The correct way to make welded splice joints in chassis members

In this video I debunk the myth that the correct way to make splice joints in chassis members is with an inclined or Z cut. These are scarf joints which have been used for centuries in timber structures but have no place in welded steel structures.
I also address the supposed regulatory requirements about this which apply in certain parts of the world.
Of necessity this is a technical video although I have tried to make the concepts understandable to non-engineers.
This is a re-posted video with the new content appearing at 16.30.

This video builds on ideas introduced in my earlier video on chassis repairs (please watch this first)

Those who want to learn more about the methods used in this video are encouraged to look at the following videos:

[johna]

As a young welder I worked at an outfit that repaired tankers that had been damaged or needed refurbishing. We lengthened frames, sometimes on brand new trucks. We cut the frame 90 degrees and butt spliced the extension piece. We backed it up with plates on the sides and stress relieved the area. That’s all we did, right or wrong and never had a problem.

[OverThickness]

This is the most eloquent, informative and educated "Shut up, I know what I am doing" I have had the pleasure of bearing witness to in some time.

[fk]

I am a welder for a Recreational Vehicle manufacturer in the USA. The plant I work at does predominantly motor homes. On some of the class A and Class C units we modify the frames by cutting them in half and adding or removing sections of the frame. When we cut a frame the cut is straight across the frame at a 90 degree angle or straight up and down much like you are advocating here. When the frame is welded back together there is a section of metal that fits on the inside of the "C" channel much like a sister rafter welded in place on the inside of the frame and if the chassis is shortened the two frame halfs are butt welded together. If the chassis is being lengthened than there is a section of steel of the same profile butt welded to the frame halfs on each end. The only reason I could see using an angle cut like you have discussed is if it is a tubular or box section frame and you make the cuts in opposite direction on the inside and outside. But I was also taught on most steel if the weld is done correctly it should be stronger than the metal itself. I've welded a few things together only to have them break in an area adjacent to the weld, but this is on repair work, not new fabrication. Sometimes we welders like to layout the weld and make it more complicated than it has to be if for no other reason to show off our skills and convince those looking at the weld in the future that we knew what we were doing when we did it.

[GordonBuck-ys]

As a retired mechanical engineer it was wonderful to be reminded of Mohr’s circle which I studied 45 years ago as part of Statics, which is the realm of Civil Engineers who design and build structures where the dynamic load s are small compared with the static loads.

For statically loaded structures the failure mode you considered, plastic deformation is appropriate. However, dynamically loaded structures often fail catastrophically by fatigue cracking.

Design and fabrication of joints to be resistant to fatigue cracking is a specialised discipline that is suggest you investigate further, perhaps in your next video.

In light duty motor vehicles, particularly the off-road variants, it is not uncommon for fatigue to be the most relevant design failure mode.

[miketrusky]

All Class 8 truck manufactures have bulletins on how to repair their frames. Great place to learn what they recommend.

[mikeitsprobablyfine]

I Stumbled on this video, Its nice to see someone using Mohr's circle in a practical example! I primarily use EC (Euro Code) myself and think the joint configuration has more to do with the allowable fatigue stress. The maximum stresses, as you very nicely calculated and presented, are irrelevant of the joint cut. However the fatigue stress is greatly dependent on the joint used. According to detail categories within the tables of chapter 8 in EN1993-1-9, longitudinal welds in joined members will have a higher Δσ (allowable fatigue stress range) than perpendicular (butt) welds. Using cover plates (fish plates or stretcher plates) further increases the fatigue resistance of the joint. Furthermore, i think the welding requirements for a full section butt weld are often neglected and dictate greatly the final strength of the joint (and for closed sections is nearly impossible to properly implement). The joint configuration to use is for me debatable, but to me the problem is more to do with fatigue resistance rather than strength analysis

[davidjohnson]

Well done. As a welder/fabricator (also with a math degree) I fully support your findings. I also appreciate your simplified direct presentation.

[stephenhans]

You are correct! Look at oil ships construction . Rarely do you see anything other than a vertical or horizontal be weld . ships must flex loaded or unloaded must be loaded and unload due stress on cargo areas. But at Sea they encounter forces that are incredible from multiple directions at the same time. Thank you for the time explaining stress and showing the process of figuring out.❤❤❤❤

[graemewhite]

I'd forgotten all the techy stuff I learned at college and the equations were going over my head a bit, but I was just starting to think "I bet this is a hangover from those 'wood botherers' habits ?" and had a little chuckle when you came to the same conclusion !

[briangough]

I agree with you. I have extended a truck chassis using a square butt joint. This happened in Queensland, Australia. about 1970. Preheating (warming), and low hydrogen electrodes used.

[brotherlove]

wonderful video. As a long term off-roader, I have read in numerous articles the same as you heard. But in most of these articles, the evidence cited was in heavy competition rigs there were broken frames and the breaks were discovered to be at the frame welds.

