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 within the beam, as well as how these load efects are resolved into forces acting on the selected STM. These forces should be equilibrated by the load path defned by the model.ACI 445.2 pdf download.