Development of a proposed design method for discontinuous columns in braced frames

Abstract

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 proposed 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. It was developed to simplify the design of square hollow sections discontinuous columns in frames using asymmetric beam (ASB) type floor construction. Floor beams are designed to carry the floor loads without interaction with the columns; columns are then designed to resist the floor beam reactions assuming a deformed shape derived from 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 and has been verified by physical tests on full-scale square hollow sections columns and finite element analysis using non-linear geometry and material properties and including residual stresses.Keywords: column design; discontinuous columns; moment shedding; plasticity; tubular steel IntroductionIn braced steel frame construction it is usual practice for a single length of column to extend over two or more stories and for the beams to frame into the continuous column and be connected by connections designed for vertical shear. Recently a new form of braced frame has been used in the UK for residential construction in which the columns are discontinuous [1]. Columns are fabricated in single storey lengths and fitted with horizontal plates (known as cap-or end-plates) at the top and bottom in order to bolt the column directly to beams below and above which are continuous over the column. Square hollow sections with the smallest possible size are used for the columns so that they can be hidden in the thickness of the walls. Because the beams are continuous, passing uninterrupted over the column lines, they benefit from the efficiency of continuity but without the extra fabrication cost associated with forming a full strength and rigid connection between discontinuous beams and continuous columns. The continuity of the beams across the the tops of the columns induces rotation at the top and bottom of the column under some loading arrangements resulting in curvature of the column, which may reduce the resistance of the column below that of an equivalent pin-ended strut, and therefore a design method for this form of construction is required. A method has been published [2] but this uses nominal moments and does not explicitly consider the magnitude of the slopes of the beams at the top and bottom of the column. This paper describes the development and validation of a new design method for square hollow section discontinuous columns which is safe, gives economical column sizes and is easy to ...

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