Take a look at this metal deck and open web steel joist floor structure. As you can see, the metal deck and the open web steel joists are severely corroded. If you zoom in, you might be able to see where the steel is starting to flake and can easily be pulled apart with bare hands. How did this happen?
First and foremost, you should understand that this particular framing is for a garage floor above it. Vehicles are parked above and there is a finished space below from where this picture was taken. As mentioned earlier, that floor is framed with a concrete slab, on a metal deck, which is supported by the open web steel joists. Even though our inspection was 30 years since the original construction, this type of deterioration likely started very early.
The main cause is due to the vehicles above it. Coincidentally, there is a corroded footprint from below of where the vehicles were located above. As you can imagine on a rainy or snowy day, vehicles truck in a ton of water. In the winter, when salt is used to maintain icy roads, that water turns into salt water and it also gets dragged in.
In a situation like this, the puddling of that water seeps through the concrete and starts to corrode the metal deck (which was originally galvanized, i.e. coated for water protection). Saltwater making it even worse, having a natural tendency to corrode steel 5-10x faster than fresh water. Once the metal deck has been compromised, the steel joists below start to experience the same effects. Even though there are floor drains, they were not properly maintained. This piled on to the problem, the drains even corroded themselves.
What is the fix? In this case, the floor structure was too compromised to be salvaged and needed complete replacement. It is an expensive fix for a poor design.
In most cases, it is not recommended to have a finished space beneath a garage for reasons described in this blog. Most are designed with a slab on grade so any trickling water can pass into the subbase below. In the cases that absolutely need the finished space below, it would be more practical to forgo the metal pans and steel joists and use a clear spanning concrete plank system.
In any case, when it comes to the integrity of corroded steel framing or the use of concrete plank systems, you should always consult a licensed Professional Engineer for their design