Columns in single storey buildings
Special consideration may need to be given to the effective length of columns in single storey buildings to account for practical construction details.
For single storey shed type buildings, columns are often fixed at the base, ie. restrained against rotation and translation, and normally carry a roof structure, such as a truss, at the top, providing partial restraint against rotation only. If the lateral stability of the building as a whole is dependent on the fixity at the base, the top of the column is not restrained against translation. In the horizontal plane of the building there may be elements which effectively restrain the column from lateral buckling at certain intervals along the height. Normally, these restraints are provided at the positions where the sheeting rails are connected, because the sheeting rails themselves can then become part of the restraining elements. The effective length for such a column will need to be carefully considered.
Major axis buckling is related to the full height of the column.
For buckling about the xx axis, the whole column is capable of distortion. Hence the length of the column which is capable of distortion in one unit is the whole length L rather than a single part of it. The base of the column is fixed and the top of the column is not fully free to distort. The top can sway laterally taking the truss with it, but cannot rotate due to the restraint from the roof truss. Hence, the effective length of the column axis xx is 1.5 L
Minor axis buckling is related to the distance between longitudinal restraints, such as may be provided by sheeting rails.
For buckling about the yy axis the column has different degrees of restraint over different sections. The bottom length L1, can be described as effectively held in position at both ends and restrained in direction at the base only. Therefore, the effective length of the column axis yy for the length L1 is equal to 0.85 L1.For the lengths L2 and L3, the column can be described as being effectively held in position at both ends, but not restrained in direction. Therefore, the effective length of the column axis yy for the lengths L2 and L3 are 1.0 L2 and 1.0 L3 respectively.The final LE to be adopted for calculation would be the largest of the three values 0.85 L1, 1.0 L2 and 1.0 L3.
