Engineering and Construction

The tied portal, a variation of the portal frame, is illustrated in Fig. 1.26.

Economy of material can be achieved, albeit at the expense of reducing the allowable headroom of the building, by provision of a tie at eaves level. Under vertical loading, the eaves spread is reduced due to the induced tensile  force in the tie.

However, although the rafter size is reduced, because under vertical loading the only possible mode of failure of the rafter is that of a fixed-end beam, deflections are more critical. Lateral loading due to wind further complicates the problem since in some cases the tie may not act in tension and becomes redundant. It is common, therefore, for tied portals to have fixed bases, which provide greater stability and resistance to horizontal loading.

Fig. 1.26 Tied portal frame