Is the roof skin structural?

[Busasaurus]

Probably a strange question, but how much of the roof's strength on a regular school bus comes from the skin? Obviously, the ribs are the main structural component, but would cutting out or modifying the sheet over top going to weaken the roof, especially on the curves? A million people have cut holes in the flat part, and even the stock emergency exits are just holes in the flat, so I can see how this would seem silly

What I'm pondering is basically a partial roof raise without disturbing any of the ribs. Think bus dormers.

Thanks!

 

[Danjo-SKO]

I think it is. I think it adds shear strength that if it weren’t there, the ribs would twist. Can you draw a picture?

 

[Busasaurus]

The simplest version of a skoolie dormer would be to detatch the roof at the sides of the bus and eliminate the curve, but only between two ribs. Attach it to a 2nd frame above the windows, bingo bango bongo.

 

[musigenesis]

The simplest version of a skoolie dormer would be to detatch the roof at the sides of the bus and eliminate the curve, but only between two ribs. Attach it to a 2nd frame above the windows, bingo bango bongo.

This is not uncommon - people sometimes do this to create a higher ceiling over a shower, for example. Structurally it wouldn't have any real effect unless you did this over the entire bus, and even then the ribs would (presumably) still be connected down the center of the roof as well as by the walls, so even in a rollover it probably wouldn't have much effect.

That being said, I don't think this would really work very well except in limited situations where you need the same ceiling height on the edge as in the center, and for spaces that are only 27" wide. The only use case I can think of is the shower stall. You can't use it to allow passageways on the side, since you'd still have to duck your head.

You would also probably end up having an easier time just doing a roof raise then building something like this.

 

[Busasaurus]

More to the point, I was imagining something like this https://imgur.com/Pmd7a74

 

[Sehnsucht-SKO]

I would say it is but in as much as everything that goes into a school bus is designed with structural rigidity in mind I probably wouldn't sweat it. Everyone rips out the seats without considering that they reinforce the walls at the knee line. Most everyone removes the interior skin to get to the moldy old insulation and install newer better insulation along with wiring and such. I don't know a precise quotient of structural compromise just those two modifications represent but considering how vastly overbuilt school buses are I'm not aware of anyone suffering a failure as a result. The only modification that seems to cause some people to think twice is a roof raise because you cut those one-piece steel bows and splice in extension pieces. Even then we have so little real world feedback to know how it holds up.

 

[musigenesis]

More to the point, I was imagining something like this https://imgur.com/Pmd7a74

If you're willing to cut a rib or two, you could end up with stick-up spaces that would be more practical.

 

[Busasaurus]

Something in my brain says that removing one rib in the middle would be worse than toy-hauler garage mods that only leave the deck in the back. But that's only gut feeling. Seems like maybe if I did another arched rib perpendicular to the originals and tied it into the ribs on either side of the removed rib, it would be as strong or stronger than the original set up.

But I am worried about twisting. When everything is strong sometimes that means any weak point will see extra load.

 

[musigenesis]

Something in my brain says that removing one rib in the middle would be worse than toy-hauler garage mods that only leave the deck in the back. But that's only gut feeling. Seems like maybe if I did another arched rib perpendicular to the originals and tied it into the ribs on either side of the removed rib, it would be as strong or stronger than the original set up.

But I am worried about twisting. When everything is strong sometimes that means any weak point will see extra load.

Your bus body is clamped to the chassis all along its length - it's not going to twist any more than the chassis does, no matter what you do to it. You'd be surprised at how much you can hack out of a bus and not have it make any difference. I drove around for a few weeks with an 8'x8' section of my floor around the back wheels entirely gone and the remainder of the body did not distort even the tiniest bit.

https://www.skoolie.net/forums/attachment.php?attachmentid=38711&d=1571618682

 

[Danjo-SKO]

If you're willing to cut a rib or two, you could end up with stick-up spaces that would be more practical.

I agree.

There’s also ways to add the rigidity that is lost by cutting a couple ribs, like for instance if some extra rails get run down the sides of the bus at the shoulder above the windows. The only problem is calculating how big the metal and how long the span.

 

[Busasaurus]

Thanks, y'all!

 

[Danjo-SKO]

But also if your dormers are designed to replace that shear, then you got it too.

 

[Native-SKO]

Food for thought ... BlueBird made RV's for a while. They only put in every other rib or maybe they only riveted to every other rib ... plus they did not put in an inner skin. At any rate, they were manufactured with less structure and the company figured it was "safe enough".

