E1.2 Basis of Design
Compared to intermediate moment frame (IMF) and special moment frame (SMF) systems, ordinary moment frames (OMF) are expected to provide only minimal levels of inelastic deformation capacity. To compensate for this lower level of ductility, OMF are designed to provide larger lateral strength than IMF and SMF and, thus, are designed using a lower factor. Systems such as OMF with high strength and low ductility have seen much less research and testing than higher ductility systems. Consequently, the design requirements for OMF are based much more on judgment than on research. Due to the limited ductility of OMF and due to the limited understanding of the seismic performance of these systems, ASCE/SEI 7 (ASCE, 2022) places significant height and other limitations on their use.
Although the design basis for OMF is to provide for minimal inelastic deformation capacity, there is no quantitative definition of the required capacity as there is for IMF and SMF systems. Despite the lack of a quantitative inelastic deformation require- ment, the overall intent of OMF design is to avoid nonductile behavior in its response to lateral load.
To provide for minimal inelastic deformation capacity—that is, to avoid nonductile behavior—the general intent of the OMF design provisions is that connection failure should not be the first significant inelastic event in the response of the frame to earthquake loading. Connection failure, in general, is one of the less ductile failure modes exhibited by structural steel frames. Thus, as lateral load is increased on an OMF, the intent is that the limit of elastic response be controlled by limit states other than connection failure, such as reaching the limiting flexural or shear strength of a beam or a column, reaching the limiting shear strength of the panel zone, reaching the axial yield strength of truss chord members at their connections to the OMF columns, etc. For higher ductility systems such as IMF and SMF, inelasticity is intended to occur in specific frame elements. For example, in SMF, inelasticity is intended to occur primarily in the form of flexural yielding of the beams. This is not the case with OMF, where the initial inelastic response is permitted to occur in any frame element.
Thus, the basic design requirement for an OMF is to provide a frame with strong connections. That is, connections should be strong enough so that significant inelastic action in response to earthquake loading occurs in frame elements rather than connections. This applies to all connections in the frame, including beam-to-column connections, truss-to-column connections, truss chord splices, truss web-to-chord connections, column splices, and column base connections. Requirements for OMF column splices and column base connections are covered in Section D2. Requirements for beam-to-column connections, truss-to-column connections, truss web-to-chord connections, and truss chord splices are covered in Sections E1.6 and E1.7.
The provisions do permit initial inelastic response to occur in beam-to-column connections for OMF provided with partially restrained (PR) moment connections. Requirements for PR moment connections are covered in Section E1.6c.
Design and detailing requirements for OMF are considerably less restrictive than for IMF and SMF. The OMF provisions are intended to cover a wide range of moment frame systems that are difficult or impossible to qualify as IMF or SMF. This includes, for example, metal building systems, knee-braced frames, moment frames where the beams and/or columns are trusses (but not special truss moment frames), moment frames where the beams and/or columns are hollow structural sections, etc.
OMF truss systems were not addressed in earlier editions of the Provisions, but recommendations were provided in the Commentary for design criteria to be used for these systems. To provide more specific direction and to incorporate these recommendations into the applicable building code, requirements for OMF systems with trusses are now included in Section E1.7.
In OMF truss systems, trusses are used as the beam elements in place of rolled or built-up shapes. The truss chords act as the top and bottom flanges and will subject the columns to localized (concentrated) transverse loads producing shear similar to panel-zone shear. These local effects are to be considered in the design of the columns.
OMF Knee-Brace Systems. Knee-brace systems use an axial brace from the beam to the column to form a moment connection. Resistance to lateral loads is by flexure of the beam and column. These systems can be designed as an OMF. The knee-brace system can be considered as analogous to a moment frame with haunch-type connections. The knee brace carries axial force only, while the beam-to-column connection carries both axial force and shear. A design approach for knee-braced systems is to design the beam-to-column connection, the braces, and the brace end connections for the forces required to develop of the beam or column, or the maximum moment that can be delivered by the system, whichever is less. is the plastic moment of the beam or column at the point of intersection with the knee brace. The column and beams should be braced out of plane, either directly or indirectly at the beam-to-column connection and at the knee brace locations, consistent with the requirements of Specification Appendix 6.
