The structural system of pre-engineered steel buildings give it its speed and flexibility. This system consists of factory-fabricated and factory-painted steel column and beam segments that are simply bolted together at site. The columns and beams are custom-fabricated I-section members that have an end plate with holes for bolting at both ends. These are made by cutting steel plates of the desired thickness, and welding them together to make I sections. The cutting and welding is done by auto or semi-auto machine system for speed and accuracy. This production line style of work makes for great speed and consistency in fabrication. The shape of the beams can be tailored to optimum structural efficiency: they are deeper where the forces are greater, and shallow where they are not. This is one form of construction in which the structures are designed to carry exactly the loads envisioned, and vice versa.