Functions of different reinforcement in a typical pile cap
Loads from columns transferring to pile cap persuade tensile forces at the bottom of cap. For example, by using truss analogy to analyse a pile cap sitting on two piles with a column at the centre of pile cap, tensile force at the bottom is proportional to pile spacing and is inversely proportional to depth of pile cap. Bottom reinforcement is designed to resist the tensile stressed produced from loads in columns.
Side reinforcement may not be essential in pile cap. As a matter of fact, primary aim of the side reinforcement is to control cracking. Though as most pile caps are hidden from view and it's considered not essential to provide side reinforcement to pile caps based on aesthetic reason.
Occasionally reinforcement can be designed at the top of pile caps that serve as compression reinforcement. This kind of reinforcement is essential in case there is a limitation on the depth of pile caps. Likewise shear reinforcement is introduced to the pile caps just in case there is a restriction to depth of pile caps.