Every year on construction projects across the United States, pretied reinforcing columns and piers collapse, resulting in added costs, project delays, injuries, and sometimes death.
The CRSI manual “Placing Reinforcing Bars” discusses the assembly of a pretied column cage. “A sufficient number of intersections are tied together to make a stiff preassembled cage that can be hoisted and placed as a unit. For large square or rectangular column cages, it may be necessary to provide diagonal reinforcing bar braces or wire bracing (twisted and tensioned) to stiffen the bars and to prevent the column from collapsing as it is lifted off the ground to a vertical position and set into place.”
The question arises as to what are the sufficient number of tied intersections and the adequate bracing necessary to prevent column collapse during hoisting, as well as when the pretied assembly is still awaiting concrete.
To identify the causes and provide safer solutions to the problem, a recent study, sponsored by CALTRANS and conducted by Dr. Ahmad M. Itani from the University of Nevada, Reno, found that several critical components in the construction of pretied columns need to be considered to arrive at a rigid, stable structure that can be safely hoisted and placed. See Figure 1 for three critical components to column structural stability.
In “Stability of Bridge Column Cages During Construction,” Itani examined both the design of the cages, the type of wire used for tying, and the experience of the iron worker making the ties. A total of 152 tests were made on the various ties from single-snap through wrap-and-saddle with all ties done by both experienced and inexperienced iron workers. The study found the quantity of ties used on a pretied column was not nearly as important as the location of the ties, the type of ties used, and the location of the internal bracing that would allow for stable and safe hoisting and guying of the completed cage.
Along with the column cage stability in its assembly, attention needs to be taken on how the assembly is braced once it has been hoisted into its final location, and how the bracing is released when forms are being set and concrete is being placed. This is especially true when a single crane is used for the hoisting and setting of the cage. There is a significantly higher number of column collapses occurring at this point in the construction process as guy cables are released to allow for the placement of forms, which may result in accidental loads being placed upon the cage. Column collapse also occurs when cables are released to allow for realignment of a cage that has become twisted as it was set. The other cables that are still attached to the column may not provide sufficient bracing to maintain the column’s vertical stability. It is critical for column cage stability and project safety that multiple points of support attachment to the column be maintained to provide sufficient stability.
To mitigate the potential for cage collapse, it is critical that the placing contractor, and the individuals responsible for column hoisting, have a clear and concise plan for the release sequence of any column supports. Along with development of greater column stiffness, this will reduce the potential for column cage failure and collapse.
Thanks to Dr. Ahmad M. Itani for sharing his research in the “Stability of Bridge Column Rebar Cages during Construction.” The complete report is available at http://go.hw.net/cc-rebar-cages.