## 1 Introduction  
Most of the current design code [1; 2; 3; 4] formulations for predicting the torsional response of reinforced concrete (RC) members are based on the space-truss model [5] and thin-walled tube theory [6]. However, due to the complex nature of the response of RC members to torque, these theories have undergone several developments and modifications, including the introduction of variable angle truss and bending phenomenon, compatibility equations, and softening of the concrete strut [7]. These developments are based on experimental observations and were introduced with the intent to develop a more uniform, rational, and easy-to-use behavior evaluation model. Torsion, which is a complicated three-dimensional problem, involves the equilibrium and compatibility of the whole 3D member [8]. Several design parameters, such as dimensions of the member, details of transverse and longitudinal reinforcement, and the strength of concrete, affect the torsional behavior of RC members. Extensive research studies have previously been carried out to investigate the effects of these variables on the torsional response of RC members. [PERSON] [9] presented an extensive experimental investigation of RC members under torsion with normal compressive strength of concrete. [PERSON] and [PERSON] [10] investigated the effect of stirrups on the torque capacity of RC beams and developed an empirical equation for estimating the stirrup contribution to torque capacity. [PERSON] and [PERSON] [11] experimentally investigated the effect of the aspect ratio and amount of reinforcement on torsional strength of RC beams, and showed that the torsional strength decreases with an increase in aspect ratio. [PERSON] and [PERSON] [12] and [PERSON] and [PERSON] [13] proposed compatibility aided analytical models capable of predicting the torsional behavior of RC members. [PERSON] and [PERSON] [14] and [PERSON] and [PERSON] [15] experimentally studied the effect of high-strength concrete on the torsional behavior of RC beams. The studies showed that the high-strength concrete beams had higher torsional strength and cracking stiffness than the normal strength concrete beams. [PERSON] et al. [16] experimentally evaluated the torsional behavior of RC beams with minimum torsional reinforcement. They found that the post-cracking reserve strength in specimens with relatively low amounts of torsional reinforcement is primarily related to the transverse and longitudinal reinforcement ratio, in addition to the total amount of torsional reinforcement. [PERSON] and [PERSON] [17] tested sixteen hollow high strength beams subjected to torsion and demonstrated that some design codes could lead to brittle torsional failure. [PERSON] et al. [18] performed experiments to study the behavior of ultra-high performance concrete RC beams under torsion with a compressive strength greater than 150 MPa. [PERSON] [19] and [PERSON] and [PERSON] [20] investigated the influence of the volume of stirrups, the location of longitudinal steel bars, and rectangular spiral reinforcement on the behavior of RC beams under torsion. A parametric study on torsional behavior of high strength concrete using nonlinear finite element analysis was also reported [21]. Recently, [PERSON] et al. [22] examined the effect of maximum torsional reinforcement limitations specified in widely used design codes. Despite these research efforts, a rational and easy-to-use torsional evaluation model has yet to be developed. This is due to (a) the complexities associated with the torsional behavior of RC members and (b) their testing limitations. Previous research has demonstrated that artificial neural networks (ANNs) can be an alternative approach for modeling of RC members under complex loading conditions. Several researchers have successfully employed ANNs to analyze the structures under different loading conditions and verified the accuracy of the approach by comparing it to the existing analytical methods.