Derivation of strut-and-tie models for the 2002 ACI Code

This paper details the decisions made by ACI Committee 318 to introduce strut-and-tie models into the 2002 ACI Code. Code statements concerning the layout of strut-and-tie models for design are reviewed. The format and values of the effective compression strength of struts are presented. The first step was to derive an effective compression strength which gave the same cross-sectional area and strength using Appendix A as required by another code for the same concrete strength and unfactored loads. The final selection of design values of the effective compression strength considered test results, values in the literature, values from other codes, and ACI Code design strengths for similar stress situations. A similar derivation of the effective compression strengths of nodal zones is summarized. Description of the geometry of nodal zones in code language proved difficult. Tie design is described, and requirements for nominal reinforcement are given. Nominal reinforcement is provided to add ductility, improve the possibility of redistribution of internal forces, and to control cracks at service loads.