A straight staircase can be designed three different ways. the first design goes from top to bottom in a straight line. the second design is l-shaped. it goes straight up and turns right or left on a landing. the third performs a t; it goes up and turns both to the right and left for a landing. spiral staircase around a fixed axis. often a floor-to-ceiling pole and stairs wrap around it. 3d modeling of the mentioned options.