## 1 Introduction  
Prefabricated rebar cages are commonly used in reinforced concrete structures for both above-ground (e.g., bridge piers and columns) and below-ground (e.g., cast-in-drilled-hole piles and slurry wall foundations) construction. These temporary structures consist of longitudinal and transverse rebars usually connected using tie-wires. Despite the increasing demand for prefabricated rebar cages in the construction industry, there are currently limited guidelines on the analysis, design, fabrication, and handling of large rebar cages to ensure their stability during various stages of construction prior to casting concrete. Rebar cage fabrication and handling often rely on experience and regional practices, which increase the vulnerability of these structures and their risk of failure. Numerous incidents of rebar cage failure have been reported [1], with consequential safety hazards, project delays, liability, and added costs. While instances of rebar cage failure are numerous, many of them have not been reported in detail, likely because of legal constraints. Given the importance of this problem and the lack of engineering guidelines, a research project was defined and sponsored by industry stakeholders to provide engineered solutions and guidelines for assembling and handling rebar cages. This study presents the full-scale experimental tests and numerical modeling performed as part of that project. The most comprehensive study on the behavior of tie-wired bridge column rebar cages is perhaps the experimental-analytical investigations conducted by [PERSON] et al. [1; 2]. In these studies, over 100 tests were carried out on six types of cross-rebar tie-wire connections to determine the force-deformation response of the connection in six degreesof freedom (DOFs). Following the tie-wire connection tests, two full-scale rebar cages were tested until failure. The full-scale test results were utilized to calibrate nonlinear finite element models, which were then used for parametric studies. The outcome of the parametric studies was a set of guidelines for the design and fabrication of tie-wired rebar cages, which are summarized in the following subsection. The studies showed that failure in rebar cages is dominated by tie-wire connection failures, rendering tie-wires as the \"weak links\" in rebar cage structural systems. Recently, the authors completed a study on the application of mechanical connectors (U-bolts) for cross-bar connections [3,4]. A series of experimental tests were performed to investigate the force-deformation response of U-bolts at a connection level (see Figure 1). Based on the experimental results, idealized bilinear models were developed for U-bolt connections in three translational DOFs [3]. The study showed that U-bolt connectors can provide stronger and stiffer cross-rebar connections compared with tie-wires. It was then hypothesized that using U-bolts for cross-bar connections in rebar cages can enhance the overall stability of the rebar cage. The present study further investigates this hypothesis. This study presents the results of full-scale experimental testing conducted on underground pile-shaft rebar cages with mechanical U-bolt connectors. The primary objective and novelty of this study involve investigating the behavior of full-scale rebar cages reinforced with U-bolt connectors under common site handling conditions such as lifting and tilting. To achieve this objective, 26 tests were performed on five different rebar cages, and their deflection profiles were measured. Detailed numerical models of rebar cages were developed. The models were then calibrated and validated based on the obtained experimental results. The validated numerical model has the capacity to predict the complex response behavior of all rebar cages with U-bolt connectors under site handling conditions, as well as traditional tie-wired rebar cages with internal stiffening elements.