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Old 02-21-2019, 05:53 PM   #1
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Adding a window at a rib

I am wanting to add a window to the curbside of a 30’ rear twin / mid bath where one did not previously exist. There is a rib that currently runs through the center of this location. There is a stringer that runs just below which the bottom window frame rivets to. There is an existing stringer above the location but it is several inches above the window.

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I have seen other windows placed at the location of a rib. Do I need to add any additional bracing or can I cut out the section of the rib?
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Old 02-21-2019, 06:03 PM   #2
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I would not do that.
That is the only real structural component for the roof load.

After looking closer I see its not that far to the next rib on each direction, but still would not do it unless adding new ones on each side
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Old 02-21-2019, 06:19 PM   #3
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This is the donor window/frame. Note that the ribs run through both the window as well as the smaller window forward.Click image for larger version

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What I’m looking to confirm is whether the stringer that runs above the windows provides the structural equivalent of a “header” and transfers the loads to the remaining ribs or if there is additional bracing that needs to be added.
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Old 02-21-2019, 06:33 PM   #4
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Looking at our 25 all ribs are at each side of each window, if Argosy trailers are built as test trailers your trailer would give some evidence of that.
the horizontals are there to keep the panels from wrinkling and have little to do with load imo.
I have no real experience with any of this but just does not seem sound to me to cut them.
maybe someone with real experience will say otherwise.
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Old 02-21-2019, 06:33 PM   #5
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Quote:
Originally Posted by AlinCal View Post
After looking closer I see its not that far to the next rib on each direction, but still would not do it unless adding new ones on each side


The other side already has a smaller bathroom window between 2 ribs.
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Old 02-21-2019, 07:48 PM   #6
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Found this advice from Andy from Inland RV (post #2)

http://www.airforums.com/forums/f425...tml#post340543
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Old 02-21-2019, 08:48 PM   #7
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I understand what Alan is talking about. It used to make me nervous to have a plumber cut a hole through a structural beam for a 4” pipe in a commercial building. But, the structural engineer had a plan. He would add steel around the hole to reinforce and restore the structural integrity adjacent to the hole. Back to Airstreams. In restoring our 55 FC, all the stringers were floating. They were riveted to the skin, but not the ribs. I asked Colin Hyde on the VAP. He said to attach the stringers to the ribs. So I did. I guess an Airstream is like a beach house. You either build to resist movement from high winds, or you build so the structure will sway with the wind. In our Airstream, I reconstructed for rigidity. I say if you cut a rib, you need to build back the structural integrity with added stringers attached to the ribs. I know that these trailers move a lot in travel. I hope a structural engineer tunes in and gives his two cents worth. I guess you could call the VAP and ask the question. Good luck
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Old 02-22-2019, 07:18 AM   #8
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Those "stringers" are generally for attaching skins and interior accoutrements and do not offer much (if any) support to the monocoque design of the Airstream shell. Not only that, but this particular rib is *right over* the axel. I would encourage rethinking the design, but if you absolutely can't, try adding a full replacement rib rather than trying to construct a "header" workaround. With as much multi-directional motion as an Airstream generates, the transfer of force isn't just "down" like with a house, so the header won't be as effective.
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Old 02-22-2019, 07:49 AM   #9
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What’s perplexing me is that there’s a rib right over the axle on 26’ that runs through the rear twin donor windows on each side as well as a rib that runs through the smaller kitchen window shown above. There must be something we’re missing.

Even though I would prefer the larger window, I had started out with the same caution and planned to use the smaller window from the donor since it will fit between the ribs, wouldn’t involve cutting an existing rib and would mirror the road side bathroom window. It wasn’t until I noticed that the ribs ran through both on the 26 that I began to wonder if it really made a difference.

I’m not an expert on monocoque structural design, but from what i have read the interior/exterior skins play an important role along with the ribs. Could it be that the window frames riveted to the skins and the bottom stringer are compensating in some way for the gap in the ribs?
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Old 02-22-2019, 08:04 AM   #10
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I meant to add there is no special horizontal framing added to the smaller or larger windows on the 26’.

Top & bottom of the kitchen window:
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Top and bottom of the larger window:
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As has been noted elsewhere, several the horizontal extrusions shown in the photos above are floating (I could move them with my hand).
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Old 02-22-2019, 08:51 AM   #11
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Do we know that the comparison example is factory original? I'm not saying Airstream/Argosy didn't/wouldn't do this, but something to consider.

Yes, the skins are a huge part of the monocoque design and contribute all of the "sheer" strength, but it's always best to make sure the skeleton is "as good as possible."

So that brings us to the "what's best vs. what will work" conversation. These forums are rife with the musing of engineers and laypeople with the luxury of hypothesizing on what would be the "absolute best scenario" (see all conversations regarding insulation!). [smile] Those "perfect solutions" aren't always realistic, inasmuch as we want to get on the road and camping! As long as safety doesn't become an issue, there is almost always a happy medium.

Also... is the window just for light, or are you planning on using it for ventilation and emergency egress? If it's just for light, and depending upon how traditional you wish to remain, another possibility that sometimes looks cool is a porthole window (or two stacked above one another) which have become much easier to find now that we have the internet. I don't know if it would work with your design, but might a porthole fit into the existing structural configuration a little easier? I know you mention wanting to use the larger window instead of the smaller "square," but maybe the added visual interest of two portholes is enough to compensate for window size?

All things to think about on an Airstream restoration adventure!
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Old 02-22-2019, 09:43 AM   #12
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Quote:
Originally Posted by kidjedi View Post
Do we know that the comparison example is factory original? I'm not saying Airstream/Argosy didn't/wouldn't do this, but something to consider.

Photos are from the donor and is a factory original. It had a single prior owner - I’m the 2nd owner, so no mods whatsoever and built in the Versailles Ohio plant.
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Old 02-22-2019, 09:49 AM   #13
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another thought

If aviation construction is any help, you might consider the effects of doubling the aluminum skin thickness as a method of reinforcement. Aircraft Inspection, Repair and Alterations AC 43.13-1B and 2B is a publication that helps figure what you'd need to think about in order to accomplish an aluminum repair that can be expected to contribute to the longevity of the whole structure. Of particular interest in this context would be rivet size and spacing, plus doubler extent and shape.

One takeaway: if you change the load bearing capacity of one area, the load finds another path through which to transmit load and vibration, and it is not always obvious.
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Old 02-22-2019, 09:51 AM   #14
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Is the window just for light, or are you planning on using it for ventilation and emergency egress?

This section in the 30’ was formerly a 3-door closet converted to a twin bed. It’s the darkest and most enclosed section (hallway curtains at both ends) of the AS since it is opposite the mid bath. So the goal was for both light and ventilation. Two smaller rectangular or portal windows do not appeal to my sense of aesthetics.
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