Pre-Stressed Steel Fabrication
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 welding distortion.) See this post about avoiding beam weakening.
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 have you welded 2 pieces together and afterword realized that 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 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 readily apparent 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 that was easy because I had a sample right there to measure. 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 can be seen here, the main beam is tacked at each corner, then after the pre-stress, the angle braces are tacked in place.
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 pre-stressed steel fabrication.
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 pre-stressing. 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.
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.