Building codes and standards could seem confusing when you just cannot find a provision that you know had to be there. In those times, the solution is to go back to the basics and see how the code provisions are laid out. Let’s illustrate that by using the following question we received recently.

Q: I’m reviewing code requirements for intermediate precast tilt-ups and can’t seem to find a reference to how to design the shear walls for shear and flexure. ACI 318-19 Section 18.5 focuses on the precast connection and Section 18.10 is specific to special walls; shouldn’t there be a reference from 18.5 to applicable sections in 18.10 or other chapters for the in-plane shear and flexure design? Any assistance would be appreciated.

A: The answer to your question can be found in ACI 318-19 Section 18.2.1.2, which states:

• 18.2.1.2. All members shall satisfy Chapters 1 to 17 and 19 to 26. Structures assigned to SDC B, C, D, E, or F also shall satisfy 18.2.1.3 through 18.2.1.7, as applicable. Where Chapter 18 conflicts with other chapters of this Code, Chapter 18 shall govern.

In other words, Chapters 1 through 17 and 19 through 26 provide minimum requirements that all structures need to meet, while Chapter 18 provides seismic requirements that apply in addition to or in place of those minimum requirements. So, when Chapter 18 is silent on any aspect of design, we need to look into those other nonseismic chapters for relevant provisions. In this case, flexure and shear design of all precast tilt-up shear walls are addressed in Chapter 11, Walls. Provisions are included either within Chapter 11 or by reference to other chapters of ACI 318-19 like Chapters 22 and 23. These provisions are deemed adequate even for seismic design of intermediate precast walls, and as a result, no additional requirements for flexure or shear design are included in Section 18.5. The only seismic requirements contained in Section 18.5 are related to the connections between wall panels, or between wall panels and the foundation, which supersede the requirements in Section 11.2.2.

Note, like all other seismic force-resisting systems, intermediate precast structural walls also need to conform to some of the requirements contained in ACI 318-19 Sections 18.2.2 through 18.2.14 depending on the seismic design category the structure is assigned to, as stipulated in Sections 18.2.1.3, 18.2.1.4 and 18.2.1.5..