Yet another view of the roof trusses at Moynihan Station, built when it was a mail-handling facility to allow for a big open floor for sorting machinery. The roof truss is at the upper right; one of the main columns is below it, two side trusses, supporting the edges of the raised skylight roof are on either side of the column, and a girder for the non-skylight portion of the roof is visible behind the column at the lower left. (The girder is covered with spray-on fireproofing, which is why it’s fuzzy. The other steel is either fireproofed with intumescent paint, or in some other manner.)
First, the roof trusses are all double trusses, as you see here, with two top chords, two bottom chords, and two sets of web members, and links between the pairs of chords to make a big box. Next, the portions of bottom chord closest to the column aren’t doing much: they help with lateral stability, but are not carrying the main roof load. You can tell because (a) the last web diagonal goes up to meet the top chord at bearing on top of the columns and (b) the connection of the bottom chords to the column are nothing much.
Some fun – in the engineering sense – can be had by looking at how the loads from the trusses reach the body of the column. The column itself ends at the bottom of the lower chords of the side trusses, but there’s an extension above that, with a bunch of flat plates as the splice. Each of the side trusses ends at a vertical that directly abuts the extension, and it looks like the load from those trusses is being transferred to the column flanges in line with the front and rear faces of the column. Similarly, the roof-truss top chords appear to be bearing midway between the flanges on the column front that are in line with the sides of the column and the stiffeners riveted to the front of the column extension. Those stiffeners continue as separate pieces onto the main column, held in place by 25 rivets each. In other words, you can see where the load is by looking to see where a lot of rivets are in line with a piece of steel.
TL;DR: Good steel design.
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