# Effects of Flexural Stiffness on Deformation Behaviour of Steel and FRP Stress-Ribbon Bridges
## 1 Introduction  
The stress-ribbon layout is one of the oldest structural systems efficient for pedestrian bridges [1]. The assemblage slenderness makes such elegant structures an intrinsic part of the modern infrastructure [1, 2, 3, 4, 5]. A slightly sagged suspension band forms a catenary shape that looks natural and aesthetically attractive. The structural system of such bridges consists of cables and ribbons and the deck slabs placed on them. Typically, spiral strands and flexible bands made of steel are used for that purpose, though the number of fibre-reinforced polymers (FRP) application examples is continuously increasing [4, 6]. The deck slabs, distributing the load to the supporting bands, often do not affect the load-bearing capacity of the bridge. Although this structural system is simple, its design is challenging. The serviceability demands limit the bridge deck slope to 8-12% [7, 8], reducing the sag-to-span ratio (\(f/l\)) to the range of 0.020-0.033. Thus, large foundations are necessary to resist tremendous tension forces acting on the ribbons.  
Another problem is related to the deformability of the stress-ribbon systems under concentrated and asymmetrical loads [1, 5, 9]--the kinematic displacement magnitude, the structural design object, can several times exceed the dead load-induced displacement component [10]. Enlarging the dead load, pre-stressing the concrete bridges, and increasing theflexural stiffness of the structural system can solve the above problem. The self-weight increase is the simplest way to stabilise the structure [1]; however, the dead load increases the thrust force acting on the ribbons. The pre-stress technologies require specialised anchorage systems and can cause a sudden bridge failure [11; 12]. [PERSON] et al. [13] revealed that the flexural-stiff suspension systems can efficiently reduce the kinematic displacements.  
This paper introduces an innovative concept of the stress-ribbon structural system with flexural-stiff ribbons. The application of hot-rolled and welded steel profiles simplifies the construction process. Furthermore, this structural system ensures the formation of lightweight decks, reducing axial forces in the ribbons. The proposed concept can be extended to the pultruded FRP profiles. However, bending stresses in the ribbons increase with increasing the flexural stiffness. Combining different materials in a composite cross-section can solve this problem, but it requires developing the connection technologies and design procedures [14; 15; 16; 17]. Thus, the optimisation of the bridge geometry and material properties becomes a crucial issue [18; 19]. Besides, conventional techniques do not apply to the design of the anisotropic material components [20].  
Numerical methods could be a powerful tool for solving a broad spectrum of engineering problems [21]. [PERSON] et al. [4] and [PERSON] et al. [5] investigated dynamics problems of the suspension bridges. [PERSON] et al. [12] and [PERSON] et al. [22] carried out numerical simulations of stress-ribbon and suspension bridges to optimise the ribbons' material parameters and geometry. However, a limited number of experimental works in the field do not determine the optimum flexural stiffness of the ribbons. The contributions by [PERSON] et al. [23], [PERSON] et al. [24], and [PERSON] et al. [25] should be mentioned as rare examples of such experimental studies.  
This study is a continuation of the researches [24; 25]. It experimentally investigates the deformation behaviour of two pedestrian steel bridge prototypes: one has flexible bands typical of such structural systems and an alternative structure with the innovative flexural-stiff ribbons. Numerical models calibrated using the test results are used for the parametric analysis of the flexural stiffness effect on the deformation behaviour of the bridge system with steel and fibre-reinforced polymer (FRP) ribbons. A practical approach to the choice of the stiffness of the ribbon-profiles is also proposed.