welding across i-beams

huangyhg

welding across i-beams?
what is the reason not to weld across an i-beam.  i have always been told that it is ok to weld along the axis of the beam but not to weld across it.  why is this?  please send reply's to
transverse welds are permitted. however, you need to be aware of fatigue since the weld acts as a stress raiser. if you have a copy of aashto, there are illustrations of the various types of fatigue categories and the allowable stress range for each.
in addition, if you are welding across an already loaded beam, you could develop a plastic tension flange and loose the whole thing.
as has been said, weld beads across the flange may be acceptable in certain applications (better have that rfi in hand first) but is not generally recommended for the reasons stated.
as to imminent failure? the possibility always exists particularly on small stuff, say something like a loaded w6. in doing the siesmic repair in los angeles before i retired we generally unloaded a beam with jacks before cutting and  welding. there is no good way to repair some of the stress cracks without welding across the flange. a great deal of ndt was  used after welding, also.
rod
hi iclampers:
okay, there's a few concerns here:
1. loaded and unloaded beams.
2. statically loaded and dynamically loaded beams.
you'll need to consider which case(s) you're dealing with.
bridgebuster is correct to cite the fatigue and stress raiser issue.  by his handle, he is involved with bridges, therefore dynamic structures.  i've never been very concerned with welding across a flange on a statically loaded structure.  i've seen hundreds of spliced beams.
i've always been concerned whenever welding on any loaded structure.  common sense should tell anyone that.  steel does not respond well to stress when it is cherry red.
evelrod is right: use jacks or whatever to get the load off the beam, then weld.
iclampers,
in the flat position, the base metal provides support for the molten pool of weld metal. therefore, this position provides for the fastest deposition rate and the most economical weld. welding in the horizontal position is similar, but slightly less efficient. welding in the vertical or overhead position requires slower deposition rates to maintain the integrity of the molten pool against the effects of gravity.
for other details refer aisc manual under welding chapter
kvram
kvram, i was a certified welder once upon a time and i still weld on a regular basis with my racing and old car hobby.  i must take issue with your assumption that in 'flat welding' "base metal provides support..." there are definate limits to how much metal you can carry without incuring contamination and thus failure in flat welds. in fact,imho, the hardest weld to pass ndt consistantly is 'flat'.  overhead is about the easiest to keep clean and verticle is able to carry the most weld metal without slag inclusion.  here in the l.a. area aws specs don't allow for "weaving" a pass. stringers only these days. with that restriction, any position is as fast as any other.  
rod