Base LOADING CONDITIONS

1.     

Unlike beam-to-column connections, which are typically designed to resist only flexural and shear loading, column base connections are susceptible to a wide range of loading conditions. The general types of loading for base connections include –

o   Axial Compression – The most typical type of column base loading, axial compression arises mainly from gravity loads on the column.

o   Axial Tension – Axial tension (i.e. uplift) may result from wind uplift or global overturning of the structure.

o   Shear – Shear loading is significant in braced frames, in which lateral loads on the structure are transferred via the braces to the base connection. Shear forces are usually accompanied by axial compression or axial tension, due to gravity loads or forces transferred through the structural braces.

o   Flexure – Flexural loading is significant in moment resisting frames. Flexure is usually accompanied by axial compression, and in some rare circumstances, axial tension (i.e. uplift). Both strong axis and weak axis bending of the column is possible. Biaxial bending may also occur, especially if the column forms part of two or more intersecting lateral-force-resisting systems. Proportional and non-proportional flexural loading is possible; the main difference between these two types of loading is related to the load eccentricity ratio (applied moment divided by applied axial load) as loading progresses, which may affect the connection stiffness and progression of failure modes. For non-proportional loading, the axial force is applied independently of the flexural moments and the load eccentricity ratio may vary substantially as loading progresses. For proportional loading, the axial force and bending moment are applied simultaneously and the load eccentricity remains relatively constant. This is consistent with the application of an eccentric axial load at the top of the column.

o   Torsion – Torsional loading at the base connection is possible, especially for monopole (e.g. traffic pole) structures. Base connection response and design for torsion may be similar to that as for shear loading.

o   Cyclic Loading – Cyclic loading due to seismic excitations may induce mechanical degradation of the connection due to damage of the connection components. Compared to static loading, cyclic loading may influence the progression of failure modes. For example, spalling of the grout pad or facture of the anchor rods or welds (from low-cycle fatigue) may affect the connection behavior.

Fatigue – Often machinery and certain building columns may be subject to vibration or cyclical loads, which may cause high-cycle fatigue failure of the anchor rods. In column base plate connections subject to fatigue, the anchor rod will typically fail before the concrete fatigue strength is reached and it is usually not necessary to consider the fatigue strength of the concrete (Fisher & Kloiber, 2006). Pre-tensioning of the anchor rods may improve the fatigue life of the connection.