Why do Structural Engineers specify steel cap plates at the top of a steel column? I get asked this all the time in the field. Installers want to know why they can’t weld the top of the column directly to the bottom flange of the steel beam.
The web or vertical portion of a typical wide flange steel beam (I-Beam) is what transfers shear or load to the column below. To adequately transfer large loads, the load must be applied uniformly over a large enough area, say several inches or more. Another way to think of this is providing adequate bearing. We can’t expect the steel beam to transfer its load to the column with a small bearing area, right? If the steel beam web does not have adequate length along which to transfer the load it could buckle, which we call ‘web yielding’ or ‘w’eb crippling’.
When we introduce a large point load from above, the given area we have to transfer the load may not be enough. It is at these locations that Structural Engineers and steel detailers specify web stiffeners to prevent buckling.
Please contact the Structural Engineer involved with your project, before you order your steel, to make sure you have the correct cap and base plates for your columns. Most engineers will specify a bolted connection between the cap plate and steel beam, to comply with OSHA regulations and for ease of construction and inspection. It is important that the steel beams be fabricated with holes in the correct location for installation prior to delivery. Discuss using slotted holes with your engineer to give more installation flexibility in the field. Our details also show a welded connection between the plate and steel beam, so that the bolts, which may potentially interfere with building finishes, can be removed once the welding is completed.
Learn more about Structural Steel by visiting other blog posts in our Structural Steel Category.