## 1 Introduction  
Recent computational advances have led to a substantial improvement in numerical modeling of reinforced concrete structures. In this context, multiscale numerical strategies [1; 2; 3] are becoming widely extended as they help us better understand relevant physical phenomena that may take place at small scales of observation but evidenced at the macroscopic scale. Examples of this are concrete structures, whose structural performance depends on certain material properties linked to lower scales [4]. For instance, its mechanical properties are strongly dependent on microstructural features (e.g., degree of hydration, sieve curve, water to cement ratio), and their influence can be directly observed on the structural performance of concrete elements [5; 6]. Therefore, it seems that advanced modeling techniques are useful for understanding the subjacent principles that govern their structural behavior, and thus, improve structural designs [7]. Multiscale approaches have been successfully implemented in a number of concrete applications such as material characterization [8; 9; 10] or structural evaluation in ordinary components, e.g., beams [11; 12] or L-shaped slabs [13; 14]. Nevertheless, its application in large concrete structures (e.g., bridges, pavements) is more recent, see for instance references [15; 16]. It is within this new scenario that we present an application of multiscale analysis in large concrete structures, namely the numerical evaluation of the shear strength of dry joints between two segments in a post-tensioned concrete box girder. Despite the fact that the shear capacity of the joint is crucial in the ultimate load of a bridge, formulations of different design codes cannot predict shear strength adequately [17; 18; 19; 20]. This is due to the fact that these do not consider the dependence of the damage behavior on the number of keys [17]. Moreover, different studies [17; 20; 21; 22] suggest that the American Association of State Highway Transportation Officials (AASHTO) code formula [23] underestimates the shear strength of multiple keys joints because the keys fail sequentially and cannot completely develop their capacity. In this work we analyze, from a numerical point of view, the influence of the introduction of steel fibers on the shear capacity and ductility of the joints. It is well known that the addition of fibers to concrete structures increases their ductility [24]; therefore, it can alleviate the problem of sequential failure while increases the capacity of the joint. For this purpose, we propose a multiscale methodology based on reference [12], analyzing the effect of fiber volume fraction, as well as prestressing values and number of keys. Two scales are covered in this work to evaluate the structural performance of dry joints, which are described next. The lowest scale of observation is the so-called mesoscale (i.e., \(\sim\)1 mm). At this scale, we use the lattice-particle model developed by the authors [12], which accounts for mixing information such as water to cement (w/c) and aggregate to cement (a/c) ratios; sieve curve; fiber size, shape, orientation, and volume content; and mechanical properties such as elastic moduli or tensile strength of the different phases. Therefore, once the material information is known, it is possible to determine macroscopic properties that will be used at the structural level. The highest scale of observation is the macroscale (i.e., \(\sim\)1 m). At this scale we use the continuum damage plasticity (CDP) model [25], which is currently available in the commercial software Abaqus [26]. This type of models usually requires a large number of input parameters, which may not be always at hand. Although some of the data may be found in the literature, commonly from experimental tests, some information could be missing. In order to overcome the partial lack of the material information, an extensive virtual testing campaign, based on the material model, is proposed herein.