I don't always get the legalese, but here we go:
http://www.texasbuildingtrades.org/viewarticle.asp?a=7380
"(5) Construction workers not admissible. Aliens seeking to enter the country to perform building or construction work, whether on-site or in-plant, are not eligible for classification or admission as B - 1 nonimmigrants under section 101(a)(15)(B) of the Act. However, alien nonimmigrants otherwise qualified as B - 1 nonimmigrants may be issued visas and may enter for the purpose of supervision or training of others engaged in building or construction work, but not for the purpose of actually performing any suc h building or construction work themselves."
However, does look like they can come under the H-2B visa
"Worker Skills: Oil and Gas Drilling, Oil and Gas Production, Pipeline Construction, Pipeline Maintenance. Inspectors, Wild Well Control and Suppression, Diesel Engine Mechanic, Gas Engine Mechanic, Manual Transmission Mechanic, Automatic Transmission Mechanic,
Welder (AMSE, AWS, etc/ Coded), Welder (Non-Coded), Electrician, Heavy Equipment Operator"
http://usah2bvisa.com/index.php?option=com_content&view=article&id=4&Itemid=6
Have not found how many visas were given to welders. BUT my guess is that welders fall under union contracts and therefore don't depress wages; programmers, DBAs, etc don't have unions.