International Building Code - IBC 2000
4" sphere – general – to a height of 34"; 6" sphere – at triangle formed by riser, tread and bottom rail; 8" sphere from a height of 34" to 42" Exceptions: 21" sphere for elevated walk for electrical, mechanical and plumbing systems and Group I-3, F, H, or S occupancies, balusters, horizontal intermediate rails or other construction.
International Building Code - IBC 2003
In areas which are not open to the public within occupancies in Group I-3, F, H or S, balusters, horizontal intermediate rails or other construction shall not permit a sphere with a diameter of 21 inches (533 mm) to pass through any opening. In assembly seating areas, guards at the end of aisles where they terminate at a fascia of boxes, balconies and galleries shall have balusters or ornamental patterns such that a 4-inch-diameter (102 mm) sphere cannot pass through any opening up to a height of 26 inches (660 mm). From a height of 26 inches (660 mm) to 42 inches (1067mm) above the adjacent walking surfaces, a sphere 8 inches (203 mm) in diameter shall not pass.