Development of Bundled Reinforcing Steel

In the construction of reinforced concrete structures, it is sometimes advantageous or even necessary to place reinforcement in bundles. Bundling may be required because of restrictions on member dimensions. Bundling of bars may result in narrower, more graceful members, or allow for easier placement and vibration of concrete.

Current codes and design recommendations allow as many as four bars to be placed in a group or bundle. There are provisions for increasing development length based on the size of the bundle, but in genral there is little guidance to aid the designer using bar bundles. The objective of this study was to provide an understanding of the bond mechanics of bundles, and to provide test data for improving development and detailing requirements for bundles bars.

Tests were conducted on two, three, and four bar bundles in an effort to understand behavior and evaluate existing codes and specifications. Analysis of a spectrum of bundle sizes provided a broad foundation for the investigation of behavior. It also aided in identifying those parameters that have the greatest influence on bond strength. Tests included in the program on individual bars having an area equivalent to the two- and four-bar bundles were to test the concept of an "equivalent bar." The equivalent diameter has been introduced in odes for use in calculations for spacing cover based on bar diameter.