Nonlinear dynamic analysis has shown shear wall design forces may be amplified by inherent wall overstrength and by apparent higher-mode effects. ACI 318-19 therefore increases earthquake shear forces for design. In some cases, the design shear force will be more than double the design shear from previous codes.
After the 1994 Northridge earthquake in California’s San Fernando Valley, many research institutions focused years of study on the seismic behaviour of precast concrete diaphragms. Study findings were used to inform changes in ASCE standards and ACI Committee 550 developed the following two new ACI standards:
- ACI 550.5-18, Code Requirements for the Design of Precast Concrete Diaphragms for Earthquake Motions; and
- ACI 550.4-18, Qualification of Precast Concrete Diaphragm Connections and Reinforcement at Joints for Earthquake Loading.
The new standards entail significant increases in seismic design forces and contain new requirements for the design and detailing of precast concrete diaphragms, particularly the connections between precast elements. ACI Committee 318 adopts by reference both ACI 550.5-18 and ACI 550.4-18.
The Chile earthquake of 2010 and earthquakes that took place in Christchurch, New Zealand, in 2010 and 2011 provided further examples of structural response to seismic forces. Observed behaviour of structural walls, as well as laboratory tests, showed unconfined volumetric areas of concrete can loosen and fall during seismic events, leading the structurally compromised walls to buckle and fail. This observation led to new detailing requirements for boundary elements of special structural walls. (Boundary elements, typically occurring around wall edges, corners, or openings, provide longitudinal or transverse reinforcement to confine concrete and provide longitudinal bar support.) Whereas previous standards permitted the use of crossties with 90-degree hooks at one end, ACI 318-19 specifies all crossties for special boundary elements now must have 135-degree hooks at both ends. New provisions also:
- limit the aspect ratio of hoops in the boundary element;
- restrict the locations of lap splices near intended plastic hinge zones; and
- require some walls to satisfy minimum longitudinal reinforcement requirements to avoid brittle fracture of under-reinforced walls.
Inspection and certification
All inspection requirements are now covered in Section 26.13, “Inspection,” of ACI 318-19.
This unification includes the relocation of anchor inspection requirements from Chapter 17, “Anchoring to Concrete.” ACI 318-19 also identifies qualification training programs for inspectors/installers and lists certification requirements. All inspectors are required to be certified if an appropriate certification program is available. Whenever an ACI certification program is cited in the commentary, a reference to that program has been included. Uniform resource locators (URLs) for the programs are included in the commentary reference list to allow a code user to review the certification program in detail to see what tasks are covered. By stating certification requirements directly in the code and linking to training programs, the process becomes easier for engineers to navigate.
Jack P. Moehle, PhD, PE, is the chair of the American Concrete Institute (ACI) 318 Building Code Committee and is the Ed and Diane Wilson professor of Structural Engineering in the department of Civil and Environmental Engineering at University of California, Berkeley (UC Berkeley). He has played a leading role in the development of building codes and professional engineering guidelines on subjects related to reinforced concrete and earthquake engineering. He is a Fellow of the American Concrete Institute (ACI), Structural Engineering Institute (SEI) of American Society of Civil Engineers (ASCE), and the Structural Engineers Association of California (SEAOC), and is an elected member of the U.S. National Academy of Engineering (NAE). He can be reached via e-mail at email@example.com.