Hi Jeffrey, thanks for using Ask Extension.

This subject can get rather muddy.  It seems there is some disagreement on materials and approaches.  This stems from some confusion with other growing systems or the different environmental conditions in the area you are growing in.

For galvanized steel raised beds, the best base layer is actually very simple. You do not need logs, branches, rocks, or gravel at the bottom. Those materials can cause settling, create air pockets, or interfere with drainage—especially in Michigan’s freeze–thaw cycles.

Here are some things to consider. Adding 1/4-inch mesh hardware cloth (no gaps) under the bed can help keep out burrowing pests like voles.  Some people like to put cardboard or newspaper down at the bottom to kill off grass and weeds, although a thick layer of soil should do that. Avoid rocks, gravel, which can create a perched water table and worsen drainage.

Galvanized steel beds drain very well on their own, so no additional drainage layers are needed.

Getting the soil mix right is probably a bigger challenge. Too much compost can cause nutrient overload and retain too much moisture. Woody compost can tie up nitrogen. A lot depends on what you can get. For your galvanized steel raised beds, a soil‑dominant mix such as 60–70% topsoil and 30–40% compost, or a balanced 30/30/30 mix of topsoil, compost, and a coarse mineral amendment (like a coarse sand) can work. These drain well, avoid nutrient overload, and stay structurally stable. A balanced raised‑bed mix of 30% topsoil, 30% compost, and 30% coarse mineral amendment (such as coarse sand or perlite) can work just fine as can a 50/50 mix. Again, availability will influence what is needed.

Additional info on the following links:

Raised bed gardens | UMN Extension

Growing Vegetables in Raised Beds | University of Maryland Extension

I hope this is helpful.