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.

The following example illustrates use of STM's for design of a pile cap. Two load cases are considered: 1) axial load only, and 2) axial load and overturning moment. The design is based on Appendix A of ACI 318-02 Results are compared to section design procedures per ACI 318-99. Compared to section design metods, STM design is more rational and leads to a more reliable structure. Because the reinforcing bars are located above the piles, overall footing depth is increased compared to traditional design in which the bars are placed between piles.

A deep beam, supporting two concentrated loads at the top, was designed in accordance with Appendix A of the ACI 318-2002, The analysis and design using the strut and tie model were performed in an efficient and straight forward manner. The strut and tie methodology provides a framework to understand and assess the flow of forces and the resisting mechanisms. Also, it is a valuable tool for achieving proper detailing of ductile concrete members.

A single corbel projecting from a 14 in. (356 mm) square column is designed using the strut-and-tie method according to ACI 318-02 Appendix A. The corbel is to support a precast beam reaction force, Vu of 56.2 kips (250 kN) acting at 4 in,(102 mm) from the face of the column. A horizontal tensile force, Nuc of 11.2 kips (49.8 kN) is assumed to develop at the corbel top, accounting for creep and shrinkage deformations. The structure and the loads are described in Fig. (3.1-1). Normal-weight concrete with a specified compressive strength, fc of 5 ksi (34.5 MPa) is assumed. The yield strength of reinforcement, fy is taken as 60 ksi (414 MPa). The selected corbel dimensions including its bearing plate are shown in Fig. (3.1-2). The corresponding shear span to depth ratio, a/d, is 0.24. A simple strut-and-tie model shown in Fig. (3.1-3) is selected to satisfy the code requirements. The main tie reinforcement provided is 5 #4 (#13 mm) bars. These bars are welded to a structural steel angle of 31/2 in. x 31/2 in. x 1/2 in. (89 mm x 89 mm x 13 mm). The reinforcement details are shown in Fig. (3.1-5).

Inadequate design of indirect supports resulted in a lot of structural damage and near failure of structural concrete beams. Most codes, including ACI 318, do not properly cover this case. However, strut-and-tie models almost automatically lead to correctly reinforcing these critical discontinuity regions. This example combines indirect supports as well as indirectly applied loads and demonstrates the application of strut-and-tie models following Appendix A of ACI 318-2002.