F1.1 Scope
Ordinary concentrically braced frames (OCBF) have minimal design requirements compared to other braced frame systems. The Provisions assume that the applicable building code significantly restricts the permitted use of OCBF and specifies a low factor so that ductility demands will be low. Specifically, it is assumed that the restrictions given in ASCE/SEI 7 (ASCE, 2022) govern the use of the structural system.
Section F1.7, Ordinary Concentrically Braced Frames Above Seismic Isolation Systems, has been removed from the Provisions. This section provided a few alterations to OCBF requirements for OCBF above seismic isolation systems. Although base isolation is expected to improve the performance of the system, it was decided that there is not sufficient research on OCBF above seismic isolation systems to warrant these alterations.
F1.2 Basis of Design
OCBF are not expected to be subjected to large inelastic demands due to the relatively low factor assigned to the system in ASCE/SEI 7.
F1.3 Analysis
Due to the expected limited inelastic demands on OCBF, an elastic analysis is considered sufficient when supplemented with use of the overstrength seismic load as required by these Provisions.
F1.4 System Requirements
F1.4a V-Braced and Inverted V-Braced Frames
In V- and inverted V-braced frames, bracing can induce a high unbalanced force in the intersecting beam. Unlike the special concentrically braced frame (SCBF) provisions, which require that the beams at the intersections of such braces be designed for the expected strength of the braces to prevent a plastic hinge mechanism in the beam, the corresponding OCBF provisions permit the beam design on the basis of the maximum force that can be developed by the system. This relief of OCBF acknowledges that, unlike SCBF, the beam forces in an OCBF frame at the time of an imminent system failure mode could be less critical than those due to the expected strength of the connection braces. See Commentary Section F2.6c.1 for techniques that may be used to determine the maximum force developed by the system.
F1.4b K-Braced Frames
K-bracing can have very poor post-elastic performance. After brace buckling, the action of the brace in tension induces large flexural forces on the column, possibly leading to buckling. No adequate design procedures addressing the high-consequence stability issues are available.
F1.4c Multi-Tiered Braced Frames
A detailed description of the characteristics of multi-tiered braced frames is provided in the commentary for SCBF. Due to the reduced level of ductility required for a multi-tiered ordinary concentrically braced frame (MT-OCBF) as compared to a multi-tiered SCBF (MT-SCBF) (from ASCE/SEI 7, Table 12.2-1, for OCBF, 3.25 versus for SCBF, ), a simpler set of design requirements is provided for the MT-OCBF. In this approach, the basis of the design is an elastic analysis of the frame with an of 3.25 . This seismic design force level is used for the braces only. The connections, struts, and columns are designed for seismic forces increased by a factor of 3 to make these elements more robust. This corresponds to 1.5 times the overstrength seismic loads-in other words, to an value equal to which is approximately equivalent to force levels associated with elastic response. Such higher required strength for the connections, columns, and struts aims at ensuring that these elements can resist the maximum forces imparted by the braces. Failure of connections or struts may induce large unbalanced horizontal loads on the columns. This, in turn, may endanger the frame integrity because intermediate tier levels are not connected to other lateral load-resisting elements of the structure. For the columns, the amplified design load is an indirect, simpler means of providing the columns with sufficient strength to resist in-plane flexural demands resulting from nonuniform brace forces and deformations in adjacent tiers. The benefits of designing the struts and strut connections to torsionally brace the columns of the multi-tiered braced frame were demonstrated by research (Stoakes and Fahnstock, 2013) and are incorporated into these provisions also. The columns must also resist the effects of out-of-plane forces due to the mass of the structure and supported items in combination with the forces obtained from the analyses required in this section. While the consideration of forces in multiple directions and from multiple sources is a design requirement for all seismic force-resisting systems, it can be particularly critical for multi-tiered braced frame systems.
For the special case of tension-only bracing proportioned such that the controlling slenderness ratio of each brace is 200 or more, it is recognized that the columns, struts, and connections are not prone to problems associated with compression buckling of the brace because these braces have little overstrength from compression or flexural strength. Horizontal unbalanced brace loads due to brace buckling are also small. As a result, the design requirements for the brace connections, columns, and struts revert to the basic requirements for an OCBF frame. However, because the frame is not connected at every tier level to the other seismic force-resisting elements in the building, no diaphragm is present at the intermediate tier levels to help distribute loads to other seismic force-resisting systems, thus, there is a potential for progressive yielding in multi-tier frames that results in flexural demand on the columns in the plane of the frame. As a result, the column is checked for in-plane bending due to the calculated difference in shear strength between tier levels. As a minimum, this force level is prescribed to be 5% of the larger shear capacity of the tier above and below that tier level. This minimum force level is intended to also capture potential differences in brace strength due to material yield strength variability. These potential in-plane force and bending demands can be shared with additional columns by appropriately connecting these additional columns to the braced frame at each tier level. It is noted that this same requirement is not applied to MT-OCBF frames with tension-compression bracing (controlling slenderness ratio of each brace less than 200) because these columns are already penalized by using the higher effective load amplification factor of 3 for these frames.
F1.5 Members
F1.5a Basic Requirements
Only moderate ductility is expected of OCBF. Accordingly, braces, except tension-only braces with slenderness ratios greater than 200, are required to be moderately ductile members.
F1.5b Slenderness
In V- and inverted V-braced frames, braces with large slenderness ratios are not permitted. This restriction is intended to limit the unbalanced forces that develop in framing members after brace buckling; see Commentary Section F2.4c.
F1.5c Beams
In past versions of the Provisions, it was assumed that beams and their connections would be treated as collectors, and thus beams would be required to be designed for the overstrength seismic load in accordance with ASCE/SEI 7. This requirement was specifically added to the 2016 Provisions (AISC, 2016) to provide greater clarity.
F1.6 Connections
F1.6a Brace Connections
Brace connections are designed for forces corresponding to the overstrength seismic load with exceptions that allow for the force to be limited to the expected brace strength. The intent is to ensure that brace yielding or buckling occurs prior to failure of a connection limit state. Net section rupture of the member is to be included with connection limit states. Allowing the required strength of a brace connection to correspond to the overstrength seismic load is considered appropriate for systems designed for limited ductility.
The Provisions permit that bolt slip be designed for a lower force level than is required for other limit states when oversized holes are used in accordance with Section D2.2(c), Exception (1). This reflects the fact that bolt slip does not constitute connection failure and that the associated energy dissipation can serve to reduce seismic response. Other limit states, such as bolt shear, bolt bearing, and bolt tearout, are required to be designed for the overstrength seismic load subject to the exceptions discussed previously.