E1.4 System Requirements
Unlike SMF, there is no beam-column moment ratio (in other words, strong-column/ weak-beam) requirement for OMF. Consequently, OMF systems can be designed so that inelasticity will occur in the columns. Where inelasticity in the columns is anticipated, stiffness reductions due to inelasticity should be considered in accordance with Specification Chapter C. Explicit requirements for OMF composed of structural steel truss and columns are included in Section E1.7; however, stability should be considered for all systems where inelasticity in the columns is anticipated. Such consideration should include the need for columns to be braced out-of-plane for stability at points likely to behave inelastically, such as connections to knee-braces; such systems should be limited to one story.
E1.5 Members
There are no special restrictions or requirements on member width-to-thickness ratios or member stability bracing beyond meeting the requirements of the Specification, except as provided in Section E1.7c for OMF composed of structural steel trusses and structural steel columns. Although not required, the judicious application of width-to-thickness limits and member stability bracing requirements as specified for moderately ductile members in Section D1 would be expected to improve the performance of OMF. In addition, Commentary Section E1.2 contains recommendations for stability bracing in OMF knee-brace systems.
E1.6 Connections
For all moment frame systems designed according to these Provisions, including SMF, IMF, and OMF, the beam-to-column connections are viewed as critical elements affecting the seismic performance of the frame. For SMF and IMF systems, connection design must be based on qualification testing in accordance with Section K2, or a connection prequalified in accordance with Section K1 shall be used. For OMF, connections need not be prequalified nor qualified by testing. Rather, design of beam-to-column connections can be based on strength calculations or on prescriptive requirements. Design and detailing requirements for beam-to-column connections in OMF are provided in this section.
E1.6b FR Moment Connections
Three options are provided in this section for design of fully restrained (FR) moment connections. Designs satisfying any one of these three options are considered acceptable. Note that for all options, the required shear strength of the panel zone may be calculated from the basic code prescribed loads, with the available shear strength calculated in accordance with Specification Section J10.6. This may result in a design where initial yielding of the frame occurs in the panel zones. This is viewed as acceptable behavior due to the high ductility exhibited by panel zones.
- (a) The first option permits the connection to be designed for the flexural strength of the beam, taken as . The 1.1 factor in the equation accounts for limited strain hardening in the beam and other possible sources of overstrength. The required shear strength of the connection is calculated using the code-prescribed load combinations, where the shear force transmitted to the connection associated with the capacity-limited horizontal shear due to earthquake loading is calculated in accordance with Equation E1-1. The available strength of the connection is computed using the Specification. Note that satisfying these strength requirements may require reinforcing the connection using, for example, cover plates or haunches attached to the beam. The required flexural strength of the connection specified in this section—in other words, of the beam—should also be used when checking if continuity plates are needed in accordance with Specification Sections J10.1 through J10.3.
- (b) The second option permits design of the connection for the maximum moment and shear that can be transferred to the connection by the system. Factors that can limit the forces transferred to the connection include column yielding, panel-zone yielding, or foundation uplift. In the case of column yielding, the forces at the connection can be calculated assuming the column reaches a limiting moment of of the column. In the case of panel-zone yielding, the forces at the connection can be computed assuming the shear force in the panel zone is times the nominal shear strength given by Specification Equations J10-11 and J10-12. For frames with web-tapered members, as typically used in metal building systems, the flexural strength of the beam, rafter, or column will typically be first reached at some distance away from the connection. For such a case, the connection can be designed for the forces that will be generated when the flexural strength of a member is first reached anywhere along the length of the member. The flexural strength of the member may be controlled by local buckling or lateral-torsional buckling and can be estimated using equations for the nominal flexural strength, , in Specification Chapter F. However, lower-bound methods of determining are not appropriate, and engineers should endeavor to establish a reasonable upper bound by considering items that contribute to the stability of the beam, even those that are typically ignored for design of the beam because they are difficult to quantify, not always present, etc. It is not appropriate to use ; a realistic value of should be used. Additionally, the stabilizing effects of the deck restraining the beam both laterally and torsionally should be included in determining this upper bound. The plastic moment, , may always be used as the upper bound.
