In AISC table J3.2, there is a foot note regarding fastener pattern length. I am having hard time understanding fastener pattern length definition. Is there a figure or something which they refer to, which shows the 38’’ dimension.
The 38" length applies to 'end-loaded" connections. In the example you describe, the load along the bolted connection is applied incrementally, along the depth of the beam web, and it leaves the angle and goes to the support incrementally, at each bolt row.
An example of an end-loaded connection would be a flange plate axial connection on this girder. The flange force is in the plate all at once, but has to distribute to the beam along the length of the connection. If the bolt pattern exceeds 38" in length, you need the 0.833 factor.
I’ve heard this described in seminars, but I don’t recall seeing a sketch that shows it.
This Steel Quiz discusses the 38" limit and what an end-loaded connection is:
Above is a snippet.