On March 19, 2019, with the installation of the last stabilizing cable in place, the tensioning operation of the cable network structure of the National Speed Skating Stadium was officially completed. With the joint efforts of all parties involved in the construction, the cable network structure of the National Speed Skating Complex was tensioned and shaped to a high standard, taking a solid step towards the completion of the main structure of the project and the topping out of the building on schedule.
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National Speed Skating Stadium Roof Rope Network Tensioning
The National Speed Skating Stadium is a landmark venue for the 2022 Beijing Winter Olympic Games and will serve as a competition and training venue for speed skating events during the games. Beijing Construction Institute, as the designer, has completed the preliminary design and construction drawing design phase of the whole professional design work.
The National Speed Skating Stadium is oval in plan, with a saddle-shaped curved roof and a curved curtain wall on the outside of the Temple of Heaven outlining a dynamic “ice ribbon” outline. In order to realize the complex building shape, our design team proposed a steel structure main force system consisting of roof cable network, ring truss and outer curtain wall diagonal cables, which cleverly built up the “skeleton” to support the gorgeous “coat” of the building.
In order to make this “skeleton” into an impeccable steel skeleton, the design team made calculations and analyses for tens of thousands of different working conditions, covering all possible scenarios for this complex Olympic building from construction to use. At the same time, the design team also commissioned the Space Structure Research Center of Zhejiang University to carry out structural model test research and build a “mini skeleton” with a geometric similarity ratio of 1:12 in the laboratory to verify the design results from construction feasibility to force performance to ensure that the entire engineering design is flawless.
Detailed design explanation
Inside the “skeleton”, the ring truss provides a closed elliptical boundary for the cable network and is supported on the concrete structure composed of 48 grandstand columns and ring beams; the upper end of the diagonal cable is connected to the outside of the ring truss and the lower end is connected to the concrete structure, which mainly plays the role of balancing the internal force of the cable network, improving the stability of the ring truss and providing support for the curtain wall structure. The cable network, ring truss and diagonal cable together form a set of rigid-flexible, rigid-flexible synergistic force-bearing system.
The main force-bearing system of the steel structure of the National Speed Skating Stadium: the “interplay” and “cooperation” of rigidity and flexibility
The roof cable network of the National Speed Skating Stadium consists of 49 pairs of orthogonal load-bearing cables and 30 pairs of stabilizing cables, with elliptical plan projection, the long and short axes measuring 198m and 124m respectively, forming the saddle surface required for roof modeling after tensioning.
Project difficulty 1: “Rigidization” process of the cable network
The cable network fits perfectly with the shape of the building and is the core element to create the architectural effect of the speed skating pavilion and form a stable force-bearing system. Unlike conventional rigid structures such as concrete and steel structures, the cable network is composed of flexible steel cables, which can only withstand tensile forces, not compression or bending, so the system itself does not have the ability to bear external loads. It is necessary to “stiffen” the system into a structure by applying pre-tension and establishing geometric stiffness based on the pre-tension and geometric dislocation to form the load carrying capacity. For a cable network structure with a strictly defined target shape after forming, the applied pretension must be precisely controlled, otherwise it will be difficult to achieve the intended building geometry.
Through morphological analysis, the structural design team obtained the magnitude and relative distribution of pretensions to be applied to the cable network, so that the pretensions are balanced between the target shape and the loads such as the self-weight of the structure, and the structure meets the performance requirements under various load conditions.
The construction process of the cable network can be divided into three main steps: ground assembly, overall lifting and synchronous tensioning, the first two of which have been completed in the early stage. Therefore, this tensioning operation is the most important part of the construction of the cable network structure and even the whole construction of the speed skating pavilion.
Project difficulty 2: Realization of complex cable network boundary
The complex cable network boundary formed by the ring truss and diagonal cables poses a challenge for the design and construction of the arena structure. In order to avoid the huge internal force caused by the tensioning of the cable network in the concrete structure, our design team proposed the idea of free sliding of the ring truss support horizontally during the tensioning of the cable network, and then fixing it after the tensioning is completed. This idea requires the steel structure main bearing system to work in a self-balancing state during the tensioning of the cable network, and the ring truss only transmits vertical forces to the substructure, thus reducing the burden on the concrete structure. However, due to the reduced support stiffness, the ring joist will have a non-negligible horizontal deformation during the tensioning process of the cable network, which will adversely affect the forming state of the cable network and the diagonal cable, and if not handled properly, it will be difficult to realize the expected building shape accurately.
The structural design team creatively proposed a morphological analysis method for the cable network structure considering complex boundary conditions to achieve accurate control of the forming state of the cable network and boundary structure under complex boundary conditions. Using this method, the design team obtained the tensioning and forming state of the steel structure main force system that exactly matched the target position and the target cable force, as well as the processing and positioning geometry of the cable network, ring truss and diagonal cable required to achieve this goal.
During the design, processing and installation of the main steel structure including the tensioning of the cable network, the above concept was effectively implemented to ensure the structural safety and the reasonable state of the tensioning system after completion. The construction team of Beijing Urban Construction Group Co., Ltd. and Beijing Architectural Engineering Research Institute Co., Ltd. is working on the fixing of the bearings at the site according to the weather conditions and construction schedule, so as to form a stable main bearing system for the whole venue and create working conditions for the subsequent construction.
Over the past 70 years, Beijing Construction Institute has always focused on its main business of architectural design, creating a large number of architectural masterpieces for society. We are honored to undertake the full professional design of the National Speed Skating Stadium, the only new venue in Beijing for the Winter Olympic Games. Up to now, we have been involved in the design and renovation of seven projects, including the National Speed Skating Stadium, the Capital Gymnasium Campus Renovation and Expansion, the Beijing Winter Olympic Village Talent Public Housing, the National Gymnasium Renovation and Expansion, and the National Swimming Center Renovation. As a large architectural design and consulting organization of the same age as the Republic, BJCC is, as always, doing its best to serve the 2022 Winter Olympics.