## 2 Multiscale Analysis of Fiber-Reinforced Concrete Structures  
In this section we present the multiscale framework proposed for the analysis of steel fiber-reinforced concrete structures. This methodology has been successfully used in the design of cementitious composites, paying special attention to the improvement of the fracture properties of the resulting material [12; 27]. In this work, we follow the same procedure for the analysis of steel fiber-reinforced concrete (SFRC) structures. Namely, we make use of a mesoscale lattice-particle model introduced in [27], which allows the introduction of fibers and extracts the material properties required by higher-scale models, such as that of the structure. Therefore, the structural behavior is characterized by means of finite element (FE) simulations whose material laws are based on lower-scale simulations. Material Modeling of Steel Fiber-Reinforced Concrete (SFRC): Fiber-Reinforced Lattice-Particle Model  
At the mesoscale, SFRC presents three well-distinguished materials phases: mortar, aggregates, and steel fibers. Moreover, in a close view, we can also identify two additional interfaces: mortar-aggregate, known as the interface transition zone (ITZ), which is a highly porous intermediate layer enclosing the aggregates; and fiber-matrix. Material properties such as the elastic modulus and tensile strength,or geometrical features such as the size or shape of these phases play an important role in the global response of SFRC. The lattice-particle model used in this work, which is described in detail in [27], accounts for this type of information. The generation of virtual SFRC mesostructures can be accomplished by different means: computational models including relevant mixing information [9; 10; 28], simulations of the casting process [29], or digitalization of actual mesostructures via X-ray computed tomography [30]. In this work, we use computational models to generate virtual mesostructures resembling actual ones using input parameters such as the sieve curve, water to cement (w/c) ratio, or fiber size and shape. In the first place, coarse aggregates are placed within the specimen using the take-and-place method [31], following a Fuller's parabola for the sieve curve. The shape of the aggregates is assumed to be spherical, although other shapes are currently possible (e.g., convex polyhedra). Once the aggregates are correctly placed (no overlap is possible), we continue introducing the fibers, which are modeled as 1D elements with a specific length (L\({}_{\ell}\)) and diameter (d\({}_{\ell}\)), until a target volume fraction (V\({}_{\ell}\)) is met. Fibers can be trimmed at the boundaries of the specimen. One important feature of fiber generation is the possibility to define a misorientation angle (\(\theta\)), which controls the maximum misorientation angle of the fiber with respect to a specific casting direction, although the maximum misorientation observed experimentally is up to \(\theta=40^{\circ}\)[32]. This allows accounting for different orientation levels, ranging from perfectly oriented to totally misoriented fibers with respect to the casting direction (Figure 1). In the present formulation of the fiber-reinforced lattice-particle model, three different types of elements are defined, namely: matrix, fiber, and bond. Matrix elements account for the mechanical interactions that take place between aggregates, while the fiber elements accounts for the mechanical behavior of the fiber itself. The bond element represents the fiber-matrix interaction. The nodes of the matrix elements correspond to the centroids of the aggregates and the interactions are identified using Delaunay's triangulation.