Topics covered in this section:
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Highly Ductile Members (ANSI/AISC 341-10, Chapter D, Pg.11-15 and Chapter E, Pg.24-43)
Member Requirements
According to these provisions special moment frames are expected to provide significant inelastic deformation capacity through flexural yielding of the special moment frame beams and limited yielding of column panel zones.
1. Classification of Sections for Ductility
The members defined as highly ductile members should comply with this section as necessary for the structural systems specified in Chapters E, F, G , H and Section D4. The rules to be followed in the dimensioning of moment resisting steel frame with high ductility level can be expressed as:
1a. Section Requirements for Ductile Members
Structural steel sections for highly ductile members shall have flanges continuously connected to the web or webs.
1b. Width-to-Thickness Limitations of Steel and Composite Sections
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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 comression elements should not exceed the limiting widht-to-thickness ratios, λhd from Table D1.1.
2. Stability Bracing of Beams
In order to restrain the lateral-torsional buckling of structural steel designated as highly ductile members, as defined in Chapters E, F, G and H, the stability bracing should be provided.
Ordinary Moment Frames (OMF)
Ordinary moment frames (OMF) of structural steel should be designed in conformance with this section.
6b. FR 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 which that are part of the seismic force-resisting system (SFRS) should 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 (LRFD) or 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 expected beam flexural strength should be determined as RyMp. The
The required shear strength Vu or Va of the connection should be based on the load combinations in the relevant building code that contain amplified seismic load. In determining the amplified seismic load the effect of horizontal forces including overstrength Emh, can be expressed as:
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where
Lcf is clear length of beam.
Mp=FyZ.
Ry is the ratio of expected yield stress to the specified minimum yield stress Fy., 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:
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(b) FR moment connections should shall be designed for a required flexural strength and a required shear strength equal to the maximum moment and corresponding shear that corresponds to it, that can be transferred to the connection by the system, by taking into account 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.