LIGHTWEIGHT novel structural skins
Opinion article by Marijke Mollaert, Lars De Laet
Lightweight structures form an increasing part of the built and urban environment. They are found in a broad range of structural typologies, from low-tech and low-cost developments to high-tech and sophisticated structural systems.
The design of tensile surface structures starts with the form finding of a pre-tensioned equilibrium shape. Pre-tension can be introduced by means of air-inflation or by stressing the surface. Typical forms are conical, saddle or wave forms. Forms can be combined and appear in all kinds of variation. There is a tendency towards almost flat surfaces, which has consequences on the deflection under loading. It is important to consider in the design the supporting structure as well as the required anchorage.
Examples of lightweight structures are shelters, large-scale membrane canopies like stadium roofs, façades, shading elements, mobile structures etc. They all have in common that they use their material optimal by being designed and conceived in an ingenious way. Their architectural design and engineering cannot be separated in different problems, but should be solved and investigated in an integrated approach.
The design and engineering of lightweight structures is a discipline with many challenges and opportunities. The research and innovation of these structures is fed by evolutions and progress in related scientific disciplines and incorporates for example new computational design methods, new materials, numerical tools and modern computer driven manufacturing processes. New advanced material models, more powerful numerical analysis tools, more accurate and sophisticated experimental equipment, but also new social and environmental needs, trigger and push further the challenging research on novel architectural lightweight structures. The development of a specific Eurocode for the design and structural analysis of membrane structures is a way to harmonize with more conventional structures and to ensure that the lightweight technology becomes a solid structural typology.
The following sample research topics illustrate the search for innovative techniques as well as the widening of the possible applications: (1) the use of lightweight structural concepts for emergency shelters for disaster relief, (2) the design of tensairity arches, (3) the design and analysis of kinematic form-active structures, (4) the use of flexible and structural stay-in-place formwork for curved concrete shells and (5) the specification of wind loading.
Lightweight structural concepts for emergency shelters for disaster relief
Some projects have a real social relevance. In the framework of the European project ‘S(p)eedkits’ (Project number: 284931) a simple ‘Clever roof’ was designed with as main objective to be fast and easily deployed. The function of this ‘Clever roof’ is to provide cover against heat and rain. It will be the first shelter product arriving on site after a disaster has stricken. Later this roof can be upgraded to a family shelter by adding walls and increasing the internal comfort of the shelter. A successful field test was performed in Senegal.
Inflatable structures have been used in civil engineering projects for several decades. Their light weight and flexibility make these structures an excellent building component for engineers and architects. However, a setback for these inflatable structures is their limited load bearing capacity. To increase this, a new concept has been developed, called Tensairity. Tensairity is a combination of slender struts, cables and an air beam under low pressure. The load bearing behaviour of Tensairity arches can be verified through experiments and numerical simulations. If the results of a numerical simulation correspond well with the experimental investigation, further improvements of the structural performance can be verified by numerical analysis.
Investigating Kinematic Form Active Structures
In search for new and innovative applications for fabric structures, the idea of Kinematic Form Active Structures (KFAS) was raised. These structures, combining the transformability of kinematic structures and the low self-weight of fabric structures, can for instance be used as light, adaptable façade elements. Currently, this research focuses on proving the viability of Kinematic Form Active Structures by investigating foldable fabric units.
Next to investigating the kinematics of these systems, important questions were raised during the development of KFAS with regards to the material properties of technical textiles. A thorough knowledge of the complex material properties is crucial to design stable kinematic fabric structures. Therefore, cruciform fabric samples are loaded under various warp-to-weft load ratios. The tests show varying material stiffness properties and crimp interchange depending on the load ratios as well as important creep/relaxation effects, confirming the high complexity of the material behaviour. However, until now, simplified linear elastic material models are used. Incorporating a more appropriate material model in the structural analysis can yield more reliable results, resulting in the design of more efficient lightweight fabric structures.
Wind pressure data
Membrane structures are subject to the natural elements and must be designed to resist external loads. As stipulated in the European Design Guide for Tensile Surface Structures and other recent international publications, accurate wind load data on doubly curved lightweight structures is still not available in standards or prescriptive documents.
The codes on wind design give upper bound values for conventional building typologies and the level of uncertainty increases if the building configuration deviates from the codified shapes. Where data is missing, experiments and tests are needed, be it wind tunnel tests or numerical CFD simulations.
Structural stay-in-place formwork for thin concrete shells
The construction of concrete shells remains a challenge in high wage economies, even though they have optimal structural behaviour and material use. On the one hand the current formwork methods used in practise are labour intensive and/or material wasting. On the other hand the shaping and placing of the rigid steel reinforcement is labour intensive, practically limiting the curvature. The combination of both flexible formwork and reinforcement by using a structural stay-in-place formwork solution in cement composite is a promising technology. The composite can consist of inorganic phosphate cement (IPC) matrix in combination with E-glass fibre mats, shortly called GFTR-IPC. As long as the IPC matrix is still wet, the composite retains the flexibility of the fibre mats and can be shaped onto any mould with a curved shape. When the IPC matrix is hardened, the impregnated mat becomes a thin cement composite formwork with sufficient rigidity to cast concrete on it. After hardening of the concrete, the stay-in-place formwork gets an additional structural function and acts as a (partial) tensile reinforcement of the final concrete structure.
The main benefits of lightweight structures are the design flexibility, aesthetic appeal, the endless possibilities in form and appearance, structural efficiency, appropriateness to cover large spans, the potential in the domain of renovation, convenient prefabrication and fast erection.
However, these structures still have some limitations, like their poor acoustic and thermal performance as well as the limited lifespan of the structural fabrics.
Further, improved strategies with respect to recycling and insulation, energy harvesting on large surfaces and the integration of ‘green’ roofs and walls are new trends to be explored.
Current applications are just a tip of the iceberg of possibilities.