Read Anna Stirane's blog: 3 ways to connect precast columns to foundation – choose yours!
To achieve a flat floor, the socket has to be installed deeper in the ground, which means more excavation works. The anchoring depth needed depends on column size, with big and tall columns requiring column extension of 1.5 meters so that the column is transferring all the forces to the socket. In this case you therefore need to go 2 meters deep into the soil, which requires lots of excavation – and filling. Some precasters produce sockets as a separate component, so the socket is an additional cost on top of the foundation casting and extra column length. Each longer column adds to column weight and transportation length, which affect crane and transportation trailer dimensions.
Starter bars upwards
Starter bars upwards – those sticking out from the foundation block for installation into corrugated tubes or splice sleeves inside the column end – enable shallow foundations. Depending on local requirements of how rebars can be bent into the foundation block, this method is feasible in those areas where groundwater is close to the soil surface.
In erection works the ground has to be so flat that counterweights needed for propping are easy to locate, or the floor slab needs to cast in advance to offer a fixing surface for propping. Special care is needed in installing the bent starter bars so that bending is fully inside the foundation and not in the connection area between foundation top and column bottom. If the starter bar is not completely straight, the capacity to transfer forces is limited.
Starter bars downwards
Starter bars can also be installed into the column bottom end, so that corrugated tubes are inside the foundation block. This leads to deeper foundations than the upwards version, which means that the foundation must be deeper than the anchoring length of the starter bars. It is important that the tubes are kept clean and empty before the column installation.
Anchor Bolt – Column Shoe Connection
ETA assessed Headed Anchor Bolts such as Peikko HPM® and PPM® allow shallow foundations, which is excellent in areas where groundwater is near the soil surface, and site conditions do not allow propping. When aiming for ultralow foundations, it is possible to use bent, long anchor bolts. A clear advantage is ongoing parallel works on site between columns, which we demonstrated in our Bolted Connections Video. The use of propping effectively prevents working between the erected columns and creates extra challenges in schedule planning.
It is up to you which method you choose – my recommendation is obvious. If your foundation block is 2 x 2 meters , every extra meter in depth means 4 m3 of excavation. A typical truck can deliver the soil of two foundations. If you have 20 foundations in your building, that is 10 truckloads of soil, and if you need to transport it 20 kilometers away, we end up with 10 truckloads x 20 kilometers x 2 (return) x 900 g/km (average emissions of a truck) = 360 kg of CO2 emissions. And this is just from selecting the foundations.