As our friend Abaci09 said, there is no such limitation. The source of this; When the plate is made, the inner height is 280-10=270 clean openings. Or 265 clear openings even if the floor is 15 cm. However, when the hollow block is made 280-32=248 cm, if the rib beam is 35 or something rather than 32, with plaster etc., this falls almost below 240. This is an uncomfortable interior height that would be considered flat for normal buildings such as residences etc. Therefore, 300-32 = 268 cm is preferred.