- (c) The third option for beam-to-column connections is a prescriptive option for cases where a wide-flange beam is connected to the flange of a wide-flange column. The prescriptive connection specified in the section is similar to the welded unreinforced flange-bolted web (WUF-B) connection described in FEMA 350 (FEMA, 2000a). Some of the key features of this connection include the treatment of the complete-joint-penetration (CJP) beam flange-to-column welds as demand critical, treatment of backing bars and weld tabs using the same requirements as for SMF connections, and the use of special weld access hole geometry and quality requirements. Testing has shown that connections satisfying these requirements can develop moderate levels of ductility in the beam or panel zone prior to connection failure (Han et al., 2007).
This option also permits the use of any connection in OMF that is permitted in IMF or SMF systems. Thus, any of the prequalified IMF or SMF connections in ANSI/AISC 358 (AISC, 2022b) can be used in OMF. However, when using ANSI/AISC 358 connections in an OMF, items specified in ANSI/AISC 358 that are not otherwise required in OMF systems are not required. For example, the welded unreinforced flange-welded web (WUF-W) connection, prequalified in ANSI/AISC 358, can be used for an OMF connection. However, items specified in ANSI/AISC 358 that would not be required when a WUF-W connection is used in an OMF include beam and column width-to-thickness limitations for IMF and SMF, beam stability bracing requirements for IMF or SMF, beam-column moment ratio requirements for SMF, column panel-zone shear strength requirements for IMF or SMF, or requirements for a protected zone. None of these items are required for OMF and, therefore, are not required when the WUF-W connection is used in an OMF. Similar comments apply to all connections prequalified in ANSI/AISC 358.
E1.6c PR Moment Connections
This section gives strength requirements for PR connections but does not provide complete prescriptive design requirements. PR connections are permitted to have a flexural strength that is substantially less than the connected beam or column. This will normally result in inelastic action occurring in the connection rather than in the beam or column during an earthquake. As described in Section E1.6c(b), the designer must consider the stiffness, strength, and deformation capacity of PR moment connections on the seismic performance of the frame. This may require nonlinear time-history analysis with accurate modeling of the PR connections to demonstrate satisfactory performance.
For design information on PR connections, refer to Leon (1990, 1994), Leon and Ammerman (1990), Leon and Forcier (1992), Bjorhovde et al. (1990), Hsieh and Deierlein (1991), Leon et al. (1996), and FEMA 355D (FEMA, 2000c).
E1.7 OMF Composed of Structural Steel Trusses and Structural Steel Columns
E1.7b Basis of Design
Design criteria for OMF composed of structural steel trusses and structural steel columns were discussed by Green et al. (2007) regarding the use of joists and joist girders in OMF systems. The design requirements in the Provisions are consistent with the recommendations in this reference.
The requirement for using capacity-based earthquake force levels for the design of truss members and connections was deemed appropriate, recognizing the negative consequences of failure of individual elements or connections within the truss. The intent of this requirement is that elastic behavior in the truss is maintained during a design-level earthquake. Truss systems used in OMF will commonly have flexural strengths that exceed those of the supporting columns. Therefore, these systems are limited to one-story structures because developing flexural hinges in columns for multistory structures creates overall instability of the structure. This height limit is also consistent with the limitation of OMF steel frames prescribed in ASCE/SEI 7, Chapter 12 (ASCE, 2022).
E1.7d Truss-to-Column Connections
For OMF systems with trusses used in lieu of beams, truss-to-column connections and the individual connections within the truss are to be designed for higher forces consistent with the minimum of the flexural capacity of the truss, flexural capacities of the columns, or shear capacities of the columns, to prevent inelastic, nonductile behavior from occurring in these connections. The high shear demand on the column is associated with the moment transfer from the truss connection and occurs over the height of the column between the top and bottom chord connections of the truss.
The requirement to use FR connections for truss-to-column connections recognizes that, while bending in the column between the truss chords will contribute to rotation in the joint, the chord to column joints will contribute comparatively negligible deformation at the joint.