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Floor failure!!!

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offgrid View Drop Down
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Post Options Post Options   Quote offgrid Quote  Post ReplyReply Direct Link To This Post Topic: Floor failure!!!
    Posted: 15 Aug 2019 at 5:20am
Originally posted by Toyanvil

If you add a frame rail you will not want to continuously welded it, it will be stronger to stitch weld it. 

Thank you Toyanvil, that is what we needed. I understand the structural requirements but I'm not a welder. 

I always thought that continuous seam provided the strongest joint and stitch welding is done where that isn't needed or to reduce heat distortion. Maybe that's the case here?  Is there something on the web you can point me to on this where I can learn more about when one is used vs. the other? 

Olddawgsrule, just for clarity, here is the formula for section modulus of a rectangular tube the "hard way" that I'm using:

Z=(b*d^3-h*k^3)/6/d  where b is the outside base, d is the outside height, h is the inside base, and k is the inside height. 

I'm suggesting another 4 inch tube (but with thicker walls) under the existing one because the taller the tube the stronger it is for a given weight per foot, and also because 4 inches is more or less the same height as the standard riser kit. 

For example, a 2x4x.1875 has a section modulus of 2.11 in^3 and weighs 6.9 lb/ft while a 2x3x.25 has a Z of 1.7 and weighs 7.1 lb.ft. Either way if you use two 3 ft sections you'll be adding about 42 lbs to the axle load but with the 4 inch tube you'll be 73% stronger while the 3 inch tube will only give you 39% more frame capacity.

Sometimes things can work out and you can kill two birds with one stone (in this case gain ground clearance and reinforce the axle to frame attachment at the same time). Like the old saying goes, if you're handed lemons, why not make lemonade? Thumbs Up




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Post Options Post Options   Quote Olddawgsrule Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2019 at 7:32am
Originally posted by Toyanvil

If you add a frame rail you will not want to continuously welded it, it will be stronger to stitch weld it. 

Thanks for the info on this. For what I've read so far the stitch weld is used so you do not compromise the parent metal. Which makes a lot of sense to this simple mind! Especially if the parent is .1 and the child is .187..

It also sounds like it's very specific in pattern. Like, 2" weld, 2" space. Did I read that correctly?

They also spoken of back-stitching. I thought it interesting that the spaces (in my example above 2") was filled/welded in the opposite direction. I never thought of a weld having directional strength. Does it truly?

Mind ya, I have no intention of doing this myself! My welding skills are close to non-existent.. It's my curious mind.
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Post Options Post Options   Quote geewizard Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2019 at 8:34am
I just cannot see how a stitch weld would be stronger than a continuous seam weld.  I think stitch welding is done just to save time, money, and filler.
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Post Options Post Options   Quote offgrid Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2019 at 9:33am
I found this on a metalworking forum. Control of thermal distortion looks like one reason for stitch welding, another being cost if a continuous weld is just not necessary. There is a 3rd reason suggested which appears to involve differences in properties between the weld metal and the base material but there isn't sufficient explanation as to why a continuous weld might result in a weaker joint in that case. Maybe Toyanvil can explain it. 

My first job as a welding engineer was as a joint detailer literally going through hundreds to thousands of welding blue prints and specifying and writing all the weld symbols. A lot of the time I was given the joint strength requirements and had to figure out the weldingdetail from that.

now as to why stitch welds are used. Distortion control is generally the first reason. In general lower heat= less distortion. And distributing that heat evenly, by skipping around, back stepping etc can help control distortion. You can also control distortion by preheating a part (another discussion)

In general the other reason joints are only intermittently welded is economics and strength.

For example a simple stiffener like a gusset on the leg of stand almost never needs a full weld. Assuming the material is the same thickness. The main member will fail by buckling or torsional stress way before the stiffener would fail. So placing full length welds gives you no strength advantages and lots of headaches and wasted cost. I remember specifically taking out pass after pass

I would say about 90% of intermittent welds (Besides distortion) are specified for this reason. The service condition stress is way lower than what a full welded joint would provide. So why bother?

finally occasionally in the post welded condition or by using a different filler material a weld joint will have a higher strength than the base metal In that case fully welding will not increase strength and actually may decrease it. In those cases intermittent welds are called out, but that requires a decent amount of pencil and paper engineering. 
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Post Options Post Options   Quote geewizard Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2019 at 9:44am
Thanks, offgrid.  Now I know.
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Post Options Post Options   Quote David and Danette Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2019 at 2:04pm
   This has been interesting and educational because on our Gulf Stream Vista Cruiser I have been thinking about strengthening the tongue. The metal bows when camper is loaded and being our model is the heaviest Vista Cruiser using this frame I am a Little concerned. For the 2020 models they use a stronger frame perhaps the engineers were a little concerned too. There has been a lot of useful information that is what is so good about this forum. Thank you
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Post Options Post Options   Quote Olddawgsrule Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2019 at 2:16pm
Originally posted by David and Danette

   This has been interesting and educational because on our Gulf Stream Vista Cruiser I have been thinking about strengthening the tongue. The metal bows when camper is loaded and being our model is the heaviest Vista Cruiser using this frame I am a Little concerned. For the 2020 models they use a stronger frame perhaps the engineers were a little concerned too. There has been a lot of useful information that is what is so good about this forum. Thank you

This may be something for you to look at. It comes from someone assisting in my trailer build design on the frame. Yes, I'm going to build my next once I get through this mess..

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Post Options Post Options   Quote Olddawgsrule Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 12:10pm
End of week 2 update:

FR has turned it over to Lippert now and Lippert is requesting more pictures.

Well, good news is no one has flat out rejected the claim/issue. 
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