Typically these towers are designed (by the manufacturer) for whatever loading conditions are specified by the end user aren't they? Depending on the configuration of the tower and how it is supporting the lines, a broken wire may or may not impact the design, as a deadend is already being designed for full line tensions. The major consideration would be the torsion that could be introduced by a broken wire, again this will depend on the geometry and may or may not have an impact on the overall design. In general, most of the designs for deadends that I have seen are not controlled by a broken wire case, however most of my experience is with bent plate, tapered tube structures, not lattice.
