Base SHEAR TRANSFER MECHANISMS

1.     SHEAR TRANSFER MECHANISMS

Under conditions of relatively low column base shear with no axial uplift, shear loading in column base plates may be adequately resisted by surface friction between the steel plate and grout/concrete foundation. However, for situations of large lateral loads or column uplift (such as in braced frames), frictional resistance may not be sufficient and shear forces must be transferred to the foundation by other mechanisms. , three popular design alternatives for shear transfer include surface friction, anchor rod bearing and shear lug bearing (Fisher & Kloiber, 2006). In addition, the base plate may be shallowly embedded into the concrete foundation to develop the shear strength through concrete bearing. Special design considerations and construction methods must be taken into account for each of these mechanisms. For example, friction resistance is only possible with axial compression in the column. In addition, under seismic loading, high shear loads and other issues such as foundation rocking, vibrations and uplift make surface friction an impractical choice for shear transfer. According to the AISC Seismic Provisions (2005), many building codes prescribe that friction cannot be considered when resisting earthquake loads. Base plate bearing against anchor rods embedded in the concrete foundation may be practical under low compressive axial loads or axial tension (i.e. uplift) at the column base, when sufficient frictional resistance cannot be developed. Fisher and Kloiber (2006) note that the use of anchor rods to transfer shear forces must be carefully examined due to several assumptions regarding the force transfer to the rods. A primary concern is the uncertainty of uniform transfer of shear loads to all the anchor rods due to lateral displacement (or slip) of the base plate. Highly oversized anchor rod holes (larger than standard oversize holes – refer Fisher & Kloiber, 2006) are typically used in base plates in order to compensate for construction tolerances. To reduce the extent of slip before engagement of all anchor rods, plate washers installed around the rods may be fillet welded to the top surface of base plate. To resist moderate to high column base shear loads, such as induced by seismic loading in low- to mid-rise structures, one or multiple shear lugs (also known as a shear key) may be attached to the base plate. The shear key is often provided in the form of a plate welded to the underside of the base plate. In some cases, a stub wide-flange section is used since it provides a higher bending resistance. The shear lug detail requires additional welding and the need for a pocket in the concrete foundation. Other details  have been proposed for shear transfer in base plates, including embedding the base plate directly into the concrete foundation, attaching it to a grade beam, using shear studs or welding steel reinforcing bar to the base plate (Grauvilardell et al., 2005). However, these details may require elaborate construction measures and are generally costly.