The direction of the main bars (T1 and B1) in a flat slab system depends on various design factors such as the span length, load distribution, structural efficiency, and code requirements.
In general, the main bars are usually placed parallel to the shorter dimension of the slab to reduce the slab's overall span length and provide better structural efficiency. This is because longer spans tend to produce higher bending moments, which can increase the size and number of reinforcement bars required to resist the loads.
However, there may be cases where the main bars are placed parallel to the longer dimension of the slab due to specific design requirements or to minimize the number of drop panels. In these cases, the design must account for the increased span length and ensure that the slab is adequately reinforced to resist the loads.
In summary, the direction of the main bars in a flat slab system depends on the specific design requirements and considerations, and it can be parallel to either the shorter or the longer dimension of the slab.