Superstructure: Three pinned arch.

The superstructure costs for a rigid-steel portal-frame shed are generally cheaper than the three pinned arch solution (see Fig. 1.2).

Differential settlement of the column pad bases will however seriously affect the bending moments (and thus the stresses) in the rigid portal, but have insignificant effect on the three pinned arch. Therefore the pad foundations for the rigid portal will have to be bigger and more expensive than those for the arch, and may far exceed the saving in superstructure steelwork costs for the portal. (In some cases it can be worthwhile to place the column eccentric to the foundation base to counteract the moment at the base of the foundation due to column fixity and/or horizontal thrust.)

Fig. 1.2 Rigid portal versus three pinned arch.