There's a lot of information out there for manual sizing. See Kelloggs Design of Piping Systems and Peng Pipe Stress Engineering. Coade/Integraph/etc have released numerous white papers on various topics, usually written by Dave Diehl and they typically have examples. See the link below.

With regards to seismic/wind, one of the issues is that the force results in significant lateral deflection of the pipe. Think of vertical supports - we put supports every so often to ensure the pipe doesn't sag. Lateral supports can be overlooked, so if you have a strong lateral event the pipe will have severe deflection.

http://www.coade.com/Mechanical%20Engineering%20News%20Index.shtml
(Apparently this link is dead...does anyone know the new link thru hexagon?)