Use a membrane element and check the "Use Special One-Way Load Distribution" box in the Wall/Slab Section dialog. Make sure the program has chosen the correct direction -- there should be an arrow shown in the direction of the span.
Why use the membrane option and not shell? I thought membrane only allows in plane stresses. You can still choose the one-way load distribution with a shell element.
Selecting a membrane option for a floor element in ETABS triggers ETABS to use its internal tributary width algorithms to calculate the load distributions as opposed to using true bending distribution as with a plate or shell element. This (using a membrane element and thus ETABS built in algorithms) results in much faster run-times than using a plate or shell element but can lead to incorrect answers if your floor plate does not behave as their algorithms assume (e.g. if the continuity effects are great etc).
