How to use the steel deck deflection calculator
Steel deck acts as the form for the slab until the concrete cures, so excessive deflection during the pour leaves an uneven slab and drives up concrete quantities. The SDI Design Manual limits construction deflection to the span divided by 180 or 3/4 in, whichever is smaller. The load used for that check is the wet concrete weight (unit weight times slab thickness) plus the deck self weight.
Deflection depends heavily on continuity. A single span uses 5wL⁴/384EI, two equal spans use wL⁴/185EI, and three or more spans use wL⁴/145EI. That is why the same deck laid in three-span sheets deflects roughly a fifth as much as a single-span layout. A steel modulus of 29,500 ksi is used, and the moment of inertia should come from the manufacturer catalog as a value per foot of width.
The results also show the longest unshored span that deflection alone would allow. A real unshored span table also has to satisfy flexural strength under the 20 psf construction live load, web crippling and shear at the supports, and the 150 lb concentrated load check, so always compare against the manufacturer table for the specific profile and gauge. Section properties vary by product, so never estimate them, and have the erection engineer or engineer of record confirm shoring decisions.
Frequently Asked Questions
The SDI deflection limit is evaluated under the weight of the wet concrete and the deck alone. The 20 psf construction live load and the 150 lb point load are strength checks, because what shows up as a permanent dip in the finished slab is driven by the concrete weight.
Adding a row of shoring to halve the span is the quickest fix. Otherwise move to a heavier gauge, a deeper profile, or a three-span layout. Shored decks need a plan for when the shoring can be removed.