AD 404: Columns in simple construction
SCI has received reports that some designers are disregarding the rules for the design of columns in braced frames (simple construction). In some cases the columns have been designed for an axial load only – even when the loading from the beams is not symmetrical. In another case with a fin plate connection the assumed eccentricity from the face of the column was the actual dimension to the bolt line, rather than the nominal 100 mm.
The rules governing the design of columns in simple construction are given in clause 4.7.7 of BS 5950-1 and – for design to the Eurocodes, NCCI document SN048, available at http://www.steel-ncci.co.uk/.
Whatever style of nominally pinned connection is to be used, the nominal moment is calculated based on an eccentricity from the face of the column of 100 mm, even if the physical dimension to the assumed location of the pin is different. A net moment will result if the beam reactions are different on either axis; the moment is distributed to the column lengths above and below.
The rules for this type of column design, including the apparently arbitrary nominal eccentricity from the column face of 100 mm have reassuring provenance – they were described in BS 449 and had been successfully used for decades. Designers should not depart from these rules without careful consideration.
Contact: Abdul Malik