Synopsis
The simulation and construction of elastic gridshells in composite materials is nowadays a technique that one can consider mastered. The bracing of the grid in its final form remains however a time consuming step with a lot of handiwork. The lack of alternative to membrane covering is also an important limit to the development of such technology. The proposed experiment tries to tackle both issues through a novel concept of hybrid structural skin made of an elastic gridshell braced with a concrete envelop. The idea is to use the gridshell as formwork for the concrete and to insure a connection between the thin concrete skin and the main grid, so that concrete assures the bracing of the grid and that the thickness of the concrete is reduced to a minimum. To demonstrate the feasibility and interest of this structural concept, a 10 m² prototype has been built.
Acknowledgement
The construction of the concrete envelop was sponsored by FibraFlex who is greatly acknowledged here. A measures campaign was also conducted in the framework of a student program at ENSG who kindly deployed its geomatic equipment to have a feedback on the geometry and mechanical behaviour of the structure.
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timelapse
watch the timelapse of the construction
3D model
visualize the 3D model of the prototype
concept
watch this trailer to understand the concept
GFRP Grid and Tchebychev Net
The single layer grid is made of 10mm composite rods assembled by 3mm screws and bolts. A plane canvas made of sewed strips of mosquito net was then set atop of the grid to deal as flexible formwork. On the boundary, small sticks were installed within sheaths to prestress the canvas in both directions and give it out-of-plane stiffness, so that it can later carry the concrete. The relative positions of the grid and canvas were then fixed at connections which are finally covered with small height gauge made with a 3D-printer.
Elastic deformation of the grid and the fabric
The grid is then deformed elastically by bending. The canvas which is fixed to the grid is thus sheared. Its initial prestress avoid the appearance of wrinkles along the compressed diagonals.
The final form of the grid is solely controlled by the positions of the members ends which have been preliminary marked on the wooden base. The ends of the main five members in each direction are thus stacked and glued in their final supports while the secondary members are fixed to metallic square by steel wires.
To reinforce the grid, three bracing cables are installed in each direction.
Fiber Reinforced Concrete Mix Design
The concrete mix used for the prototype was formulated especially for this application. Due to the reduced thickness (15 mm), reinforcement could only be done by incorporating fibers in the fresh concrete. To ease the flushing of the concrete, soft stainless metallic fibers were selected, more precisely FibraFlex® fibers FF15E0/10 type, 15 mm length, from Saint-Gobain. It had then to be easy to place on the formwork, and to remain on inclined surfaces without slipping. Therefore we sought to obtain a high shear threshold that equilibrates shear stresses on inclined surfaces and low dynamic viscosity which facilitates the placing operations. These specifications have led to the following guidelines:
- Small aggregates (diam.<4mm). The concrete is in fact a mortar.
- Water to cement ratio not too low (e/c>0.4) to minimize the dynamic viscosity.
- Incorporation of stainless steel fibers, the proportion of which was experimentally adjusted in order to obtain a satisfactory post-pic behavior in terms of ductility (0.5% mass).
- Use of admixtures which improve sticking on lost formwork and diminish the risk of early age cracking.
Remove the temporary parts of the gridshell structure
Before pouring the concrete, a small layer of quick cement was set on the canvas, in order to harden the soft formwork and to avoid significant sagging under the weight of the wet concrete. The concrete was then poured manually in two passes starting from the four supports. Once the concrete has hardened, the four temporary parts of the gridshells are cut off and removed, so that the hybrid shell can stand freely on its four supports. Finally, the concrete skin was protected with one layer of lime whitewash to increase its durability and give it its beautiful white color. Supports were also paint in blue to recall cycladic churches and also the amasing seabird named booby…
