Long-span Timber Gridshells

Editorial Team, June 1 2020
Thanks for the input from Diogo Teixera (DMAA)

The challenge we faced at the onset of the project was to pool the necessary technical expertise and to gear the design development towards the architectural principles of our scheme. The particular areas of expertise include energy design and thermal performance, structural integrity and glazing, as well as drawing construction and logistics. To acquire an intimate understanding of local climate conditions, the thermal requirements inside the structures, the structural performance and availability of suitable construction resources are the essential components to achieve the overarching goal of minimizing the ecological footprint of the project. Nature is the main actor here.


Climate Control

The building envelope is optimized with regard to light transmission, solar gains and heat loss and is composed of individual modules, which are specified according to their particular position in the envelope and orientation to the sun. The degree of transparency gradually changes along the north-south axis of the building, varying from highly transparent on the south facing areas to completely opaque on the north side. Other energy design strategies include optimal building orientation, specially developed climate control systems which are integrated into the façade design, automatically controlled natural ventilation, evaporative cooling, high efficiency sensible and latent heat recovery systems, thermal rock stores and modeled topography for energy storage and biomass fueled combined heat and power plant (CHP).

Air circulation, Light radiation, Thermal Heat gaining's and storage, Water collection for plant irrigation are considered for the Taiyuan Domes.
Illustration: Energy Design Cody

3D-renderings: DMAA


Structural analysis

The largest of the three domes has a clear span of over 300 ft, making it one of the largest timber gridshells worldwide. All three parabolic gridshells comprise doubly-curved glulam beams, arranged in two or three crossing layers. The domes are glazed with doubly-curved glass with operable windows in some areas. When viewed from above the timber structures resemble seashells, with the primary members closely bunched on one end and then fanned out across the surface of the domes, driven by a desire to optimize solar gains by creating a gradient in skin transparency. The geometric generation of these domes presented a particular challenge, as they are not spheres. In order to structurally optimise the doubly curved geometry, the team had to consider all of the constraints, including daylighting, structural performance, shipping, and fabrication and pre-assembly.

A clamshell is the paragon of the structural design for the domes, illustrations: Bollinger + Grohmann


Loads, load cases and respective deformation shapes of desert
illustrations: Bollinger + Grohmann

Reaction forces on the supports
illustrations: Bollinger + Grohmann


Notable facts:
Domes range from 11 to 30 m in height;
diameters from 43 to 90 m
2381 glulam beams for the three domes
60.000 screws and steel dowels for cross connections
Typical length of glulam members:
7–8 m

Illustrations: Bollinger + Grohmann


Preparation of the Glulam Modules

The members were binned into fabrication groups based on their width and strong-axis radius so that the timber laminations could be pressed and glued on a constant radius jig, and then the final parabolic shape was milled into the curved glulam billets. C# scripts were used to automatically create BTL files, which drove two different types of 6-axis CNC machines that cut and shaped each beam at the two Glulam manufacturing facilities in Germany and Austria which produced for this project. The orientation of the beams was optimized to limit the amount of milling that was required, while still achieving the doubly curved shell geometry envisioned by the architectural design. It was critical to work closely with the glulam manufacturer at an early stage to determine what CNC processes would be possible while achieving the tight timelines for getting all material to site. In addition to milling the overall beam profiles and end connections, the CNC line predrilled each screw hole, notched the beams to align snugly at their intersection, and marked the north or east top ends of each member to help orient them during panel assembly.

At Hasslacher Norica Timber, Sachsenburg, Carinthia, Austria


© aerial-drone – stock.adobe.com

Drawing Production and Container Planning

All three domes have been parametrically generated and analyzed in Grasshopper and Karamba, allowing for iterative form finding and structural optimisation.
Similar to the steel shop drawings, control plans for the glulam fabrication and the panel assembly were generated automatically from the 3D models using custom C# and Grasshopper scripts. The arranging and packing of the shipping containers was also done in a semi-automated process. The containers were planned in the order in which they would be required on site, and aligned with the order in which the glulam beams were fabricated. This allowed the material to be efficiently organized and shipped to Taiyuan. Each beam was labelled with a series of numbers which was used to identify which container it was in, as well as the final position of the beam in its dome.

© StructureCraft


Construction

The foundations and concrete ring beams, complete with cast-in steel plates, were constructed over the course of several months prior to the arrival of the glulam. StructureCraft carpenters led the installation process working closely with SKF construction crews. Each gridshell was discretized into panels that could be pre-assembled on site or in a nearby warehouse and then trucked and craned into place. The entire footprint of each dome was filled with temorary steel scaffolding, which was primarily used to provide access to all points of the dome surface, and to provide lateral support for the panel support columns. The preassembled panels were craned into place, and set on the custom adjustable support points. After the main panels were erected, the rest of the connections were in-filled piece by piece - a process which helped to minimize errors in construction, and provided sufficient tolerance to ensure that all pieces could be accurately fit together. After completing the glulam structure, key survey points on each dome were recorded. Then the dome was de-propped and the scaffolding was removed, and then the survey points were rechecked. This process continued for the small and medium domes several times while the glazing was installed to check on any significant settlement or deformation of the glulam structure.

Acknowledgements

We thank the following partners for sharing their expertise in creating these structures: Energy Design Cody Consulting GmbH, Bollinger + Grohmann, StructureCraft, SKF Builders, Hasslacher Norica Timber, Mule Studio, One To One

Further reading:
https://www.timberdesign.org.n...

About

TAIYUAN BOTANICAL GARDEN The greenhouse represents the centrepiece of the new botanical garden. It is composed of three domes destined to accommodate plants of different climates, together reinventing the silhouette of the garden. Two of the three domes accommodate the pavilions for tropical and desert plants.

dmaa.at/work/taiyuan-botanical-garden