This is a generalized answer to a question in general.

By code, the requirement of stirrups or links is based on shear demand on the cross-sectional area of the wide beam (one-way beam shear), or on the surrounding cross-sectional areas (two-way punching shear) as in flat slabs.

In practice, depending on floor geometry and loading pattern, special concerns may arise over the potential of "stress concentration" over a narrow bend in the wide beam/floor element. Although the pile cap is not a good example (usually wrapped in heavy reinforcement in all directions), it does help to visualize uneven stress distribution on the cross-section. Similar to the T beam, the stress is highest near the pile (as support) and decreases to zero at the free edges. While overall shear capacity is adequate, but the high stress is concentrated in the center that can cause cracks locally; weaken the performance and durability of the continuous pile cap that behaves like a wide beam. So very often the shear reinforcement is utilized to compensate for the high stress in the concrete near the support.

The concept above has carried into the design of the flat slabs. In practice, you might encounter the so-called "embed beam", which is a narrow band of longitudinal reinforcement enclosed in the transverse reinforcement to form a rigid strip that acting similar to a physical beam (without stem). The code takes care of the phenomenon of stress concentration in two way flat slabs (considered as a very wide beam) by specifying the column strip and the interior strip, however, other than one or two-way shear, the transverse reinforcement is left to the judgement and care of the individual designer/engineer based on the effects of special structural layout, load pattern has had on the durability and appearance of the slab (wide beam).

The stress variation in a wide beam is best visualized through the stress diagrams from a FEM or plate analysis. Often the transverse reinforcement is not mandated by code, but personal judgement and preference, also his/her experiences. Hope this write-up helps.

This is a generalized answer to a question in general.

By code, the requirement of stirrups or links is based on shear demand on the cross-sectional area of the wide beam (one-way beam shear), or on the surrounding cross-sectional areas (two-way punching shear) as in flat slabs.

In practice, depending on floor geometry and loading pattern, special concerns may arise over the potential of "stress concentration" over a narrow bend in the wide beam/floor element. Although the pile cap is not a good example (usually wrapped in heavy reinforcement in all directions), it does help to visualize uneven stress distribution on the cross-section. Similar to the T beam, the stress is highest near the pile (as support) and decreases to zero at the free edges. While overall shear capacity is adequate, but the high stress is concentrated in the center that can cause cracks locally; weaken the performance and durability of the continuous pile cap that behaves like a wide beam. So very often the shear reinforcement is utilized to compensate for the high stress in the concrete near the support.

The concept above has carried into the design of the flat slabs. In practice, you might encounter the so-called "embed beam", which is a narrow band of longitudinal reinforcement enclosed in the transverse reinforcement to form a rigid strip that acting similar to a physical beam (without stem). The code takes care of the phenomenon of stress concentration in two way flat slabs (considered as a very wide beam) by specifying the column strip and the interior strip, however, other than one or two-way shear, the transverse reinforcement (for the effects of shear or torsion) is left to the judgment and care of the individual designer/engineer based on the effects of special structural layout, load pattern has had on the durability and appearance of the slab (wide beam).

The stress variation in a wide beam is best visualized through the stress diagrams from a FEM or plate analysis. Often the transverse reinforcement is not mandated by code, but personal judgment and preference, also his/her experiences. Hope this write-up helps.