The concepts of pre-stressed steel fabrication have been around for ages, but they are not often applied for simple welded structures like trailer frames or crane stands. 
I’m not talking about huge pre-stresses structures like bridges. And, I’m not referring to full pre-stressed beams that normally dominate this kind of conversation. This is more about the practical implementation of some of the basic principles to get results we want in our DIY projects.
Feeling the Heat
For purposes of this Mechanic’s Tip, we’ll limit the discussion to areas of heat distress from welding — causing weld warp. (Or, sometimes we call it “Weld Pull” or “Draw” or welding distortion.) See this post about avoiding beam weakening, and this post for the engineering.
We all know that the heat of welding causes materials to “move” or warp, and if you’ve been welding a while you’ve learned how to compensate. Sure, there are lots of small steps that can help, so these ideas do not overshadow those. Things like lot’s of tacking and moving around the framing are always important. (Setup is also a key — like this example with tips for a trailer frame.) However, these pre-stressed steel fabrication techniques can certainly help in some circumstances. Think of it as another tool in your box.
What is “Weld Warp”
How many times do you weld 2 pieces together and afterword realize one is no longer straight. The classic case is the “T” connection. After the weld, the crossing piece is no longer straight. This is particularly noticeable when the crossing piece is thin material, but it’s also a problem when welding heavier stuff like angle or channel.
The amount of weld warp (distortion) has a lot to do with where the welds are placed, and what the beam sections are. Obviously beefier beams and/or smaller weld areas affect this less.
The first time I welded a tube to the center of a piece of channel, I was horribly disappointed that the channel (the crossing piece) was not straight. I really couldn’t see the warp, but it was obvious when I stood it up. The channel worked better as a rocking chair than as a sturdy stand. Heat distress had caused the channel to warp slightly. So, here’s how I solved the problem for future builds.
Solutions with Pre-Stressed Steel Fabrication
The idea is to pre-stress the member in a direction opposite how it will bend under welding heat. To do this, First, take a guess as to how much the part will warp. It’s usually not much. For me the measurement was easy because the early sample was right there. In this case, it rose about 3/32″ from the center (point of welding) to each free end.
Second, set up the welding so the beam is “bending” that amount in the opposite direction. See the photo for details of how I do it for this gantry crane leg situation. The 2 bottom beams are the same. The 2 chunks between them in the middle are each 3/8″ thick. The angle parts at the ends are 3/32″. By clamping the beams together on each end, it makes a 3/32″ pre-bend for each side.
Third, weld it in place as you normally would to minimize warp — still with the jig. As in the photo, tack the main beam at each corner, then after the pre-stress, tack the angle braces in place.
(Using lots of clamps – or weights – to hold things in place is always a good idea.
Welding while in stress will do 2 things: First, the heat will relieve the stress at the point of weld allowing it to “relax” some. Second, it will pull anyway as it cools, but it will pull from a position of stress relief — making it come back to nearly flat (if you’ve done the pre-stressing correctly). This is the technique of pre-stressed steel fabrication.
Aluminum
A word of caution when welding aluminum (or for that matter when welding other materials). The metals act different when heated, so play and test it before trying to copy the Pre-Stressed Steel Fabrication methods. Also, read this article too about Aluminum Trailers.
The Best Way To Learn
As usual, the easiest (and best) way to learn is to try it. In the photos you can see the way I jig things. The bar clamps secure perpendicularity. The shims (wood) under the beams assure vertical position. The (parallel) beam at the bottom clamps to the real bottom beam with proper spacers between so it bends and pre-stresses properly. The pre-stressed steel fabrication never turns out perfectly flat, but in this case, it’s more important that the tips of the channel are the points of contact, so just a little over-bend works well. Since this is a crane stand, weight from the load flattens it right out making it very stable.
In the final photo, the stands are side-by-side to check perpendicularity and the pre-stressing effect. After this photo, each leg was set again with the clamps to finish welding. I like to verify that things are correct before completing the welds. It’s more work, of course, but any mistakes or oddities are much easier to correct — but that’s just me.
Experimenting like this is especially good if you’re just starting out with welding. When you choose a new welder and begin to experiment, you learn a ton.
Good luck with these pre-stressed steel fabrication techniques. Apply them as needed. This also works very well with certain areas of a trailer frame.
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