Fixed and pinned bases - Connection of the steelwork.

The function of a column baseplate is to distribute the column  forces to the concrete foundation. In general a plain or slab base is used for pinned conditions or when there is very little tension between the plate and the concrete.A gusseted base is used occasionally to spread very heavy loads but more generally for conditions of large moment in relation to the vertical applied loads, the principal function of the gusset being to allow the holding-down bolt lever arm to be increased to give maximum efficiency while keeping the baseplate thickness to an acceptable minimum. Gusseted or built-up bases give an ideal solution for compound or twin crane stanchions in industrial shed buildings.

Fixed bases are used primarily in low-rise construction either in portal buildings specifically designed as ‘fixed base’ or in industrial sheds in which the main columns cantilever from the foundations. They are also used, though less  frequently, in  multi-storey rigid-frame construction. In each of these cases it is assumed by definition that no angular rotation takes place, and although this is unlikely to be achieved it is generally accepted that sufficient rigidity can be obtained to justify the assumption.

Pinned bases are those in which it is assumed that there is no  restraint against angular rotation. Although this is also difficult to achieve it is accepted that sufficient flexibility can be introduced by minimizing the size of the foundation and  similarly reducing the anchorage system. Pinned bases are used in portal and in multi-storey construction.

Typical pinned and fixed bases are shown in Fig. 27.6.

Typical column base connections
Fig. 27.6 Typical column base connections

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