Foundations. Warning upon joining pad footings with strap or tie beams between columns with foundations at different elevations

A strap beam, from the footing of a column which starts at a higher foundation elevation to a column or wall whose foundation elevation is below the first, induces forces in this column or wall which cannot be taken into account, such as axial forces and moments, due to the footings being designed after the analysis. In other words, the upper part of the structure is analysed first and, once the forces at the column/wall starts have been obtained, the pad footings are designed.

An iterative analysis would be required to solve the foundation. The program does not carry out this process. Hence, even though the program emits a warning and allows the user to continue designing the foundation, the strap beam is not taken into account. Nonetheless, to be able to neglect the induced forces, detail CSZ007 can be executed. This can be modelled in the program by introducing a column start at the higher foundation elevation, close to the column or wall whose foundation is below, as if it where a building dilatation joint, then selecting this start as the final end of the strap beam.  

Logically, the floor plan drawing must then be edited so this column start does not appear. The detailing of the strap beam must also be edited so its length reflects its real length which is that between the footing at the top elevation and the footing at the bottom elevation.

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