Florian Förster of Buro Happold describes the basic principles of cardboard structural design As structural design with cardboard and paper products is not yet codified, the designer relies not only on empirical knowledge, project specific tests and the understanding of first principles of engineering but also on his willingness to take extra design responsibility.
As a result, cardboard allows the designer to pursue structures that are not based on precedent and go beyond conventional building structures.
Cardboard and paper products are available in a variety of standard forms, mainly manufactured for the packaging industry. The following cardboard products are available.
Tubes - manufactured by rolling multiple layers of spirally wound paper plies over a spindle. The layers are glued together by starch or PVA.The tube wall thickness depends on the number of plies but can range up to 16mm.
Tube diameters up to 600mm are commonly available.
Panels - manufactured by laminating sheets of paper or particles for solid boards.
Honeycomb boards can be made by pressing paper pulp into a honeycomb mould and then sandwiching the honeycomb structure between sheets of paper: by gluing multiple sheets of paper together and pulling them apart or by gluing two halves of moulded honeycomb panels together.A number of Land T-shaped and RHS sections are also available in cardboard.
Like other structural materials cardboard is best used in forms that exploit its inherent strength and material behaviour. Due to the manufacturing process cardboard is an anisotropic material, hence the material strength varies greatly depending on the direction of the stresses. It is most efficiently used to transfer axial and in-plane stresses only, a point which should be kept in mind when deciding the structural form and load path.
Columns - axially loaded columns can be designed from cardboard tubes.Load-bearing columns are generally of a large diameter and the ratio between the tube wall thickness and diameter is high.Hence tubes tend to fail locally in buckling.Overall buckling of the tubes is less likely due to the low slenderness ratio of the sections.
Beams - can be designed using sheets of honeycomb cardboard or sections. The support conditions of beams need to be considered carefully to avoid stress concentration and minimise shear deflection and shear creep.The use of tubes as beam elements is not recommended; their bending capacity is low as the outer surface layer is not continuous.
Walls - flat panels can be used for the design of walls, either load-bearing, self-supporting or mounted to a primary frame. In all cases the stiffness of the wall and its performance under lateral loads are critical.Stiffness can be enhanced by stiffeners, cross walls or by designing the wall as a folded plate. If the panels are mounted on a primary frame, the cardboard acts as a cladding material.
Buro Happold has established a number of tentative design parameters for cardboard.
They are based on project-specific tests and particular products.As there are no general structural requirements and standards for cardboard products, the parameters need to be reassessed prior to each new project.
lMaterial properties of cardboard:
Cardboard tubes Tensile/compressive strength 8.1 N/mm 2Design tensile/compressive strength taking account of creep effects and a factor of safety (FOS) of 10: 0.8 N/mm 2E-value: 1,000-1,500 N/mm 220mm-thick honeycomb sheets Bending strength: 6.9 N/mm 2Design tensile/ compressive strength taking account of creep effects and a FOS of 10: 0.6 N/mm 2E-value: 1,000 N/mm 2All values relate to a stress direction parallel to the surface.Stresses perpendicular to the surface have not been tested.