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Topics covered in this section:

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

For members designated as highly ductile members, the Member Requirements

1- Width-to-thickness ratios of members for OMF are no limitations.

2- Stability bracing of beams in OMF is no requirement.

According to AISC 341-16, the program does not control width-to-thickness ratios of comression elements should not exceed the limiting widht-to-thickness ratios, λ_hd 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 Moment Connections

FR moment connections which are part of the seismic force resisting system (SFRS) should 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 multiplied by 1.1 (LRFD) or by 1.1/1.5 (ASD). The expected beam flexural strength should be determined as R_yM_p. The required shear strength V_u or V_a 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 E_mh, can be expressed as:

...

where

L_cf is clear length of beam.

M_p=F_yZ.

R_y is the ratio of expected yield stress to the specified minimum yield stress F_y.

(b) FR moment connections should be designed for a required flexural strength and a required shear strength equal to the maximum momentand shear that corresponds to it, that can be transferred to the connection by the system, by taking into account the effects of material overstrength and strain hardening. and stability bracing of beams or joints.

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