Interior skin necessary?

[J. Morgan]

There is a thread in here somewhere where the owner of the crushed trailer discusses the sad situation.

He says snow load.


Brevi tempore!

[WayneG]

This is the roof over the cab of my 84 Motorhome. I bought it this way from a fellow in Vermont. I am not sure if it was snow or something else, he did not say. There are no ribs in front or the rear sections of the motorhome, and only the plastic liner inside.

[rugjenkins]

I stand corrected! 😋

[J. Morgan]

He was very despondent about the whole situation. He had put a ton of work into the trailer, and was planning to do the interior in the spring.


Brevi tempore!

[ArtisanAir]

Originally Posted by rugjenkins

I don't believe that was from a snow load unless u can show before and after pics. That looks like a tree damage or a wreck.

Just think about physics. Most likely the middle rib near the a/c unit went beyond its tensile strength and stressed the rib out until a catastrophic tear occurred. That then distributed the displaced load onto the adjacent ribs. When that happens they will fall like dominoes. Provided evidence in the background suggests around not just 2+ feet of snow but ICE also (far more dense). If that powder is actually snow holding water (slush) it could be even heavier. I would imagine that there is close to a ton of weight placed on top of the trailer. Its kind of like that funny mayhem commercial....

https://youtu.be/5kqLcpkdUTw

[Cardinal283]

It makes sense. Without the inner skin the ribs are allowed to twist. As soon as they do their strength is gone. I don't know about on a newer trailer but on my '63 the ribs are very flimsy. I can twist them with my hand (yes my inner skins are currently out).

[ArtisanAir]

Originally Posted by Cardinal283

It makes sense. Without the inner skin the ribs are allowed to twist. As soon as they do their strength is gone. I don't know about on a newer trailer but on my '63 the ribs are very flimsy. I can twist them with my hand (yes my inner skins are currently out).

Well in Texas I am almost positive you will not see that situation.

I know i'm sounding like a "Monday morning quarterback" but you would think maybe to brace the exterior shell from within, via jack and lumber. That would have crossed my mind well before that happened. But what am I saying, that's probably Canada where they get that in a days time. I still feel deeply saddened for this owner, man I could not even imagine what I would say after walking out and seeing that sight ..... Man

[J. Morgan]

I see it happening as the weight pushed down primarily on the center of the roof, pushing the sides outward at the base of the arc of the roof, once the walls moved outward a certain amount it was game over, the roof fell in a rapid cascade effect starting at near the center of the roof.

These trailers are strong because of the arch, and most of the strength comes from the outer skin, (the ribs main contribution to strength is holding the skin in place), once the arch is compromised, it is done.

I am of the opinion that inner skins would not have helped to prevent this as much as the wall bulkheads would have.

That said, my trailer has only one wall... Very near the back of the trailer.


Brevi tempore!

[ArtisanAir]

Originally Posted by J. Morgan

I am of the opinion that inner skins would not have helped to prevent this as much as the wall bulkheads would have.

Agreed, he should have done some preventive measures before this happened. Though the skin does indeed give strength (not more than the bulkheads), it will not overcome mother nature to that degree.

[J. Morgan]

I assert that there was SEVERAL thousand pounds of crystalized water on the roof of that trailer.


Brevi tempore!

[Shacksman]

Originally Posted by J. Morgan

I assert that there was SEVERAL thousand pounds of crystalized water on the roof of that trailer.


Brevi tempore!

Right, it was spring and raining into the snow. The owner was out of the area for awhile and found this when he came home.

[Tim O]

Originally Posted by Belegedhel

I have seen this topic argued on many threads, and it always boils down to an apparent matter of opinion, supported by resumes, anecdotes, and conventional wisdom on either side. So just for a thought experiment, I decided to get out my slide-rule and put some numbers to this problem (bear with me, this gets exciting at the end).

The strength of both the inner and the outer skins themselves is greater than the rivets that hold them together. That is to say, if I have two sheets of aluminum riveted together at a seam, and I pull them apart axially, the rivets will shear before the aluminum sheets will tear. The rivets themselves, being interior or exterior are the weak links in this system. Therefore, one way to approximate the relative contribution to the strength of the shell “system” is to compare the strength of the rivets on the exterior shell to the interior skins.

