Statement of Problem
Rebar cages are the skeleton of reinforced concrete components and are commonly used in the building construction, especially in deep foundation of high-rise buildings and bridges. Deep foundations in many types of buildings and civil works utilize Cast-in-Drilled Hole (CIDH) piles and/or slurry wall foundations (SWF). The CIDH and SWF cages are usually the largest and heaviest cages on the job site. In the absence of any formal engineering guide documents to ensure the stability of these rebar cages, their collapse during construction is an imminent danger. Potential collapse of rebar cages can lead to critical safety hazard for construction crews, litigation, construction schedule delays, and thus, excessive cost and losses that are otherwise preventable. The industry currently lacks proper engineering design and detailing procedures to safeguard the stability of the rebar cages in the construction stage. This is a critical shortcoming that motivates this study.
Based on the current state of practice, rebar cages are usually built by tying the longitudinal and the transverse reinforcement bars using black annealed steel tie wire connections. Without any engineering procedure to determine the strength, stiffness, and detailing requirements, the rebar cages are usually built based on the practical experience and knowledge of steel fabricators using a “rule of thumb” process. While the process may perceive to be adequate, the increase in size (diameter and length) of the rebar cages (as for example in CIDH and SWF cages), and the increasingly more complicated construction site handling process can increase the risk of unpredictable rebar cage collapse. Failure and collapse of rebar cages has had a history of past occurrence in the bridge constructions and the likelihood of collapse is undoubtedly higher for large CIDH and SWF cages. This is an important hazard threatening the construction practice. The industry currently lacks proper engineering knowledge to design and fabricate the rebar cages to ensure their stability in various construction stages in order to mitigate the risk of collapse and the associated damage and cost. This is the main objective of this research study.
With proper engineering insight to analyze rebar cages behavior, the fabrication of large rebar cages can benefit from innovative techniques to improve their stability. The use of mechanical connectors (e.g., U-bolts and wire rope connectors) at critical location on the rebar cage can enhance their stiffness and strength. This can provide an innovative solution to save fabrication complexity, material cost, and improve the stability and safety of large CIDH, SWF and large above grade column rebar cages. Disseminating engineering knowledge and procedures to use mechanical connectors in the fabrication of rebar cages is another objective of this study.
Figure 1. Mechanical connectors
Figure 2. Wire rope connectors
Rebar cages are rather complex structures composed of interconnected elements with flexible connections and subjected to different combinations of internal forces during various stages of construction (lifting, tilting, picking, and placement). Providing engineering guidance for fabrication, detailing and handling requires a thorough understanding of the mechanics-based behavior of the rebar cage system under different loading scenarios. Recent investigation by Itani et al. has revealed that the current construction practice of rebar cages using various type of tie wire connections has low strength and stiffness. Two full scale cage tests performed at the University of Nevada, Reno has demonstrated the vulnerability of the current practice. Figure below shows one of the rebar cages and its failure mode. Following up on experimental tests, nonlinear computational models, calibrated using the experimental results, were used to conduct parametric analyses on rebar cages. Based on this investigation, engineering guidelines and design procedures were established to improve the stability of fixed base column rebar cages. This study has provided useful information to analyze and design rebar cages in the entire construction industry.
The past studies were focused on above grade column rebar cages. Extending the engineering knowledge for stability analysis and design of large CIDH and SWF rebar cages necessitates additional experimental and analytical research. To date, no systematic study has been performed to provide design guidelines for fabrication and site handling of these large rebar cages. The application of innovative mechanical connectors, which can improve the stability of these rebar cages, is another gap in the current state of knowledge that this proposal will address.
Figure 3. Rebar cage tested to failure at UNR Large Scale Structures Lab.
Figure 4. Rebar cage tested to failure at UNR Large Scale Structures Lab.
Cast-in-Drilled Hole (CIDH) piles have gained popularity in high rise building and bridges as deep foundation since they reduce the required number of piles and simplify the details of the pile cap. Moreover, deep foundations in many types of buildings and civil works utilize slurry wall foundations (SWF) to allow for top down construction techniques, reducing the construction time. The CIDH and SWF cages are relatively longer and heavier than above grade rebar cages and present additional challenges in handling them at construction sites. Similar to any column rebar cage, the CIDH/SWF cage is most often prefabricated on site or in a steel fabricator shop and shipped to construction sites. At the site, the cage is rigged to be lifted, tilted, and set in place. Figure 5 shows the three construction handling stages of a CIDH rebar cage, which include lifting from the horizontal, tilting, and setting in place vertically. Each of these stages represents a different loading condition and has distinct structural demands on the cage. During the lifting phase, the stability of the cage depends on the internal braces and tie wire connections to the template hoops and pick-up bars. The distribution of the forces among the braces is expected to be equal. During the tilting phase, the distribution of the forces among the braces is not equal and will represent the extreme loading condition on the cage. Significant bending of the cage at this stage may serve as a visual indicator of the overall stability of the cage. The last phase is cage setting. At this stage, the internal braces do not affect the overall response of the cage while the pick-up bars and the tie wire connections are responsible for the cage stability. Any interruption in the load path or sudden loss of the stiffness in the cage during these phases will result in cage failure. The consequence of any cage failure will have a detrimental effect on project cost, schedule, and unfortunately, may lead to injuries or death. Therefore, it is important to better understand the distribution of forces and stiffness in CIDH/SWF cages under the three loading conditions. Lifting, tilting, and setting cages cannot be based on experience, guess work and ‘feeling’ but it should be based on engineering principles, analytical procedures, and specifications. Currently, the knowledge on behavior of and the load path in CIDH/SWF rebar cages during various loading condition do not exist. A recent innovation in construction of rebar cages has been the use of mechanical connectors including U-bolts and plates with threaded rods at the template hoops that are spaced every 10 to 15 ft along the length of the cage. These mechanical connections replace the use of tie wire connections at the template hoops and reduce or eliminate the need for internal bracing. It is expected that using these mechanical connections at strategic locations (template hoops) along the rebar cage will increase its stiffness and thus, increase the construction productivity, improve safety, and reduce cost.
Figure 5. Construction handling of rebar cage.
The proposed research will examine the behavior of CIDH/SWF cages using mechanical connectors (U-bolts, threaded rod with plate, and wire rope connectors) to better understand the stability of such cages during the three loading conditions. The research will build on the knowledge and the information that was gained during the investigation of the stability of above grade column rebar cages. Analytical and experimental investigation will be conducted to develop and understand the behavior of CIDH/SWF and large column rebar cage stability during construction. The objectives of the proposed work are:
1. Establish rapid assessment and safety evaluation for CIDH/SWF rebar cages.
2. Develop a procedure to predict the distribution of the internal forces in CIDH/SWF cages during all phases of construction.
3. Establish guidelines and better practices for constructing and handling CIDH/SWF and large column rebar cages.
4. Publishing values for various mechanical connector strength.