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Basis of Design
OMF designed in accordance with these provisions are expected to provide minimal inelastic deformation capacity in their members and connections.
Member Requirements
There are no limitations on width-to-thickness ratios of members for OMF beyond those in the Specification. There are no requirements for stability bracing of beams or joints in OMF, beyond those in the Specification. Structural steel beams in OMF are permitted to be composite with a reinforced concrete slab to resist gravity loads.
Connections
There are two type of beam-to-column connections as fully restrained (FR) and partially restrained (PR) moment connections.
FR. Moment Connections
FR moment connections that are part of the seismic force-resisting system (SFRS) shall satisfy at least one of the following requirements:
(a) FR moment connections shall be designed for a required flexural strength that is equal to the expected beam flexural strength, RyMp, multiplied by 1.1 and divided by αs, where αs = LRFD-ASD force level adjustment factor = 1.0 for LRFD and 1.5 for ASD.
The required shear strength of the connection, Vu or Va, as applicable, shall be determined using the capacity-limited seismic load effect. The capacity-limited horizontal seismic load effect, Ecl, shall be determined as follows:
(b) FR moment connections shall be designed for a required flexural strength and a required shear strength equal to the maximum moment and corresponding shear that can be transferred to the connection by the system, including the effects of material overstrength and strain hardening.
(c) FR moment connections between wide-flange beams and the flange of wideflange columns shall either satisfy the requirements of Section E2.6 or E3.6, or shall meet the following requirements:
(1) All welds at the beam-to-column connection shall satisfy the requirements of Chapter 3 of ANSI/AISC 358.
(2) Beam flanges shall be connected to column flanges using complete-jointpenetration groove welds.
(3) The shape of weld access holes shall be in accordance with clause 6.11.1.2 of AWS D1.8/D1.8M. Weld access hole quality requirements shall be in accordance with clause 6.11.2 of AWS D1.8/D1.8M.
(4) Continuity plates shall satisfy the requirements of Section E3.6f.
(5) The beam web shall be connected to the column flange using either a CJP groove weld extending between weld access holes, or using a bolted single plate shear connection designed for the required shear strength given in Section E1.6b(a).
For options (a) and (b) in Section E1.6b, continuity plates shall be provided as required by Specification Sections J10.1, J10.2 and J10.3. The bending moment used to check for continuity plates shall be the same bending moment used to design the beam-to-column connection; in other words, 1.1RyMn/αs or the maximum moment that can be transferred to the connection by the system.
PR. Moment Connections
PR moment connections shall satisfy the following requirements:
(a) Connections shall be designed for the maximum moment and shear from the applicable load combinations as described in Sections B2 and B3.
(b) The stiffness, strength and deformation capacity of PR moment connections shall be considered in the design, including the effect on overall frame stability.
(c) The nominal flexural strength of the connection, Mn,PR, shall be no less than 50% of Mp of the connected beam.
Exception: For one-story structures, Mn,PR shall be no less than 50% of Mp of the connected column.
(d) Vu or Va, as applicable, shall be determined per Section E1.6b(a) with Mp in Equation E1-1 taken as Mn,PR.