Membrane Stiffness, F11, F22 Deck section properties are modeled with membrane behavior having in-plane stiffness but no plate-bending stiffness. The membrane stiffness for the in-plane forces F11, F22, and F12 of composite floors that consist of concrete on metal deck is based only on the concrete above the flute with thickness "tc", as shown in picture below. In other words, membrane the membrane stiffness contribution from the metal deck and the concrete within the flutes "hr" are is ignored by ETABS.
