Cantilever (gerber) girders.

The issues on lateral stability for such things you can study in Galambos V edition on Strength and Stability of Metal Structures. But you need not go so far and can use any proper code for the beams, Gerber or not, using the extant bracing as data for how well design the beams.

You can refer to the BS5950-part-1 which gives the effectve
length of cantilevers with various end conditions.

Canadian Institute of Steel Construction in 1989 published 'Roof Framing with Cantilever (Gerber) Girders and Open Web Steel Joists'. Excelent pub. re bracing, lateral instability, column/beam/owsj/steel deck interaction modelling, connections, frame stability, etc... and a good reference section too. especially interesting wrt. top of column/fixity/condition - effective length - moment - buckling. Suspended spans and cantilevers simple enough. Its the connection of all the components at the column and overall frame stability thats interesting and most important. The OWSJ manufacturers needs your design assumptions re column/owsj connection. We don't bother with Gerber systems unless the single storey 'Box' is greater than 40,000 sq. ft., the fabricators tell us the cost to fabricate all the connection details @ col. & ends of suspended span, isn't set off by the weight savings in the suspended span.