Hello homealone, your questions are very open-ended. All of these variables would likely be different for the various welding processes and also the applicable codes and uses in various industries and applications. Pipe, structural steel, equipment and machinery, various types of materials and alloys can all have different specifications and settings depending on a host of factors. Generally there are engineering considerations for justifying certain gaps, lands, and bevel angles. Bevel angles will also have a great impact on required weld deposition, which can affect stresses and heat issues for alloys and materials. Joint access can determine joint type and fit in many instances, if you can only have access to one side for welding the joint may be quite different than if there is access to both sides of the joint. Generally, a two sided access allows for less weld metal deposition than a single sided joint and can also help to limit stresses in certain cases. If you post some more specific scenarios, it is likely that the responses that you receive will be more exact. Sorry for the generalizations, but this is the best that I can do with the information that you have supplied. There are many on here who can answer very specifically with regard to bevel angle, land and gap dimensions and give you very specific reasoning for why these items are important and relevant. Best regards, aevald
By swnorris 
Date 02-15-2008 15:40
Edited 02-15-2008 15:46
There are joints you're describing that can be found in the AWS D1.1 Structural Welding Code. These are either prequalified complete joint penetration details or prequalified partial joint penetration details. The minimum "root gap" (root opening) is based on whether you're prepping for a CJP or a PJP weld, and whether you'll be backgouging or not. Both the PJP joint details (no backgouging), and the CJP details that are to be backgouged require less "gap" (0" to 1/8") as opposed to CJP joint details that require 1/4" gap for 45 degree bevels, or 3/8" "gap" for 30 degree bevels. The "root land" (root face) generally doesn't apply to CJP joint details unless the joint is to be backgouged. The "root land" also applies to PJP joint details. These joint details will vary some between the different prequalified welding processes (SMAW, GMAW, FCAW, SAW), and, there are various allowable tolerances for "root gap", bevel, and "root land" based on the specific joint detail and the welding process used. Other CJP or PJP joints with the prequalified welding processes, or other joints with other welding processes can be used, but if D1.1 is invoked on a project, the joint and process would have to be qualified by testing and may be subject to approval by the Engineer of Record for the project.
