Column design for axial compression and end rotation.

A column design method has been developed for use in braced frames with discontinuous
columns using flexible cap and base plates and floor beams that are either simply supported
or continuous. The method is intended to be used with shallow floor construction with
concrete or steel/concrete composite slabs in which the floor slab occupies the depth of the
floor beams and is fully grouted to the beams so that the slab restrains the full depth of the
beams and was developed to simplify the design of discontinuous columns in frames using
Corus ASB type floor construction without resorting to methods using nominal moments.
Floor beams are therefore designed to carry the floor loads without interaction with the
columns. Columns are designed to resist the floor beam reactions with column end-rotations
equal to the slope of the floor beams at the top or bottom of the column, whichever is the
greater.
The method incorporates the elasto-plastic behaviour of columns subject to axial
compression and large end-rotations. The design procedure uses rectangular stress blocks of
classic plastic cross-sectional resistance and includes the end-rotations of the columns and
the effects of imperfections. Therefore the approach is similar to classic second-order rigidplastic
analysis but with the important additions of actual end-rotations and an initial
imperfection. The method has been verified by physical tests on full-scale columns and finite
element analysis using non-linear geometry and material properties.
The design method
1. is derived from simple geometry and simple statics
2. accounts for initial imperfections by deriving a design value from the strut resistance
of whatever design code is specified
3. is limited to sections that cannot be affected by torsional, lateral torsional or
torsional flexural buckling
4. is not applicable to sections with thin walls
5. has been verified for use with hot-finished square hollow sections by testing fullscale
columns
The design method is defined in detail and illustrated by a worked example so that it can be
used in normal design practice. Connection design and bracing requirements are discussed to
enable the column design to be used to design complete frames.