Beams and girders
Because the sizes of rolled beams continued to be limited for many years, in some buildings other means were found for forming the supporting girders.
There were a number of notable improvisations to create beams with the required properties. These included the following examples:
· in the Cooper Union building a deep beam was formed by fastening two rail sections together by bolts through the flanges.
· Cooper and Hewitt show a floor constructed of rail section joists resting on wrought iron box beams assembled from plate with channel sections to form the top and bottom flanges
Until deep broad flange rollings became available, the floor joists had to be carried on girders which were made by riveting angles and plates together. This was also before the use of suspended ceilings and so an ideal layout for the structure was to arrange the girders to coincide with the positions of internal walls to avoid them projecting below the ceiling.
In the 1920s improvements were possible in the design of plate girders because universal plates were introduced.
There is an interesting cycle of development because deep rollings were a more efficient way of forming girders than riveted construction although the limitations of rolling meant that plate girders continued to be used for longer spans. In the 1920s improvements were possible in the design of these because universal plates were introduced, ie. plates rolled on their sides and edges simultaneously. More recently, however, welding techniques have improved. Standard deep sections were once fabricated by cutting I-section rollings and welding plates into the web but now it is economical to have sections made to order by welding them from plates so that tapered beams, and beams of any section, can be obtained.
Bolted joints were used in some early frame buildings but were not satisfactory.
Joints in some early frame buildings were bolted, but these were not satisfactory because their use led to cracking of finishes. As a consequence rivets became universally used. Today improvements in bolting have led to the obsolescence of rivets.
