Random Frame Welding Question

[JHZR2]

I am replacing the front bumper on my 96 Ram, so was looking through the frame and bumper section of the FSM. I happened upon a section on frame reinforcement (which is why this thread is titled “random”), which caught my interest because of the picture.

The FSM shows as follows:

I think this is called a fish plate.

The instructions say this:

Note the comment and image showing the top and bottom flanges shorter than the frame, and not welded to it.

If I undersrand frames correctly, the c channel type frame is similar to an I-beam. The top and bottom are under compression and tension, while the vertical part (the web as I know it) is just there to hold the thing together. That’s why you can’t drill or damage the flanges on a beam, but can drill through the middle.

My curiosity is two things:

1) why would they want the flanges to be shorter (less wide) than the original? I can appreciate that if a frame was cracked and they did this, the top (uncracked) might be 2x as strong, but the bottom (cracked) would have minimal strength. Wouldn’t you want it as wide as the original to have more strength?

2) I can understand why you wouldn’t weld the front/rear portions of the overlapping reinforcement on the horizontal (flange) sides. Since the frame compresses and goes under tension, those points that are perpendicular to the flange would crack. But wouldn’t you want to weld the reinforcement to the flange in the primary direction (front to back)?

It seems like the FSM approach only stiffens the webbing, which is not what gives it strength.

Where am I wrong?

 

[wwillson]

1) why would they want the flanges to be shorter (less wide) than the original?

Could it be that it gives you a better weld surface?

 

[Trav]

All I can think of is they want space for applying rustproofing. You can make a good seal this way but just on the edge it wouldn't be very good.
You wouldn't weld the upper and lower as the weld can weaken the metal causing it to crack again in front of the new weld.
As you can see on the side they do not weld in a straight line for the same reason.

I am not a pro welder just some thoughts.

 

[JHZR2]

Could it be that it gives you a better weld surface?

That would be my initial thought, but look at the drawing, no welding allowed on the flange sections... Only on the (veritcal) web.

Thus the question. Seems strange that you wouldnt weld along the long sections of the flanges, especially if you cut them short.

 

[JHZR2]

All I can think of is they want space for applying rustproofing. You can make a good seal this way but just on the edge it wouldn't be very good.
You wouldn't weld the upper and lower as the weld can weaken the metal causing it to crack again in front of the new weld.
As you can see on the side they do not weld in a straight line for the same reason.

I am not a pro welder just some thoughts.

OK that would explain the no welding on upper and lower flanges. If Im correct, that's where all the forces are. Certainly dont want that cracking due to weld-induced weakening of an already questionable part.

 

[Danno]

Does the manual say anything about what is a repairable crack vs an unrepairable crack?

 

[Trav]

OK that would explain the no welding on upper and lower flanges. If Im correct, that's where all the forces are. Certainly dont want that cracking due to weld-induced weakening of an already questionable part.

I would assume many of these cracks are due to plowing in this area anyway.

 

[JHZR2]

Does the manual say anything about what is a repairable crack vs an unrepairable crack?

Honestly, I don’t recall. I would need to go back through the fsm. I don’t have a cracked frame fortunately. I found the image interesting which is why I took the screen shots. I’ll have a look again.

edit: nothing in FSM.

I did find this interesting graphic from International Harvester:

Interesting that this one was placed inside, was not a complete “C”, and welded parallel on the flange!

 

[Trav]

I have a feeling this is a case of there is more than one to skin cat as they say depending on the location of the crack and location of the piece.

 

[Cujet]

Welding is generally forbidden on frames, mostly due to the lack of sufficient skillset and methods, and not for engineering reasons. However, when done properly, it's not a problem.

A great resource for welding of structural components is the AC 43-13. The methods are simple, easy to understand and do apply.

 

[DuckRyder]

Without actually reading the pertinent sections of the FSM or associated manufacturer or ICAR bulletins it is hard to say with certainty.

However generally there are two considerations in these sorts of things 1) Heat and 2) Strength. (and heat really is a concern because it can affect strength.)

You do not want the frame weaker NOR stronger than the original unaltered part.

 

[JHZR2]

Welding is generally forbidden on frames, mostly due to the lack of sufficient skillset and methods, and not for engineering reasons. However, when done properly, it's not a problem.

A great resource for welding of structural components is the AC 43-13. The methods are simple, easy to understand and do apply.

Yeah, somehow they extend frames for limos and upfit trucks.. my truck didn’t have any frame adaptations, the big aluminum bed was welded and fit to the mounting points on the frame. I’m not sure if my truck was a chassis cab or a truck with bed removed. Some day I’ll pull the build sheet.

Without actually reading the pertinent sections of the FSM or associated manufacturer or ICAR bulletins it is hard to say with certainty.

However generally there are two considerations in these sorts of things 1) Heat and 2) Strength. (and heat really is a concern because it can affect strength.)

You do not want the frame weaker NOR stronger than the original unaltered part.

Well I provided the FSM section. Interestingly it also talked about use of heat for frame straightening. As I recall, it allowed 500C or so for heating the frame to straighten.

I get it that welding heats local spots much higher, but that’s just in the web per the FSM, and that doesn’t really handle any of the load, which is why you can drill through it...

This was all general interest because some of the info caught my eye... I would like to learn to weld (not frames) someday...

 

[Ihatetochangeoil]

Does the manual say anything about what is a repairable crack vs an unrepairable crack?

No cracks are unrepairable. You can build a bridge of solid metal across the Mississippi river. How much money do you want to spend? I can tell you some things you don't want to do; don't weld transverse (across) the direction of computed stress...This is a prohibited weld type in AWS D1.1 Structural welding code.
This is illustrated in JHZR2's 4th post above as the original crack. Very likely why they recommend a reinforcing plate to extend 4" each side of the crack. BTW, the word "crack" is about the nastiest word that can be used when we are talking structure.