When a basement wall reaches another basement wall perpendicularly to it (or with an angle less than 15º with respect to the perpendicular direction, and less than 5m in length), the basement wall reaching it acts, regarding the analysis and design of the first footing, as if it were a strap beam.

So, suppose there is a wall with a total overturning moment of M. Two walls reach it at its sides and 3 strap beams. The programme shares the total M amongst the 5 elements (3 beams + 2 walls) in its initial design, which results in a moment share of M/5 for each strap beam and perpendicular wall. The footings of the walls and the strap beams are designed for that moment. A second iteration is then carried out which distributed the moment, M, depending on the real stiffness of each strap beam, considering the wall footings to be 40 x 100 cm beams. They are redesigned, etc. This is the reason why in a particular structure, it may seem that the strap beams are not contributing or if they are not placed, the result of the footing of the wall is the same.