A steel bar joist system is a floor or roof system made of light, open-web steel joists spaced closely together, spanning between steel beams or girders, and topped with a corrugated metal deck and a thin concrete slab.
Bar joists are standardized by the Steel Joist Institute and are economical for offices, retail and especially roofs, where loads are light and the structure stays dry. They are rarely used for parking decks. The thin slab and light joists respond poorly to concentrated wheel loads, deflect and vibrate under traffic, and offer little protection against the water and deicing salts that drive deterioration in parking structures.
Why parking avoids them
- Wheel loads. Concentrated vehicle loads cause local deflection and cracking in thin slabs over metal deck. See concentrated load.
- Corrosion. Salt-laden water penetrates cracks and attacks the metal deck and joists from above, where it cannot be inspected easily.
- Vibration. Light, flexible floors bounce under moving vehicles.
- Repair. Deteriorated deck and joists are hard to repair without replacement.
Where they do appear
- Roofs and canopies over parking, and floors of other uses above or beside a garage.
- Retail or office buildings with a small amount of interior parking, where the joist floor should be checked carefully before cars are allowed on it.
The Parkonomics view
A bar joist floor is an office floor. If a building proposes to park cars on one, ask for the engineer’s check of wheel loads, deflection and corrosion protection, and budget for the deck the way you would budget for a roof.
Related terms: Structural Steel System, Hybrid Steel System, Concentrated Load, Structural Systems of Parking, Deflection
Further reading: The Evolution of U.S. Parking Structures in 2026