## 1 Introduction  
### Background  
As of today, concrete is one of the principal materials used in construction. Normal concrete, however, has some demerits (i.e., its poor ductility and tensile strength) that make it unsuited for use in advanced infrastructure [1]. The drawbacks of conventional concrete have been largely overcome through the development of steel-fiber-reinforced high-performance concrete (also known as ultra-high-performance concrete (UHPC). In the 1990s, the first class of UHPC (known as reactive powder concrete) emerged [2], and the properties of this concrete have been extensively investigated [3, 4, 5]. The structural performance of the ductility, bearing, and shear capacities of normal and high-strength concrete beams have been significantly enhanced by the inclusion of steel fibers [6, 7, 8, 9, 10]. The introduction of steel fibers in high-strength concrete (FRC) beams also enhanced their deformability, moment capacity, and flexural rigidity [11, 12]. At the material level, abundant information is available on the special rank properties of UHPC compared to normal concrete. For instance, the development of cement-hydration indexes and their relationships with the strength characteristics of UHPC were previously studied [13]. There is, however, a gap in the literature regarding the preparation of UHPC mixes that contain ternary hybrids of microsteel fibers of various aspect ratios in the presence of a binary mixture of desert sands at ambient temperatures. These sands are found in the Arabian Peninsula, which is considered the global tank for desert sands. In the literature, a single type of desert sand available in Saudi Arabia has been proposed for use in manufacturing UHPC mixes with single types of steel fibers [14]. Further, a research team in Algeria developed a UHPC mix that incorporates dune sand to strengthen a multipurpose structure [15]. In another study [16], steel fibers, macro-polypropylene fibers, and dura fibers were combined to create high-performance concrete (HPC). Additionally, the flexural, self-healing, and sound-release properties of UHPC were reported [17]. The modified Andreasen and Andersen packing model, D-optimal design, and Artificial Neural Networks have recently been employed to optimize UHPC mixtures [18]. In the last decade, studies on the mechanical behavior of UHPC beams have gradually broadened at the structural level, and full-scale bar-reinforced UHPC beams with various fiber types and reinforcement percentages were experimentally investigated [19]. Additionally, the validity of current approaches to predict the loading capacity of UHPC beams was investigated by comparison with the experimental results of small-scale specimens [20]. In practical applications, the superiority of UHPC (e.g., its high strength-to-weight, ductility, and durability properties) has contributed to the emergence of new prospects for concrete. The use of UHPC, for example, can help reduce the weight of a structure by up to 70% [21]. The source for the superior ductility of UHPC is its discontinuous fibrous system, which controls cracks and can minimize the need for continuous bar reinforcement [1]. Moreover, the use of a low water-to-binder percentage and high content of fine-constituent materials has resulted in the higher durability to UHPC, allowing this concrete to tolerate harsh environmental conditions [22]. Due to these unique properties, UHPC has been effectively employed in infrastructure (e.g., pedestrian [23; 24] and road [25; 26] bridges), buildings (e.g., [27; 28; 29]), and the retrofitting of pre-existing structures [30; 31]. Notably, encouraging findings were reported on the use of UHPC in coastal facilities [32; 33], earthquake-prone areas [34], and composite piling systems [35]. More recently, the structural responses of UHPC beams with different bar areas and fiber dosages to four-point loading tests were reported [36]. In the present literature, there is no information on the behavior of hybrid-fiber-reinforced UHPC beams with various effective depths.