[colvinator]

Well ! Very interesting indeed. Whilst I have a 50 year history of electronic and electrical engineering, I'm convinced by your analysis of the chassis joints. I would have thought an angle joint would be required. Not any more !. Thanks a lot for the video. Colin ( Wakefield )

[daviddroescher]

Square cut seams to be top notch for stationary items. Avation chassis repairs, eg dented portion of tube, are called out for angle cuts and fish plates.
This is where some of the idea that angle cuts are better. The problem i see with this approach is that round tube, Square and c channels are different in there bending properties.
Thank you for the clear explanation. It will beapplyed when i splice a chevy diesel 4x4 front frame section into my 2wd 1st gen dodge Cummins

[petermcneill]

I agree completely, while I wasn’t able to follow all your maths, I can agree from nearly 20 years experience on and off of doing chasing repairs that all the straight mig welded but joints iv done have been so far completely trouble free

[timothywilliams]

With the correct filler and proper welding techniques you are right. The reason some area's do this is because poor and improper welding. More overlap gives more weld area to bind it together. Also long term fatigue is probably being considered.
In my area it's also common to see reinforcing plates welded on over the joint.

[jeanphilippepoirier]

The most important point to remember is the overall strenght depends on the base materiel. In an old rusted frame, this can be a concern. As a precaution, it might be interesting to double up the material. So adding a backing plate might be more useful than the type of joint.

[kevinmartin]

I served my apprenticeship in the 70s on Foden trucks, and their inhouse built chassis were legendry. The only inclined joint in the chassis was for flitch plates ends, and these were only bolted in. So I have always butt welded my chassis repairs!

[BasementEngineer]

Hmmm, allow me to chime in here with my experience as a DIY and Professional Engineer now retired.
I had a case where an overhead travelling crane was shut down by the DoL and I got a call for rescuing the little fabricator.
To cut a long story short, the main beam of the overhead travelling hoist had a perpendicular "square" welded joint in the middle of its 30 ft span!
I advised the owner to contact an inspection firm that specialized in structural steel examination and have them do a magnetic particle examination of the weld on all surfaces of the beam, and to evaluate any indications in compliance with the appropriate code.
If all was well with this examination, it was to be followed by radiographic examination of the weld to insure that the weld interior was also sound metal (No partial penetration welds here!). Again, any indications were to be evaluated in accordance with the applicable code.
Only with this examination and sound results would I certify the load capacity of this hoist.
Moral of the story: For a butt joint to be of the same or better strength than the parent metal, the weld must be examined to ensure its quality.
And structural welds for buildings, bridges, oil platforms etc. are thusly examined to ensure this required quality.
The same cannot be said for DIY work on automotive frames. Here a longer weld with greater cross sectional area is worthwhile to make up for lower quality welding and unknown steel properties.
As an aside, LENGTHENING a chassis without due analysis using known values as shown herein, is wrought with its own difficulties.

[tano]

As an engineer, I entirely agree from a stress analysis point of view.
I suspect that the guys advocating for splice style joints are doing so more because (in a "working in your shed" context)
1) the longer joint offers more opportunities to clamp the two sides to ensure good alignment, which would not so much make it stronger than a properly executed full penetration butt weld, but reduce the risk of misalignment between the two sides that would introduce various problems with stress concentrations and load eccentricity etc that would weaken it,
and
2) a longer weld at a gentle angle is closer to a horizontal position weld (and is therefore easier for an amateur to get a good weld) than the vertical up or vertical down alternative for welding the beam web. Obviously this isn't a concern in a factory for a properly trained welder, but for the guy in his shed working in an awkward position it may be easier, and therefore it may reduce the risk of a poor weld, and again this would potentially reduce risk of failure from introduced defects. Not so much making it stronger as reducing risk of making it weaker.

Of course, I am not advocating for back yard DIYers to attempt chassis mods; I can't think of a more dangerous field of endeavour for an MBA (mediocre but ambitious) welder... but people do try it, and in that backyard world, the splice joint might in fact have a slightly lower chance of failure and hence be part of the accepted wisdom for a reason. But (to repeat for emphasis) obviously I would infinitely prefer the DIY guy to use some common-sense and bring in a qualified welder for the frame splice welding at least.

[annelarrybrunelle]

Used to work in an auto frame shop. Repairs of a rusted or broken frame usually require patch pieces, and these in sometimes very difficult locations. There also may be unmeasurable consequences to the welding heat of the strength of the parent metal. Consequently, the experts I observed (I am not one for this kind of work) used a strategy of smaller patches and much weld bead to achieve multiple connections of surfaces and, of course, best penetration of the weld, along with a distribution of load among several weld joints. Additional considerations arise from the particular channel cross section of the frame, its thickness, and accessibility at the point of repair. I also would suppose that the kinds of loads on a vehicle frame differ from those in a building.