To understand the structural function of pile caps in foundation systems, we need to explore the role of pile caps within the context of structural engineering.

1. Definition of Pile Caps: Pile caps are thick concrete mats that sit atop piles driven deep into the ground. They are used to hold a cluster of piles together and provide a single structural unit for load distribution.
2. Primary Function: The main purpose of pile caps is to distribute the structural loads of a building or structure evenly across the individual piles and into the ground. This ensures that the loads are not concentrated on a single pile, which could potentially cause structural failure.
3. Explanation of Options:
   • **Connecting foundation piles directly to floor beams:** Pile caps do not directly connect piles to floor beams. Their main function is not to serve as a direct linkage but rather to distribute loads.
   • **Protecting exposed pile surfaces from environmental damage:** This is not a primary function of pile caps. Environmental protection is generally managed through different means such as coatings or casings.
   • **Providing waterproof protection around foundation piles:** While waterproofing is an important aspect of foundation engineering, it is not the primary purpose of pile caps.
   • **Distributing structural loads among supporting piles:** This option accurately encapsulates the primary role of pile caps, which is to transfer and distribute the loads evenly across piles.
4. Conclusion: The correct answer is Distributing structural loads among supporting piles. This function ensures that the entire foundation supports the structure effectively and safely.

In summary, pile caps serve a crucial role in foundation engineering by ensuring the even distribution of loads among piles, thereby maintaining structural integrity and stability.