Welding Group 4 and 5 shapes definitely require some special attention. These are generally in compression as columns. These heavy sections have been known to exhibit segregation and a coarse grain structure in the mid-thickness region of the flange and the web. Because these areas may have low toughness, cracking might occur as a result of thermal cutting or welding. To minimize the potential of brittle fracture, the current AISC ASD Specification includes provisions for material toughness requirements, methods of splicing, and fabrication methods with an element of the cross section more than 2" thick intended for tension applications. You might want to refer to that if you have access to a manual. Here's a couple good articles that may be helpful to you.

The Challenge of Welding Jumbo Shapes

http://www.jflf.org/pdfs/papers/jumboshapes.pdf


Fabricator’s and Erector’s Guide to Welded Steel Construction

http://www.jflf.org/pdfs/papers/fabguide.pdf