Tianfu Agricultural Expo
Five Timber Vaults
The Tianfu Agricultural Expo is part of a major development program in the greater metropolitan area of Chengdu, which aspires to compete with other major economic hubs in China. Located on the west border of the Sichuan Basin, where up to 5000m high mountains meet Sichuan’s great plains, the wide views of waving rice fields and mountain peaks on the horizon inspired the architect Cui Kai with the concept for the curved timber enclosures.
At over 75,000m2, the new Tianfu Agricultural Exposition is the largest timber structure in Asia, and one of the largest timber structures in the world. This series of five vaults (G1-G5) use unique Vierendeel-inspired trusses which are a hybrid of timber chords and steel webbing, achieving clear spans up to 110m and heights up to 44m.
The unique wave of the building ensemble blends gently into the landscape, but also provided challenges for the engineering team to materialize the architectural scheme into a bold project on a very tight schedule. Housing museums and displaying agricultural products from the region, the roofs of these halls are clad with ETFE but are open-ended, encouraging a direct connection with the surrounding farmland.
The result is a unique series of long-span timber structures, created through cooperation of team members on three different continents in a year and a half, delivering indeed the owner’s desire to showcase the economic power of the Chengdu agrarian region, and provide a world-class attraction through innovative engineering and design.
Structural Engineering
Early design discussions yielded the concept of an “expanding” arch with varying triangular cross sections. With a deeper, wider truss near the base, each cross section is comprised of two single bottom chords and a double top chord. The triangular push-pull nature of this design is structurally more efficient than a bending-active Vierendeel web.
The diagonal webbing integrated in each arch is structurally designed to allow slender web elements, which almost disappear and minimize the visual connection between top and bottom truss chords. The Larch double top chord is exposed to the weather, creating the need for a carefully developed weather protection and flashing concept throughout the project. On top of the truss bottom chords lie glulam purlins, supporting the exterior ETFE weather membranes which are pre-tensioned against the two neighboring arches.
All vaults have the same structural system, which consists of independent truss arches, laterally tied together by purlins and global cable bracing. Vertical and local orthogonal forces (due to wind) are transferred by the purlins to the bottom chords where they are translated into axial and bending chord forces. The steel base connections transfer the chord forces through steel embed plates into the massive concrete foundations which were designed by the local design institute.
All five buildings have a great geometrical variation of the same structural system. This fact makes it a prime application for automation and parametric generation. The center lines for the structural models were generated simultaneously with the geometry exploration in the main Grasshopper script, which allowed a direct model export for structural analysis.
This project involved the cooperation of team members on three different continents throughout the Covid-19 pandemic, delivering indeed the owner’s desire to showcase the economic power of the Chengdu agrarian region through a world-class attraction.
Fabrication
The assembly line nature of the project had a special role for efficient detailed design. Efficiency in this case was a fine balance between saving through material cost reduction and easy to install repetitive connections to ensure a smooth preassembly process.
Due to the necessity of shipping the glulam pieces from Europe to China by rail, single members were constrained by container sizes. In this case, the parametric project tools were used to optimize container packing and shipping sequence.
Once the material arrived on site, temporary fabrication facilities took over to assemble the so-called sub-assemblies. These ~11.5m long assemblies consisted of un-spliced glulam chords and associated steel webbing. Between two and four sub-assemblies were connected near the final arch location and formed the up to 35m and 25t heavy lift-assemblies. One arch consists of up to five lift-assemblies.
Installation
The construction of the superstructures started after all internal steel buildings were finished, to ensure crane access below. After exploring different erection methodologies, it was shown that the shoring towers could be supported by the steel structures below where necessary. In addition to providing temporary support, the surveyed shoring towers also played an important role during the construction process by controlling the arch geometry.
Each arch erection started with the base lift-assemblies, supported by a shoring tower at the tip. The base connections were temporarily fixed to the foundation embed plates during this phase.
After a sequence of arches were erected and tied together by purlins, most shoring towers could be lowered and reused for the next arches. Finally, after a vault completion, the LED screen cables were installed and pre-tensioned in specified load steps and order.