NorthEgressStair-DeckDefl.zipI have a model where I modeled the steel roof deck (with plates) using the equivalent thickness of deck based on the ratio of equivalent shear stiffness to the shear modulus of steel (tequiv = G' / G). The equivalent thickness was approximately 0.0464 inches. I had the modulus of elasticity set for 29,500 ksi (standard steel deck modulus). When I did this I was getting huge vertical deflections of the deck from the seismic load combinations even with no vertical load on the deck.
I then changed the deck thickness to 1/2", but changed the modulus of elasticity to 2737.6 ksi (29500 * 0.0464 / 0.5) to get approximately the same stiffness (EA). This time there was hardly any vertical deflection in the deck.
Does STAAD.Pro not like really thin deck? What is going on here?
The pictures below show the displacement for the exact same load case (notice the displaced shape difference though).
Picture 1 shows the results with the thin deck: max vertical deflection in the steel deck is approximately 5000" for a load case with zero vertical load.
Picture 2 shows the results with the thicker deck: the deflection for the same node of 1191 is 0.013".
The horizontal deflection/drift is about the same for both thicknesses of steel deck. Why the high deflections for the thin plates?
Anyone?
I didn't look at your actual files yet, but it's important to note that out-of-plane stiffness of a plate is related to the thickness cubed (^3). So if you are comparing 0.5" deck to to 0.013" deck, the first is 56,895 times as stiff.
I understand that but there is zero vertical load on the plate so I wouldn't expect any deflection at all or minimal at the most.