Where nonlinear analysis is used to verify the seismic design, ASCE/SEI 7 (ASCE, 2022), Chapter 16, stipulates that the structural actions in members, connections, and other components should be designated as either “force-controlled” or “deformation-controlled.” Force-controlled actions are required to be designed with sufficient strength to stay essentially elastic under the combined earthquake and gravity loading. Deformation-controlled components are required to be modeled inelastically, where the deformations should be checked to confirm that they are within the allowable range of modeling—that is, that the inelastic response is reliably captured by the nonlinear analysis model. ASCE/SEI 7 further stipulates that the component actions be designated as either “ordinary” or “critical,” depending on the consequences that failure of the component action may have. Based on this designation, ASCE/SEI 7 stipulates demand and capacity factors, similar to load and resistance factors, for evaluation of the component actions that are intended to help ensure a sufficient reliability in the design. The tables in this section stipulate the default designations for the specified system types. Exceptions to these requirements may be taken, provided that the engineer of record provides evidence to substantiate the exceptions, which is subject to review and acceptance by the authority having jurisdiction.
Commentary Sections 1.5 and 1.6 provide guidance on component modelling.
1.7.1 Special Moment Frames (SMF)
In special moment frames (SMF) utilizing seismic capacity design principles, most of the inelastic behavior is expected to occur through flexural and/or mechanical hinging of the beams and limited shear yielding in the panel zone. Limited yielding is also possible in the columns and column bases. Further guidelines on modeling steel moment frames are included in the NIST documents (2017a, 2017b).
1.7.2 Special Concentrically Braced Frames (SCBF)
In special concentrically braced frames (SCBF) utilizing seismic capacity design principles, most of the inelastic behavior is expected to occur through tension yielding and compression buckling of the braces. Limited yielding is also possible in the beams and column bases. The flexural action for brace connections can be designated as either force- or deformation-controlled as indicated in Table A-1.7.2. The modeling of the connection should be consistent with this designation.
1.7.3 Eccentrically Braced Frames (EBF)
In eccentrically braced frames (EBF) utilizing seismic capacity design principles, most of the inelastic behavior is expected to occur through flexural or shear yielding of the links. Limited yielding is also possible in the beams outside the links and at column bases.
1.7.4 Buckling-Restrained Braced Frames (BRBF)
In buckling-restrained braced frames (BRBF) utilizing seismic capacity design principles, most of the inelastic behavior is expected to occur through yielding of the braces in tension and compression. Limited yielding is also possible in the beams and at column bases.
1.7.5 Special Plate Shear Walls (SPSW)
In special plate shear walls (SPSW) utilizing seismic capacity design principles, most of the inelastic behavior is expected to occur through tension yielding of the web plates in conjunction with flexural hinging of the horizontal boundary elements (HBE); limited yielding is also permitted in the vertical boundary element (VBE) panel zones and at VBE bases. Those actions are therefore classified as deformation controlled. Due to the consequences of column failure, VBE flexural deformation at the base is designated as a critical action.
Consistent with the expected system behavior described here, other actions on SPSW are categorized as force controlled. For some of these, failure is considered to compromise the gravity-load-carrying capacity of the element, and these are therefore designated as critical. Critical force-controlled actions include shear in HBE and VBE and axial and shear actions at the VBE base. VBE subjected to large, sustained gravity loading have significantly greater vertical deformation due to ductility demands; thus, axial action in such members is a critical force-controlled action, while it is ordinary for VBE with a lower gravity load.
Nonlinear analysis is permitted as an alternative to the capacity-design requirements for VBE. Analysis should confirm that the anticipated mechanism forms in SPSW and that VBE are not subjected to inelastic action other than limited panel-zone and base flexural yielding.
Commentary Section F5.5c addresses system proportioning to prevent in-span hinges of HBE. Nonlinear analysis should confirm that HBE hinges form effectively at the connection to the VBE, or through inelastic deformation of the panel zone.
1.7.6 Composite Plate Shear Walls—Concrete Filled (C-PSW/CF)
In composite plate shear walls—concrete filled (C-PSW/CF), the intent is for most of the inelastic behavior to occur through flexural yielding and hinging at the base of the composite wall. In coupled C-PSW/CF utilizing seismic capacity design principles, the intent is for most of the inelastic behavior to occur through flexural hinging at both ends of the coupling beams and flexural yielding at the base of composite walls.
Given this anticipated inelastic behavior, wall axial and flexural actions and beam hinge flexural actions are designated as deformation-controlled. Walls are designated as critical due to the potential loss of gravity load-carrying capability should the limits be exceeded. Beams are designated as ordinary because loss of gravity load-carrying capability is low.
Wall and beam shear and wall-to-foundation connections are intended to stay in the elastic range resulting in their critical, force-controlled designation.
1.7.7 Gravity Framing Systems
In general, gravity framing systems should be modeled in the nonlinear analysis when they are expected to significantly affect the overall drift response of the structure and the distribution of forces in the structural members. To the extent that the gravity system is generally expected to improve the building response, it may be permitted to exclude the gravity system in the analysis. However, in cases where the gravity system may negatively influence the performance, it should be modeled. This may include situations where (a) the gravity framing introduces irregularities in plan or elevation; (b) the gravity framing provides incidental outrigger action, which can increase forces in the gravity and/or lateral framing system; and (c) other cases.
Whether or not the gravity framing is modeled in the analysis, it is important to confirm that the gravity framing has sufficient ductility to maintain its strength under the earthquake story drift demands. Connections that are nominally designed for gravity loads can be designed as deformation-controlled with respect to flexural deformations, provided that they can maintain their required shear strength under the imposed deformations from earthquakes. This may require that certain components of the connection be either detailed to accommodate the imposed deformations (e.g., using slotted bolt holes for shear tab connections) or, alternatively, that any nonductile failure modes (e.g., bolt shear or tension failure, weld failure) be prevented through capacity design (e.g., sizing bolts such that bolt bearing or plate yielding controls and avoids bolt failure and sizing welds such that plate yielding controls the design).