If I examine a linear foot of seam in my exterior shell, I observe that there is a rivet approximately every ¾”, so for 12” there would be 16 bucked rivets, the original diameter of which was 1/8”. I perused around the internet and found a table of shear strengths of aluminum solid rivets, and not knowing the actual alloy of these “soft” rivets, I chose the weakest/softest solid rivets on the table (just for a “worst case scenario), which were made of 5056-H32, and have a shear strength of 363 lbs. So that 12” section of shell seam has a total shear strength of 363*16=5808 lbs.

Now, if I look at a seam joining the interior skins together, I could generously observe that there is approximately one blind rivet (with aluminum mandrel), also with a 1/8” diameter, every 4 inches (three rivets per linear ft.). A similar table of shear strength values puts the strength of these rivets at 120 lbs each, so my 12” length of interior skin seam has a total shear strength of 360 lbs.

So, the total shear strength of the one linear foot of the “system” (inner skin + outer skin) is 5808 + 360 = 6168. Interior contribution amounts to 360/6168, or just 5.8%. My conclusion: both sides are technically correct, inner and outer skins contribute to the system, but the truth is their contribution is profoundly lop-sided toward the outer shell.

Is 6% enough to care about? Well, I did a back-of-the-napkin “wind load” calculation. I assumed that my trailer has 49 square ft of surface exposed to the on-coming wind, and that I am driving at a leisurely 75 mile per hour in a super low profile Lamborghini that has been rigged up to tow by Can-Am. I assume the car’s super sleek profile does not block the wind on the face of the trailer substantially. For simplicity, I’ll treat the trailer as a square box, with a drag coefficient of 2. Under these circumstances, the wind is putting 1411 lbs of force against the front of my trailer, trying to rip the shell off of the frame. Since the trailer is about 7ft wide, that means I have 7 * 5808 = 40,656 lbs of shear strength between my exterior shell and the U-channel in front. I believe I have two rows of about two feet long that form the front hold-down plate (but these rivets are spaced out), so let’s add another 2 * 5808 = 11,616 lbs. I end up with a total 52,272 lbs of shear strength holding the front of my exterior shell to the trailer frame (assuming the bolts that hold the U-channel down don’t just pull right through the channel). So the wind force on the trailer on an average Sunday drive is just 1411/52,272 =2.7% of the available strength of the riveted structure.

Take-away: Make sure your rivets that hold the exterior shell to the front hold-down plate are all intact, and ensure that your subfloor is not rotten, your outriggers are intact, and that the bolts going from C-channel to frame/outriggers are solid.

I can also offer my own actual experience, in which I towed my trailer with absolutely no interior skins for hundreds of miles and observed no ill effects (no guarantees, individual results may vary….). The feeling of towing interior skin free, is quite liberating, something akin to going commando.

Thanks for this very thoughtful post. Given your conclusions about the relative contribution of the interior skin, would you consider that the interior skin could be replaced with a somewhat softer or thinner skin without an appreciable compromise?

[Aerowood]

Be careful here with going with thinner and softer, you want to be able to lean on the wall without it denting. Another consideration is if you want to mount something on it thicker is better. I replaced all the inner skin in my Globetrotter and went thicker and a little softer. I used .040 5052 H32. Thicker can be easier then thinner to work with as it isn't as flopinzee (I can make up words on my posts)..

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[Belegedhel]

Tim O--Welcome to the Forums!

I'll second Aerowood's approach described above. This is exactly what I did as well. In my case, I was trying to go with the bare, shiny aluminum look, so I really didn't want a bunch of ripples that might have resulted using a thinner material.

Most people when rebuilding their trailers will end up following the same construction techniques that were originally used, that is, cabinetry and gauchos that just mount to the interior skin, without much regard for proximity for ribs, hoops, or other "framing" structures.

I have seen examples of folks that have completely redone their interiors in wood (seems like J Morgan, above, did that). I don't know whether they put the wood over the aluminum skin, or if the wood is all there is. It would be interesting to hear how this holds up over the long haul, as there is a lot of vibration/motion in the shell as you go down the road, and I wonder if the rivets in a softer interior (like thin plywood) would wallow out the holes.