The primary framing system
In Conventional steel buildings, mill-produced hot rolled sections (beams and columns) are used. The Size of each member is selected on the basis of the maximum internal stress in the member. Since a hot rolled section has a constant depth, many parts of the member (represented by the shaded area), in areas of low internal stresses, are in excess of design requirements.

Frames of pre-engineered buildings are made from extensive inventory of standard plates stocked by the PEB manufacturer. PEB frames are normally tapered and often have flanges and webs of variable thicknesses along the individual members.The frame geometry matches the shape of the internal stress diagram thus minimizing material waste and reducing the total weight of the structure.

Conventional Steel Frame	Pre-Engineered Steel Frame

The secondary framing system
"Z" shaped roof purlins and wall girths are used for the secondary framing. They are lighter than the conventional hot-rolled "C" shaped sections in conventional steel buildings.
Nesting of the "Z" shaped members at the frames allows them to act as continuous members along the length of the building. This doubles the strength capacity of the "Z" shaped members at the laps where the maximum internal stresses normally occur.