I have seen maybe 6 or 8 of these in the last 25 years. They come in many styles like solid reinforced concrete slabs, precast voided concrete panels, concrete on steel decking supported by steel beams are a few I have seen. Besides the obvious point, they need to be engineered there are a few issues inspectors need to consider. They all have a weak point which is water and chlorides entering through concrete pores, cracks, and poorly sealed joints. Another issue I have is these suspended slabs are no different than a highway bridge slab and should be inspected every two years and should have a load rating posted IMO. If you inspect a house with a suspended slab I would recommend the following:
- Ask for engineered plans or recent inspection report from a structural engineer.
- Looked for cracks and water intrusion under the slab.
- Check section loss in visible steel beams, steel panels, and any support columns.
- Look for floor slab deflection or low areas that hold water, snowmelt, etc.
- Remind homeowners or buyers these slabs were designed with a specific load or vehicle. It’s important to know what the maximum load rating is and IMO this should be clearly posted.
- Reinforced steel and prestressing strands in concrete slabs and precast voided concrete panels are hidden. Typically rusted rebar or strands will show up as excessive deflection under normal design loads, but could be sudden collapse if overloaded.