I assume you are using the Eurocode (EN 1993-EC3) and the connection type called Smart FinP. In my sample with a singe column of bolts, if I use zp = 50mm, I also see 50mm used in the design report for the Bolt group shear. When switching to two columns of bolts the user is still specifying the distance from the outer, closer, column of bolts to the edge of the girder, but the calculation report will be based on the distance between the centroid of the bolt group to the same point. In other words, the zp on the report is larger by 1/2 p2.
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