Effect of Tripping Prefabricated Rebar Assemblies on Bar Spacing

Prefabrication of steel reinforcement (rebar) assemblies can reduce construction costs and time while improving fabrication quality and safety. However, changes in rebar spacing (i.e., shifting of rebar from their original tied positions) can occur during the transportation and final placement of these assemblies, potentially leading to violations of code-required spacing tolerances and changes in structural behavior. The primary objective of this paper is to investigate these spacing changes. Many factors and conditions can contribute to the shifting of rebar during the transportation and final placement of prefabricated rebar assemblies. Specifically, this paper describes the effect of tripping of horizontally prefabricated rebar assemblies on rebar spacing, focusing on heavy construction in nuclear safety-related reinforced concrete (RC) shear walls. Two-dimensional rebar mats and three-dimensional rebar cages are prefabricated horizontally at grade by professional ironworkers, laterally translated, and tripped to a vertical position to simulate the construction sequence based on the results from an industry survey. Relative movements between the bars are measured using the photographic measurement technique digital image correlation and compared to code-required tolerances. As a main finding, the largest movements are measured in horizontal bars directly involved in lateral translation and tripping. Additionally, it is observed that diagonal bracing is more effective in restricting bar movement than is increasing the percentage of tied intersections. Number of rebar layers, type of transverse reinforcement, and rebar cage thickness are found to have no significant effect on bar spacing changes. As a major contribution to construction engineering practice, this is the first study to investigate the movement of rebar in prefabricated rebar assemblies during tripping and to make quality control recommendations for these assemblies.