Tied foundations - Design.

Introduction
Tied foundations are often adopted as a means of exploiting to advantage opposing forces. This is achieved by linking them together via a tie or tie beam. The effect this has on  the design is to reduce the horizontal force requiring to be resisted by the ground (see Fig. 12.1 (a)).

The use of a tie can reduce the amount of movement likely to occur in developing the reaction and reduce the cost of the foundation.

Fig. 12.1 Tied foundation.

Design decisions
In any situations where horizontal forces, such as thrusts from portal frames, etc., act in opposite directions, consideration should be given to connecting the forces via a tie in order to reduce or totally react a horizontal force. For example, if the forces are equal and opposite then the total force can be reacted. On the other hand, if the forces are opposite and not equal, the smaller of the two forces can be tied and the remainder left to be reacted by foundation 1 or, if a higher tie force is used, foundation 2 can also be utilized, thereby reducing the force to be taken in passive pressure (see Fig. 12.1 (b)).

Sizing the foundations
The main pad foundations are designed in the same way as those previously discussed in Chapter 11 but taking into account the tie force reaction in accordance with the above considerations. The tie itself must be designed to resist the force H1 or H2, as the case may be, and must be detailed to
transfer this force without excessive slip or failure between the bases of the stanchions.

This is usually achieved by designing a tie rod for the total force using appropriate permissible tensile stresses for the steel and ensuring that suitable mechanical anchorage or bond anchorage is achieved in the details between the  stanchion and tie (see Fig. 12.2).

In detailing these ties, the detailer should ensure that the  tie acts on the centreline of the horizontal thrust force or that any eccentricity produced is designed into the foundation by the designer. The tie rod itself could contain a turn-buckle for tensioning in order to reduce lateral movement due to possible slackness in the rod, alternatively, if adjustment is not required, a reinforced concrete tie beam as shown in Fig. 12.3 could be used. Care should be taken  to ensure axial tension across any connections which may  be required in the tie by the use of turn-buckle or male/ female-type plate connectors. In the case of portal framed factories it is often desirable to construct the floor slab after
erection and cladding of the building. In this case the engineer must ensure that all tie members are constructed and covered prior to the erection of the steelwork, in order that the presence of the tie members does not impede the construction process.

Fig. 12.2 Tie anchorage.

Fig. 12.3 Reinforced concrete tie beam.