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Interesting question: 1. Yes you can model them as inclined shell, and detail the stair and slab connection accordingly. Half landings make the modelling and force transfer a bit tricky. 2. In many high rise buildings, the staircase is enclosed in a vertical concrete shaft (walls)- for fire protection reasons. Here the concrete shaft (housing the stairs and lift cores) tends to performs building stability function also. 3. Nowadays the core walls are cast by "slip-forming" hence many stairs are precast, lowered down into the shaft by crane and fixed to the shaft walls. In the past as the core walls were 'jumped' level by level, the staircases were cast in-situ with slab/walls. Speed of construction and project programme become a consideration too. 4. Apart from a structural engineering role, stairs also perform a safe protected fire escape route function, a legal requirement in many countries' building regulations, hence concrete or block wall enclosures are provided around stairs. 5. Therefore, if the staircases are cast in-situ (not precast) then detail and model them accordingly.