# Improving the Structural Behavior of Tied-Arch Bridges by Doubling the Set of Hangers
## 1 Introduction  
In tied-arch bridges with a single arch, the most common method of linking the arch and the deck is by suspending the deck from the arch by means of a single set of cables, pinned at both ends, anchored to the centerline. This configuration, when properly designed, can successfully help to meet the strength and stiffness requirements of a bridge, provided that the acting live loads are transversally symmetric, i.e., when the resultant live loads are contained within the plane of the arch, as described by [PERSON] [1] or [PERSON] [2]. For example, in addition to the classical vertical hangers arrangement (Figure 1a), the arch and the deck can be connected by means of a Nielsen-type arrangement (with V-shaped cables, resembling the diagonals of a truss), which, as it is known ([PERSON] [3]), significantly reduces both deflections and forces within the vertical plane (Figure 1b). Further reductions of the arch's bending moments in the vertical plane can be obtained by refining the arch elevation so that it is as close as possible to the antifunicular geometry, as described by [PERSON] [4]. However, the cable arrangement contained within a central vertical plane has virtually no capacity to help the bridge meet certain serviceability or failure criteria under nonsymmetric loads. i.e., not contained within the plane of the arch, such as eccentric live loads, wind loads ([PERSON] [5]), or dynamic and seismic loadings, as described by [PERSON] et al. [6] or [PERSON] [7]. This happens, for example, for vertical deflectionsat the edge of the deck or out-of-plane buckling of the arch, as shown by [PERSON] [8] and [PERSON] [9] and detailed in Section 7. Of course, these problems can be solved by increasing the size of the arch or the deck, as studied by [PERSON] [3]. If the deck has a hollow-box cross-section, which is very common in bridges with a central hanger arrangement, the torsional stiffness can be increased by enlarging the area enclosed by the median line (i.e., its size). However, this is not always possible, e.g., due to clearance restrictions under the deck. Alternatively, the torsional stiffness of the cross-section can be increased by using thicker steel plates at the perimeter of the cross-section. Whatever the solution, it implies an increase in the cost of the bridge. The cost increase can be very high since resisting equilibrium torsion is extremely expensive, as it happens in some bridges designed by [PERSON], where the equilibrium torsion is resisted by an eccentric torsion bar located at one edge of the deck, such as in the La Devesa footbridge (see [PERSON] [10] and [PERSON] [11]). Another and more drastic approach to these problems is by duplicating the arches and placing each arch on an edge of the deck, a solution which also implies a significant increase in cost and, in addition, leads to a substantial alteration of both the structural behavior and the aesthetic appearance of the bridge. Another possible strategy is to use stiffened hangers, such as those shown in the bridge in Figure 2, which have the capacity to support shear and bending forces in addition to axial forces. This solution has been studied in detail by [PERSON] and [PERSON] [12,13] and involves additional structural systems (such as out-of-plane bending and torsional stiffness both of the arch and the deck) to help limit forces and deflections. Finally, another possible alternative within the designer's possibilities, consists, simply, of doubling the set of hangers and attaching the arch to both edges of the deck (Figure 3b) by means of two lateral sets of hangers. This strategy is the focus of the research shown in this paper. Thus, the objective of this paper is to study how a cable arrangement, composed of two sets of lateral hangers, that links the arch to the two edges of the deck can be used by the designer as a strategy to satisfy serviceability and failure requirements in a tied-arch bridge under eccentric loads, i.e., whose resultant is not contained in the plane of the arch.