BS 5950
With the Criteria - Effective Length command, major and minor effective length factors (Le/L) are set for the columns, beams and brace struts in the structure.
For columns, the Le/L values can be calculated by the program using Appendix E provisions of BS 5950:1990 and BS5950:2000, or can be specified by the engineer. These Le/L factors apply to all lateral columns in the structure. The engineer can specify an appropriate effective length factor per table 24 in BS 5950. Otherwise, the engineer can specify that the effective length be calculated according to the requirements of Appendix E. This appendix considers the members framing into the column, and whether a section is braced against sidesway (See Criteria above for making this designation) to calculate the effective length of the column. The equations used to calculate the Le/L factor are taken from Annex E in the Draft Amendments for BS5950, dated April 1998, and from BS 5950-1:2000. The structure is assumed to be comprised of rigid joints where moment connected, and is either fully braced against sidesway (k3=infinity) or has unrestricted sidesway (k3=0), no partial lateral support is considered. The effective length factor used to calculate the axial capacity of the column is also used to calculate the lateral torsional buckling capacity for the beam when subject to bending. The following assumptions are made in implementing Appendix E:
A column pinned top and bottom (or where all beams and columns framing into the column in question are pinned) will be assigned an effective length factor of 1.0. This is true even when the column is indicated as being subject to sidesway.
A column fixed at the foundation will be assigned a k2 value of 0.5 per BS5950, Section 5.1.2.4. (a). A column pinned at the foundation will be assigned a k2 based on a base stiffness equal to the column itself per BS5950, Section 5.1.2.4. (b). Any other columns with a pinned end will be assigned a k value of 1.0 for that end.
In the BS5950-1:2000 implementation, several issues related to effective length and notional loads need to be considered. These are discussed further at the following links:
The Engineer provides the effective length factor for RHS and I section beams. If the Use LTB (Lateral Torsional Buckling) Factor is selected then the Le/L factor for the beam will be calculated according to the data provided in the LTB section of the dialog. Connection information is required to determine the effective length of channels subject to axial load. The LTB section of the dialog is used to calculate the effective length of the unbraced beam for Lateral Torsional Buckling consideration. This information is required whenever a beam that is subject to bending has an unbraced flange.
The compression capacity of I section and RHS beams subject to axial load is calculated using the effective length factors provided by the engineer. For channels the end connection details are used to determine the effective length factor according to Section 4.7.10.4 of BS 5950. Double channels are not currently accommodated in the structural system. In calculating the bending capacity of an unbraced beam the impact of end fixity on the lateral torsional buckling (LTB) capacity of the section is considered. Loads are assumed to be normal loads (applied at the shear center). For a segment continuous through brace points the Le is equal to the unbraced length. Where a beam is continuous through a column or supporting girder an effective length factor of 0.7 is assigned to that end of the segment. This is indicative of the column providing the beam a measure of restraint for bending about the minor axis. Where a beam is pinned at a support, or where a cantilever tip occurs, the effective length factor is taken as the value provided by the engineer in the Criteria-Effective Length dialog. The effective length factor for an unbraced segment is taken as the average of the factors at the segment ends (Refer to Table 10). For I and RHS struts the minor axis effective length factor can be the same as the effective length calculated for the segment for lateral torsional buckling if the engineer indicates as such in the Effective Length dialog described previously.
For I, RHS and CHS brace sections the Le/L factor must be entered directly, for double angles connection details are required to be input from which the effective length factor will be determined. For double angles the connection detail must be provided and the effective length factor is then determined according to section 4.7.10.3 of BS 5950. Single angles are not currently accommodated in the structural system. The Le/L factor used to calculate the axial capacity is also used in the calculation of bending capacity lateral torsional buckling where a brace is subject to end moments.
Modifying the Effective Length criteria after a Code Check has been performed will invalidate the results of that Code Check. The model will be redrawn on the screen in its original colors (rather than the Code Check colors) indicating that the Code Check must be re-run.
For columns the engineer can specify an appropriate effective length factor per table 24 in BS 5950. Otherwise, the engineer can specify that the effective length be calculated according to the requirements of Appendix E. This appendix considers the members framing into the column, and whether a section is braced against sidesway (See Criteria above for making this designation) to calculate the effective length of the column. The equations used to calculate the Le/L factor are taken from Annex E in the Draft Amendments for BS5950, dated April 1998. The structure is assumed to be comprised of rigid joints where moment connected, and is either fully braced against sidesway (k3=infinity) or has unrestricted sidesway (k3=0), no partial lateral support is considered. The effective length factor used to calculate the axial capacity of the column is also used to calculate the lateral torsional buckling capacity for the beam when subject to bending. The following assumptions are made in implementing Appendix E:
A column pinned top and bottom (or where all beams and columns framing into the column in question are pinned) will be assigned an effective length factor of 1.0. This is true even when the column is indicated as being subject to sidesway.
A column fixed at the foundation will be assigned a k2 value of 0.5 per BS5950, Section 5.1.2.4. (a). A column pinned at the foundation will be assigned a k2 based on a base stiffness equal to the column itself per BS5950, Section 5.1.2.4. (b). Any other columns with a pinned end will be assigned a k value of 1.0 for that end.
The effective length factor for brace members subject to axial load is provided by the engineer for all shapes except double angles. For double angles the connection detail must be provided and the effective length factor is then determined according to section 4.7.10.3 of BS 5950. Single angles are not currently accommodated in the structural system. The Le/L factor used to calculate the axial capacity is also used in the calculation of bending capacity lateral torsional buckling where a brace is subject to end moments.
The compression capacity of I section and RHS beams subject to axial load is calculated using the effective length factors provided by the engineer. For channels the end connection details are used to determine the effective length factor according to Section 4.7.10.4 of BS 5950. Double channels are not currently accommodated in the structural system. In calculating the bending capacity of an unbraced beam the impact of end fixity on the lateral torsional buckling (LTB) capacity of the section is considered. Loads are assumed to be normal loads (applied at the shear center). For a segment continuous through brace points the Le is equal to the unbraced length. Where a beam is continuous through a column or supporting girder an effective length factor of 0.7 is assigned to that end of the segment. This is indicative of the column providing the beam a measure of restraint for bending about the minor axis. Where a beam is pinned at a support, or where a cantilever tip occurs, the effective length factor is taken as the value provided by the engineer in the Criteria-Effective Length dialog. The effective length factor for an unbraced segment is taken as the average of the factors at the segment ends (Refer to Table 10). For I and RHS struts the minor axis effective length factor can be the same as the effective length calculated for the segment for lateral torsional buckling if the engineer indicates as such in the Effective Length dialog described previously.