The effective width of the concrete flange is determined in accordance with ANSI/AISC 360-10, section I3.1a.
Orientation of the slab, as determined by the direction X of the slab in the Robot model, is used to specify the effective thickness of the slab (in accordance with ANSI/AISC 360-10 I3.2c). The concrete below the top of the deck ribs is ignored for any slab with an orientation perpendicular to the beam. If the deck is parallel to the beam (or nearly so, the assumed tolerance is 10 degrees from parallel and cannot be modified), the concrete in the deck ribs is taken into account with Deck Area Factor. This factor represents the concrete proportion in the deck cross-section, and is calculated automatically based on the geometry of ribs. In determining the strength of the concrete deck and the location of the Plastic Neutral Axis, the thickness of the concrete slab t sand the thickness of the deck t d (that is, the separation between the slab and the steel section) are used. When the deck is parallel to the beam, t s is increased by the depth of the deck ribbing multiplied by the deck area factor, and t d is reduced by the same amount.
After this correction is made, the strength of parallel and perpendicular slabs are treated identically.