to ensure that rotation could take place between the strips. Several points of the grid were then jacked upwards until they took on the designed shape.

 

　　The final form transformed the original square grid into a diamond shape compared to the initial form. At the same time, the stiffness of the single 5cmx5cm batten was small enough to allow for large enough bending deformations to occur. After the design shape was achieved, the edges of the shell needed to be trimmed; and additional cross ties were added to the diamond shaped grid to make the soft grid a strong structure.

 

 

In 2000, Otto and collaborated with Shigeru Ban on the Japan Pavilion at the Hanover World Expo, which utilized a mesh-shell structure similar to that of the Mannheim Hall, except that the timber of the roof grid was replaced with paper rolls.

Soliday forum Cafe, Paris was built with GFRP in 2011. The GFRP pipes with a rod diameter of 42mm and a wall thickness of 3.5mm were now laid on the ground and subsequently finished in just a few days with two cranes.