## 1 Introduction  
Reinforced concrete (RC) structures often incorporate linear members for which the torsional effects can be critical for designing or for assessing the actual cross-sectional resistance. This situation is very common in bridges and building structures which incorporate beams and columns under high eccentric loadings, or any structural system whose equilibrium depends on reinforced concrete elements resisting torsion. For the assessment of the capacity of such members, an accurate estimation of the torsional capacity in their critical cross-sections is usually required. To do so, structural designers often apply the provisions from codes of practice, which incorporate simplified models and semi-empiricalequations, allowing to estimate the resistance to internal forces for a given RC cross-section. As far as torsion is concerned, and unlike what is observed for the bending capacity, recent studies have reported that current codes of practice can still fail to estimate the torsional capacity of RC beams. This conclusion arises from comparing the estimates from codes and the results from experiments [1; 2; 3]. For RC beams with somewhat high reinforcement ratios, unsafe estimates for the torsional capacity were also reported. This can explain why structural failures related to primary torsional effects still continue to be reported in the literature [4]. The aforementioned problem has been well-known by the technical community for a long time. For this reason, efforts have been made in the past decades to propose alternative and more accurate models for RC members under torsion. One such model is the skew-bending theory, proposed by [PERSON] in 1968 [5] from the observation of the experimental torsional failure pattern in tested RC beams, and further developed by other authors [6]. For common and small RC rectangular cross-sections, simple equations were derived from this model to compute the torsional capacity, which were incorporated in some reference codes of practice, namely the American building code prior to 1995 [7] and the current Eurasian code [8]. Nowadays, most of current reference codes of practice for RC structures (such as the American, Canadian and European codes [9; 10; 11; 12]) base their provision rules for torsion on the space truss analogy. This more rational model was proposed by [PERSON] in 1929 [13] and further refined by other authors mainly in the second half of the last century [14; 15; 16; 17] based on the results of extensive experimental programs. When compared with the skew-bending theory, the space truss analogy provides simpler equations for the design of RC beams under torsion and for members with a much wider range of geometrical cross-sections. With the aim of obtaining simple rules for design to be incorporated in the provisions from codes of practice, the original equations from both the skew-bending theory and space truss analogy have been simplified by using some empirical hypotheses. As a result, codes of practice incorporate many semi-empirical equations, which can still fail to provide always accurate and safe estimates for the torsional capacity of RC beams, as referred before. Hence, more refined or alternative models for RC beams under torsion are still needed and have been proposed in the past two decades. Among the refined models proposed in the literature, some of them are still based on the space truss analogy. Some of these refined models allow to predict very well the torsional capacity of RC beams with a wide range of design attributes and torsional loadings [18; 19; 20; 21; 22; 23]. Alternative and more advanced analytical or numerical reliable models, including models based on the finite element method, have also been proposed [24; 25; 26]. However, many of these models need advanced calculation procedures and demanding computational efforts just to compute the actual torsional capacity of RC beams. Hence, they cannot be easy to use by most of practitioners.