Socket connection
This is undoubtledly a tried and true method – to the extent that even the ancient Romans used it. They dug a hole in the ground, erected the column in the hole and used grout made out of volcanic ash to secure the column in place.
Today the process is more or less the same, except that the hole in the ground is often substituted with a socket made of concrete. This socket can be precast or it can be cast in situ. Either way, columns need to be supported and set in the exact position with wooden wedges or similar. After grouting, it’s impossible to make any adjustments if the need arises.
Starter bars
Starter bar column connection relies on rebars protruding from the foundation.
In the column, you need to have holes for the starter bars and grout. Typically, this is done in the precast factory during casting with corrugated pipes or similar. When the column is erected, it has to be supported and non-shrink grout needs to be manually poured to secure the connection. Care must be taken to position the column properly while the grout cures.
Some markets require that the rebars project from the column. In this scenario, holes for the starter bars and grout need to be fabricated in the foundation.
Bolted connection
As its name implies, a bolted column connection is made with anchor bolts. Cast into the foundation using a template, the anchor bolts transfer tension, compression and shear forces to the reinforced concrete base structure. The bolt counterparts – column shoes – are cast in to the columns at a precast factory.
A moment-resisting connection is made quickly by lowering the column in place and tightening the nuts to specified torque with readily available hand tools. The installation process takes on average 20 minutes per column and requires only a crane operator and two people on the ground.
The connection is finalized by grouting the anchor bolt recesses and joint underneath the column with non-shrink grout. Note that the connection is moment-resisting and safe as soon as the nuts are tightened – there is no need to use wedges and braces while waiting for the grout to cure.
A bolted connection can also be used to create a column to column splice, which is impossible to do with a socket method or with starter bars.
Also note that a bolted connection allows you to reduce foundation thickness and to avoid interfering with existing structures. Choosing a bolted connection will also reduce the excavation depth and costs.
Column connections in seismic areas
Bolted connections can also be used in seismic areas, where the main objective is to ensure that the building sustains earthquakes without collapsing. Most design approaches solve this by keeping a certain reserve of the strength in the structures. In other words, they are often overdesigned and oversized. An energy dissipating bolted connection can help you save up to 20% of concrete thanks to slenderer cross-sections of the column compared to overdesigned and oversized connections.
