ACI 445.2-21 Strut-and-Tie Method Guidelines for ACI 318-19— Guide

ACI 445.2-21 Strut-and-Tie Method Guidelines for ACI 318-19— Guide. 3.3—Conceptual example 3.3.1 Introduction—This section presents an introductory discussion of the strut-and-tie method based on a concep- tual example, which is presented to introduce and illus- trate general strut-and tie modeling concepts. Subsequent chapters include details of STM application. The example structure consists of a simply-supported beam shown in Fig. 3.3.1. The self-weight is neglected for simplicity in this conceptual example. In general, several load combinations may need to be investigated for any given member or structure and each may require a diferent geometric confguration of the STM to maintain static equilibrium at the nodes. Further, the pres- ence of movable gravity loads or reversing lateral loads may necessitate the use of a diferent STM for each load pattern or combination. Given the simplicity of this example struc- ture, a single load case is sufcient for discussion. 3.3.2 Identifcation of B- and D-regions—After the applied loads and corresponding reactions are determined, the struc- ture is divided into B-regions and D-regions. The B-regions can be designed and detailed with cross-sectional strength methods described in Chapter 22 of ACI 318-19. The use of an STM as described in Chapter 23 of ACI 318-19 is also allowed in these regions. In the D-regions, conventional beam theory from Chapter 22 of ACI 318-19 that assumes a linear variation of strains over the cross section is not a reasonable basis for design. Instead, STMs are used to repre- sent idealized load paths in these D-regions. ACI 318-19 contains additional guidance on the determi- nation of D-regions for the common types of geometric or loading discontinuities depicted in Fig. 3.3.2b, along with the forces acting on the D-region boundaries. The geometric discontinuities in Fig. 3.3.2b(a) are so named because the disturbance arises from an abrupt change in the geometric shape of a member. Loading discontinuities shown in the left three cases in Fig. 3.3.2b(b) arise due to the presence of a localized application of load, including support reactions or applied concentrated loads, as is the case in this example. A corbel, as shown at right in Fig. 3.3.2b(b), is infuenced by both geometric and loading discontinuities. 3.3.3 Boundary forces and basic strut-and-tie model— The forces acting on each of the boundaries of each D-region are determined next. Figure 3.3.3 indicates the internal sectional load efects (shear force V and bending moment M) computed at the D-region boundary...