All stairways shall be constructed to sustain, with a safety factor of at least four, a live load of one hundred pounds per square foot. The width of stairways shall be at least four feet, the treads shall not be less than eleven inches and the risers not more than six and one-half inches. All stairways shall be on straight runs with all changes in direction made with platforms. There shall be a substantial handrail on each flight. No door shall open immediately on a flight of stairs but there shall be a landing at least the width of the door.