## 1 Introduction  
Among different bridge typologies, steel truss bridges are widely diffused because of intrinsic steel properties as well as time- and cost-savings during the construction process [1]. It is widely acknowledged among practitioners and academicians that steel structures allow people to design slender structures resulting in low structural weight and economic cost [2]. Aiming to exploit the performance of both steel and concrete, the use of composite joints in steel truss bridges has emerged, which may show some advantages over conventional practices [3]. In particular, this specific bridge typology is composed of a steel truss system whose elements are connected by composite steel-concrete joints aiming to enhance fatigue strength during the cyclic loads caused by traffic [4] as an efficient solution to cover large spans and preserve low structural complexity during the assembly and erection stage. In this way, the resulting structure presents an improved structural behavior while maintaining the advantages of a steel bridge [5]. Currently, many countries in Europe have bridges with a lifespan of more than 50 years, and the connections of these structures are usually one of the most critical aspects due to fatigue phenomena caused by excessive cyclic loads or changes in the vehicle's weight [6; 7; 8; 9]. In such cases, the proposed retrofitting approach of connections can be very useful in improving the overall strength of the existing bridges [10]. Specifically, the method consists of encasing the steel connections of the steel truss structure with concrete. In this way, the interaction between concrete and steel leads to a new behavior and can guarantee the required safety level for variable traffic loads. The implementation process of the strengthening of the connection starts with the preliminary design of the joints, followed by the concrete mix design, and finally molding and injecting the concrete into the connections [11]. The efficiency of the retrofitting approach has been demonstrated by [PERSON] et al. [12] and [PERSON] et al. [13] adopting the Deshenglu Bridge (China) as case study for the researches. A sample of the composite connection type of the bridge was reproduced in the laboratory and fatigue tests were performed aiming to detect the level of damage under cyclic loads. Moreover, analytical and numerical models were developed by the authors showing good accordance with the experimental results. Other experiments, conducted on composite tubular K-joints of steel bridges, confirmed the beneficial effect of concrete encasing to preserve steel connections under fatigue phenomena [14]. One of the most famous real-scale applications of this strengthening approach can be observed on the Minpu Bridge in Shanghai (China). It is a road and railway steel cable-stayed bridge with a total length of almost 1208 m and a main span of 708 m [15]. It is realized with a truss steel deck and stiffened by concrete joints [16; 17]. Laboratory simulations and an experimental campaign conducted by other authors will be discussed in the following to better understand the global behavior of the joint and the role played by concrete encasing. A new methodology that emerged in the last few years is the use of post-tensioned metal straps (PTMSs) [18; 19]. This practice consists of placing steel belts/straps around the concrete and prestressing them with an initial tensile force [20]. This approach has been widely adopted for civil buildings where the performance of the post-tensioned metal strips was first investigated [21; 22]. The optimal placement and the areas to be retrofitted by adopting this approach can be detected through numerical and finite element analyses as well as laboratory tests [23]. Several authors focused on investigating the mechanical performance of this specific retrofitting system. [PERSON] et al. [24] took the first steps in introducing prestressed steel belts.