|“Less is more”
- Mies van der Rohe
|“More with less”
Applied to architecture and structure, the former is primarily an aesthetic position; the latter is a principle of economy.
Designing lightweight space frames requires rigorous discipline following a few basic rules.
- No bending
- Symmetry and geodesics
- Hierarchical systems for the transfer of loads
- Remember – God is in the construction details
Take a bamboo cane, and with your hands, subject it to tension, compression and bending. This very simple experiment demonstrates that bending is an indirect inefficient transfer of forces, as compared with the direct-action (vectors) of tension and compression.
Remember, in any structure that is capable of transmitting forces, tension (pulling) and compression (pushing) always coexist! (It is impossible to imagine one without the other).
A truss, which is an assembly of triangulated struts and ties, is much more efficient and lighter than a beam on girder.
A space frame is a three-dimensional truss, transferring forces in an axial manner. Designing highly efficient space frames requires a clear differentiation between these two states of stress, using hollow or angle steel or aluminium sections for the struts (bones) and wires or cables for the ties (ligaments).
Prestressing is a particular effective way of achieving lightness as it enables undesirable compression stress to be converted into tensile stress! The bicycle wheel is a classic example.
Symmetry and Geodesics
The optimal solution of the enclosure of space is the spherical geodesic dome (the sphere encompasses the given volume with a minimum of surface). Increasing the size of this structure will increase its efficiency!
Imagine the straight edges of the 5 regular Platonic solids and the 13 Archimedean solids expanded into curved edges, as if each polyhedron has been blown up like a balloon. These curved edges will form a grid of great circles arcs, i.e. geodesics (the shortest line between two points on a spherical surface is an arc of a great circle). Each geodesic polygon can be divided again into smaller geodesic polygons, i.e. triangles. The number of the subdivision of the polygon edge define its frequency.
Buckminster Fuller recognised that a pattern of great circles arcs with a 5-fold symmetry, based on the icosahedron, is the best solution to the problem he set himself, i.e. how to enclose space with a structure that requires the least amount of material. His iconic design for the U.S. Pavillion, Expo ‘67, Montreal, Canada, is a double grid geodesic dome, 76 meter diameter, with a 5-fold symmetry. The outer triangular grid has a 16-frequency.
Geodesic domes with a 5-fold symmetry are difficult to aggregate in two directions, since the geometry along the cross-section where they join lack order and is messy. This problem can be neatly solved in designing domes with 3-fold, 4-fold or 6-fold symmetries.
Hierarchical systems for the transfer of loads
The strategy in designing lightweight space frames is straightforward. Think of curvatures that are synclastic and anticlastic with “geodesic” triangles. Choose a geometry with symmetrical patterns.
Employ hierarchical systems for the transfer of loads – it is Nature’s way”! Differentiate between the primary structure, its secondary structure and the tertiary structure. Size their elements accordingly. Where it is not absolutely essential – remove inactive material in the structure. Employ elements that act mainly in axial tension or compression as against those acting in bending. Tensile ties are much more economical than compression struts, especially when the former are prestressed!
A minimal space frame can be defined mathematically!
At any chosen cross-sectional area of the structural element, the fraction-stress under maximum load divided by the permissible stress of the material – is equal to one.
God is in the construction details
“God is in the details”
Mies van der Rohe
Indeed, good construction details are the mark of a healthy construction. The Victorian engineers were masters in construction details, made from wrought-iron. The structural forms they used, like trusses, arches and portal frames were primarily one-directional load carrying arrangements. Space frames carry loads in a two-way action. This makes the design of the connection a challenging problem. There are numerous space frame connectors on the market. They are usually made from steel.
The most successful and also conceptually satisfying , is the MERO connectors. It was invented by the engineer and good businessman Max Mengeringhausen (1903-1988). The connector has the form of a rhombicuboctahedron (polyhedron with 26 faces). It is an ingenious spatial nut and bolt fixing. An 18-way joint at angles of 45 degrees. This did put a constraint on the possible space frame configurations that could be built with these connectors.
However, being able to vary the angles between the struts and ties and their lengths in the MERO system via computer numerical controlled (CNC) processing, any imaginable configuration can be built. The MERO connectors come in a range of different sizes (the MERO system was used for the iconic Eden Project in Cornwall). For low-tech DIY space frames my deceptively simple universal connector is hard to beat!