Limitations of current design procedures for steel members in space frames

The use of more advanced methods of analysis to design steel frames may lead to substantial material savings, in addition to simplicity in the design procedures. However, these benefits do not yet appear to be a powerful incentive for many structural engineers to abandon the familiar linear elastic analysis (LEA) based design procedures, even when dealing with steel structures that are not regular rectangular frames. This paper uses a heuristic example to demonstrate the serious limitations of the LEA based design procedures, whether alignment charts or system buckling analysis is used to determine the effective lengths of the compression members. It is shown that LEA based design procedures may lead to unsafe structures due to their inability to account for bending moment amplification in the rigidly connected tension members of a space frame. Furthermore, there is no allowance for the amplification of axial forces due to changes in the structure geometry, which is significant for the space frame example. Confidence in the system buckling analysis method for determining the effective lengths of compression members, based on linear buckling analysis, is shown to be potentially dangerous for certain types of frames. For the space frame example, the elastic buckling load is overestimated by over 200%. The conservatism inherent in the member capacity check equations specified in steel design standards is also illustrated.