AD 304: Columns in Braced Bays and Nominal Moments

Figure 1: Typical Braced Bays in Simple Construction

Following the publication of AD 275 concerning columns in simple construction further questions have been asked about the design of columns in braced bays of simple structures and the use of nominal moments. This AD is written on the assumption that the centre lines of the members in the braced bays are coincident at each node and that the beam-column-bracing connection is designed and detailed so that the only nominal moments that arise are from the assumed eccentricity of the beam end reactions.

Unless some additional factors arise which makes the column a special case, see AD 275, columns in braced bays of simple structures should be designed in accordance with clause 4.7.7 of BS 5950-1: 2000 taking account of nominal moments.

Figure 1 shows  typical braced bays which are usually analysed as pin-jointed frames with all the members at a node intersecting at a common point on their centre lines in order to determine the load effects in the members under vertical and lateral loading.

Assuming pinned joints in order to determine forces in members ignores the nominal moments described in clause 4.7.7 of BS 5950-1: 2000 which must be taken into account in the design of columns in simple structures. In most cases the value of the nominal moment is calculated from the assumed 100mm eccentricity of the beam end reaction from the face of the column and the net nominal moment at any level distributed as set out in clause 4.7.7.

Traditionally, when calculations were routinely performed by hand, the nominal moments for the columns were combined with the axial loads in the columns from the analysis of the pin-jointed frames. This should still be done even when the frame analysis is carried out using software.  At least one commercially available software package combines the axial loads and nominal moments in the columns automatically. However, designers should be careful not to ignore the nominal moments in the design of the columns when analysing the pin-jointed frames for braced bays using general plane frame analysis packages.

Another AD will follow shortly which will describe the common cases where the settings out points of the diagonal bracing members in braced bays are displaced from the intersection of the centre lines of the beams and columns.