Typically columns are attached at the base using holding-down (HD) bolts. These pass through a base plate attached to the concrete by means of 'starter bars' welded to the top base. The most efficient layout is for the base plate to extend beyond the face of the concrete as this gives a greater lever arm for the HD bolts. However, this is very inefficient for precasting since the base plate will not fit within the mould. It is useful therefore for the base plate to be flush with the column profile.
An alternative to the base plate is to use column shoes. These are in effect ‘mini’ base plates, taking a single HD bolt, and incorporating the starter bars. Usually associated with each column shoe is the corresponding element that is cast into the foundation. Some systems have a male insert that stands proud of the foundation. Others have a female insert and a connecting bolt. The advantage of the female insert in the foundation is that the surface remains flush, and free from trip hazards or projections that could become damaged by vehicles.
Column shoes are equally suitable for round, rectangular or unusual shapes (see above), and are very cost effective relative to base plates, especially where column sizes are large.
Column shoes have several advantages over a single base plate.
They weigh far less, especially for a large column. For a column (say) 1000 x 750, a 50mm thick base plate with starter bars would weigh over 300 kg, requiring a crane to handle it. The alternative of several column shoes can mean individual shoes, easily man-handleable.
The use of column shoes simplifies the design process. Unlike ‘traditional’ base plate designs, there is no need to check bending in the base plate, weld sizes, bar lengths etc. The products are sold as a standard item having a stated capacity. Capacities range from some 60 kN to over 1000 kN per shoe.
Several manufacturers supply the products and there is little significant difference in the basic appearance. Most suppliers have software to choose and justify the sizes.
Column shoes can be used just as easily with ‘traditional’ HD bolts in cone recesses. This gives even greater tolerance in setting out.
Shoes are primarily designed to take tensile loads, although they will also take some horizontal loads. The limiting factors are either the HD bolts, or the starter bars, and each should be checked if loads are high. If higher loads are required then a dowel into a pocket may be used. This can be a screwed-in dowel, or a stub projecting from the underside of the column. For the highest loads, the stub may be a steel section or a concrete ‘pin’. The clear area between the column shoes allows this, where a traditional base plate would not.
Column shoes are especially useful to connect a precast column to the top of a steel column. This example occurred during the construction of a large shopping centre when volatile steel prices changed the choice of materials. The splice point was positioned above the floor level to minimise bending moments.
They may also be used to provide horizontal ties at the ends of beams as shown below. For specialised cases such as these, the supplier should be involved at an early stage.