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The bridges both have two decks, a 6m-wide deck which is to be used by both pedestrians and cyclists and a secondary 3m-wide glass deck reserved just for pedestrians. The footbridges are curved and are suspended only along the outside edge of the curve by hangers and suspension cables. These two bridges are believed to be the first of their kind in China and are also claimed to be unique in the world in terms of their scale, which stretches to 120m in developed length with the equally impressive 9m deck width.

GSA was used for the structural static and dynamic analyses of the footbridges and for the form-finding of the cables. An erection simulation study was also carried out using GSA, focusing on cable form-finding and pedestrian-induced vibration studies.

Because the geometry of the bridge is well defined, all the node coordinates of the model could be calculated in Excel and a system of clear node numbering established. In addition, since the node geometry was defined, the same principle was able to be applied to the elements linking two nodes. A control parameters table was created, enabling the model geometry to be updated, and a component object model interface was used to generate the parametric model and initialise the cable form as 'flat' before the form-finding process.

After generating the model, the form-finding process was started. This is a procedure which contains four main steps based on each stage of the different types of analysis available within GSA.

The first step was to find the hanger force; this process calculates and sets the vertical force that the hangers will be subjected to. In order to find those forces, vertical pinned supported 'helper-hangers' were built into the model. The 'shape' of the corresponding created node loads is a typical distribution for this kind of curved structure, one which is supported only along its external side.

Node loads were created in a particular load case and were used to carry out the second step of the procedure, which was to find the form of the suspension cables. This sets prestress loads for the edge suspension cables and the back-stay cables in order to reach the minimal target radial and tangential displacement for the top of the two inclined masts, as an inherent result of the force equilibrium at the top of the mast.

Prestress - given as prestress force loads in GSA - in the central suspension cable was one of the input parameters of the model and dictated the final hanger cable length and the form of spatial catenary at the central area of the bridge. A fast iteration process enabled the user to set the prestress loads and to reach the target displacements and force equilibrium.

The third step of the process was to find the force vector of the hangers. This was used to adjust the prestress in the hangers to achieve a minimal geometry deviation from a nominal coordinate at the lower node - constrained in the longitudinal direction - which was essential to establish the correct forces to link the suspension cables with the curved deck. An iteration process enabled the user to set the prestress loads in the hangers to reach the target displacements for the lower nodes.

As the inclination of the hangers interacted with the form of the suspension cables, the second and third steps were repeated in order to reach a higher level of precision of force equilibrium at the mast top.

The fourth and final step summarised the prestress forces calculated from the previous steps of the separate model and set them in the assembled model with both cables and decks. The prestress forces were summarised into a single load case as the form-found geometry with force flow built in. These four steps worked on different stages in the GSA model. The use of the advanced stage features of GSA was one of the key points for this form-finding process.

The final cable geometry was achieved as follows: the radial alignment of the hangers was established by repeating the form-finding process with updated initial geometry depending of the results of the iteration n-1. After three run throughs, the radial alignment of the hangers was considered to be acceptable.

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