If a basement wall is reached by another basement wall perpendicularly (or at an angle of less than 15° to the perpendicular wall and also with a length of less than 5 m), the footing of the latter acts as an additional centring beam for the design and sizing of the footing of the former.
For example, consider there is a wall with a total overturning moment M. Its footing is reached by two walls on the sides and 3 centring beams. The program distributes the total M moment between 5 elements (3 beams + 2 walls) for a first sizing, which results in a distribution of M/5 for each centring beam and perpendicular wall. The wall footing is sized and the beams are sized with this moment. Then, in the second iteration, the M moment is distributed according to the real stiffness of each centring beam, the wall footing being considered as a 40 x 100 beam. It is then sized again, etc. This is the reason why in a given structure it may appear that the centring beams are not acting or that, if they are not used, the result of the wall footing is the same.
