E6.1 Scope
Cantilever columns can be used as an individual column, as a single row of columns, or as multiple rows of columns. Cantilever columns can be used in one or both directions of the seismic force-resisting system. Examples include, but are not limited to, columns of buildings, canopies, and pipe racks. When the stability of the mass supported by a cantilever column relies on rotational restraint at the top of the cantilever column, the cantilever column is an inverted pendulum-type structure.
E6.2 Basis of Design
ASCE/SEI 7 (ASCE, 2022) includes two types of cantilever column systems, ordinary and special. The special cantilever column system (SCCS) is intended to provide a limited level of inelastic rotation capability at the base of the column. This system is permitted in seismic design categories B through F but is limited to heights not exceeding 35 ft (11 m). A relatively low seismic response modification coefficient, , of 2.5 is assigned due to the system's limited inelastic capacity and lack of redundancy.
E6.4 System Requirements
E6.4a Columns
The requirements in this section are identical to those in Section E5.4a. See Com- mentary Section E5.4a for further discussion.
E6.4b Stability Bracing of Columns
Cantilever columns behave as beam-columns. Cantilever columns may require bracing to control lateral-torsional buckling so that flexural yielding can develop at the column base. The bracing requirements of this section are not intended to provide column stability or to prevent sidesway. The lateral-torsional buckling limit state does not apply to round hollow structural sections (HSS), square HSS, square box sections, or any cross section bent about its minor axis; thus, the exception states that bracing may be omitted for these cases. The maximum spacing for bracing is based on the unbraced length limits for inelastic analysis in accordance with Specification Appendix 1. For cantilever columns that are used in inverted pendulum-type structures, the ratio should be at least 0.5 . For cantilever columns that are not considered to be inverted pendulum-type structures, where the calculated moment at the top of the column is zero, the ratio may be taken as zero. An exception has been included to allow the usage of short columns without bracing by conservatively limiting the column length to half the maximum bracing spacing.
Point torsional bracing is likely to be the best choice for bracing as lateral bracing may cause the seismic force-resisting system to become something other than a cantilever column system. Point torsional bracing can be achieved by encircling the column section with the roof deck or by attaching beams to the column that provide sufficient bending strength and stiffness as required for beam torsional bracing in accordance with Specification Appendix 6. Cantilever column systems can act as a cantilever column in one direction and can be used with other systems (moment or braced frames) in the orthogonal direction. The seismic force-resisting system in the orthogonal direction can be used to provide the required lateral or point torsional bracing for the cantilever column system.
E6.5 Members
E6.5a Basic Requirements
The column members are required to satisfy the width-to-thickness and other provisions for highly ductile members. The intention is to preclude local buckling at the hinging location (bottom of the column), which in this type of structure, with little redundancy, could lead rapidly to collapse.
E6.5b Column Flanges
Abrupt changes in beam flange area in locations of high strain, as occurs in plastic hinge regions at the base of SCCS columns, can lead to fracture due to stress concentrations.
E6.5c Protected Zones
For commentary on protected zones, see Commentary Section D1.3.
E6.6 Connections
For general commentary on demand critical welds, see Commentary Section A3.4. For additional commentary relevant to column splices and column-to-base plate con- nections, see Commentary Section E3.6a.