That's an excellent question about the often-overlooked details of urban furniture! The attachment of wooden slats to a bench frame is crucial for safety, durability, and comfort. Typically, the slats are not directly nailed or screwed in a way that would split the wood or allow moisture entrapment.
The most common and robust method involves using specialized hardware. Pre-drilled holes are made in the wooden slats. Then, bolts or machine screws are passed through these holes and into threaded inserts or nuts embedded within or underneath the metal frame. A washer is almost always used between the bolt head and the wood to distribute the pressure and prevent the head from digging into the surface. This creates a strong, mechanical connection.
Crucially, there is almost always a small gap left between the slat and the metal frame itself. This is achieved by using a spacer or a shoulder on the bolt. This gap is vital for drainage and airflow, allowing water to run off and the wood to dry quickly, which drastically reduces rot and prolongs the bench's life. The hardware used is almost always stainless steel or galvanized to resist rust.
For benches with a more streamlined look, you might find a "hidden fastener" system. Here, slats may have a groove on the underside that clips onto a continuous rail on the frame, or special brackets are used that are not visible from the top or sides.
So, in short: it's a deliberate, engineered connection using rust-resistant bolts and spacers, designed for strength, safety, and longevity in all weather conditions. Next time you sit on a public bench, you can appreciate the smart engineering right beneath you!