SP-208 With the addition of the new Appendix A on strut-and-tie models in ACI 318-02, "Building Code Requirements for Structural Concrete," engineers have an alternative code procedure for designing structural concrete members where the usual sectional design assumptions for flexure and shear do not apply. ACI SP-208 contains selected papers presented at the 2002 ACI Fall Convention by members of ACI Subcommittee 445-A, Shear and Torsion: Strut-and-Tie Models. This publication shows engineers how to apply strut- and-tie modeling in accordance with Appendix A. The papers also trace the development of Appendix A of ACI 318-02 and summarize important tests that confirm strut-and-tie modeling as a rational basis for the design of structural concrete. Most of the examples have been taken from practice. In addition to explaining the approach of determining a model, they point out where problems can occur in dimensioning or in detailing and anchoring of the reinforcement and how the design can be improved.

In this example, application of the new strut-and-tie modeling provisions of ACI 318-02 to the design of a wall with openings is summarized. Because the openings constitute a significant portion of the wall, earlier Code versions provide little relevant guidance fro ensuring that the wall provides adequate resistance to the applied loads. Previous examples fo the application of strut-and-tie models (STM's) to multiple load cases and/or lateral loads are rare. The wall in this example is designed to resist multiple combinations of both gravity and in-plane lateral loads. Construction of the STM for each load eombination is outlined. In addition, employment of statically indeterminate STM's to improve the efficiency and serviceability of the wall design is discussed. The example also covers selection and anchorage of tie reinforcement, as well as capacity checks for struts and nodal zones.

After a brief summary of the contents of the SP and the examples, several general points are discussed which are based on observations made about the examples. The choice of a strut-and-tie model is a major issue and different engineers may propose various modles for the same task. This leads to a discussion of the uniqueness of models and whether it is acceptable that different engineers may choose different models and thus different reinforcement arrangements and detailing for the same D-region. A further issue identified in some of the examples was the transition of a B-region to a D-region, and the procedure of modeling is shown. Finally the role and the importance of detailing is emphasized and some examples for this are given. Also some observations are made which led to recommendations for reconsidering some code provisions.

Strut-and-tie models make the design of portions of complex structures transparent. This example, a pier table from a cable stayed bridge, is developed to show how strut-and-tie modeling can be used for an area that may be exposed to cyclic loading and how the results from alternate loads may be superimposed upon one another. The pier table transmits forces from the pylon, through an integral superstructure connection, to individual support legs. The pier table also creates an area for the transmission of superstructure forces. This example briefly describes the model development based upon the perceived flow of forces within the structure. The tie reinforcement is then detailed and the nodal zones checked.

This paper documents the decisions made by ACI Committee 3 18 to introduce strut-and-tie models into the 2002 ACI Code. Sections 3 and 4 of this paper review code statements concerning the layout of strut-and-tie models for design. The format and values of the effective compression strength of struts are presented in Sec. 5. 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 same unfactored loads. The final selection of design values of the effective compression strength considered test results, design values from 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 in Sec. 6 of the paper. The description of the geometry of nodal zones in code language proved difficult. The design of ties is described in Sec. 7 of this paper and requirements for nominal reinforcement are in Sec. 8. Nominal reinforcement is provided to add ductility, to improve the possibility of redistribution of internal forces, and to control cracks at service loads.