Preventing bends and bows when welding?

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I had some slight bow in two of the piece at work today. The base material was 5" channel, and I was welding three pieces of 1 1/2x1 1/2x1/4" angle to the bottom (flat) side of the channel (channel was 6' in length). It was bowed after I was done welding, even though laid down some stitches at both ends and in the centre as well, so it was tacked to the table really good. But after taking it off of the table it was still bowed? Does anyone have any tips to help with this? 3/16" fillets completely around all 3 pieces of angle iron (which my boss said is a bit excessive, but that's what the drawing calls for so that's what needs to be done).
Reply:I had a PM about distortion, tried to answer it, but my explanation wasn't good.  My fault.Look at what made the metal bend!  Try to imagine the force that screwed it up, then counteract it.  Most times you can't prevent the bend, but you can remedy it by heat shrinking.  (Forget the advice of the master bender)  You have to heat, in conjunction with opposing force, to make it right.  I've tried over the years to put up tutorials on this crap, but it seems to fall on deaf ears.  You can either pre-bow the work, or bring it back after welding by heat shrinking it.The process is simple.  Look at what happened, imagine where the metal stretched to make the bow, then heat in that spot to shrink it back to shape.  Coax it back with some applied force.Sounds like Pablum , but it works.Got some heat shrinking threads out there, can't quite remember, but one coming up tomorrow.  AND IT WORKS"Any day above ground is a good day"http://www.farmersamm.com/
Reply:Strongbacks only work some of the time.  In many cases the stored stress will show up when the strongback is removedYour job is to "normalize" the metal, and that's actually a bad word.........normalizing infers heat treating with soak times....etc.  You just have to use heat to counteract the heat you put in."Any day above ground is a good day"http://www.farmersamm.com/
Reply:Oldie but goodie. Prevention and Control of Distortion in Arc Welding
Reply:It is always a problem when seal welds are called for.  Often the pieces are galvanized and any open crack will invite water, rust and staining.  If you are doing more than one you should just clamp back to back with a shim in the center so that they are prebent before welding.  Guessing how much to shim is the trick.  After that you have to flame shrink unless you have a big press.