 

[Ronnie-SKO]

Food for thought ... BlueBird made RV's for a while. They only put in every other rib or maybe they only riveted to every other rib ... plus they did not put in an inner skin. At any rate, they were manufactured with less structure and the company figured it was "safe enough".

Yes but school buses have to meet rollover requirements, RV's do not. Most RV's in a rollover completely collapse. I would expect the Bluebird Wanderlodge to be somewhere in the middle.

My inclination would be to do like home construction and if needing to cut a rib out for a window,door, partial roof raise, then put a header to span between the ribs.

 

[musigenesis]

I was surprised by the wall structure around my side exit door since it does not actually have a header like what you find above a door or window in a house (sorry, having the sun behind me does not make for a very clear pic). The wall panel above it sort of looks like a header, but underneath it you can see that the cut rib just ends where it connects with the weak piece running front-to-back. The space between the cut end and the top of the door frame is just empty (in a wooden house this would be two solid pieces of 2X10 or similar - it's basically a short beam that needs to resist the downward force applied by the stud or studs directly above it).





There's also way more vertical replacement structure than you would expect if the concern was just to replace the weight-bearing capacity of the cut rib. There are two additional rib pieces added, plus two additional stiffener pieces right next to the door.

I think the concern with this door framing is resisting a side impact rather than bearing the weight above it. If you're cutting a rib or ribs somewhere in the bus, as long as you don't have people riding in the bus right next to these cuts I don't think you really have to worry about it - in terms of just holding itself together the bus will be fine.

 

[banman]

I'm sure a lot of thought went into how easy it is to open that emergency door after an accident occurs -- keeping the frame opening intact so the door can still easily open with the bus on it's side for example...

The only reason to be concerned with maintaining all the structure of all the ribs is if you'll routinely have people riding back there -- like a bus full of kids...
Or you plan to build an observation deck, or other heavy roof load...

Has any Skoolie been in an accident where the entire build was not hauled to the scrap yard? Huge bummer for the individual -- I don't wanna get into that...
Just wanna ask, has any Skoolie been in a rollover and put back into service?
Struck a tree or phone pole and been put back into service?

Make sure you don't have glass chandeliers flying through the air to hit you in the head from a minor impact -- but there may be no reason to maintain a roll cage level of strength for an area with no people to protect...

 

[Sehnsucht-SKO]

Re the every other rib concern: it's also highly improbable that during a rollover event there will be some large irresistible object will exist and end up in the path to impact precisely at the point where a structural rib would exist save for the proposed modification and therefore that point becomes the Achilles heel of the build. The roof as a whole will be called upon to bear up the weight of the vehicle in a rollover AND only to the extent that it not collapse and crush the occupants but some distortion is no doubt expected and perhaps engineered in some manner much like a crumple zone for cars to sacrifice the impact area in order to preserve the passenger compartment. This is the one unknown factor in my opinion regarding roof raises because if you cut and splice all or part of the ribs you've essentially introduced a front to back Achilles heel which may not deform but actually fail catastrophically in a rollover event. Again, to my knowledge we have no real world skoolie rollover data with which to formulate a comparison of OEM versus aftermarket so I'm just theorizing. I have yearned to crush three skoolies though in order to have a definitive answer - one fully original school bus, one with seats and inner skin removed, and one with a raised roof.

 

[banman]

Re the every other rib concern: it's also highly improbable that during a rollover event there will be some large irresistible object will exist and end up in the path to impact precisely at the point where a structural rib would exist save for the proposed modification and therefore that point becomes the Achilles heel of the build. The roof as a whole will be called upon to bear up the weight of the vehicle in a rollover AND only to the extent that it not collapse and crush the occupants but some distortion is no doubt expected and perhaps engineered in some manner much like a crumple zone for cars to sacrifice the impact area in order to preserve the passenger compartment. This is the one unknown factor in my opinion regarding roof raises because if you cut and splice all or part of the ribs you've essentially introduced a front to back Achilles heel which may not deform but actually fail catastrophically in a rollover event. Again, to my knowledge we have no real world skoolie rollover data with which to formulate a comparison of OEM versus aftermarket so I'm just theorizing. I have yearned to crush three skoolies though in order to have a definitive answer - one fully original school bus, one with seats and inner skin removed, and one with a raised roof.

I'm sure you can get the data you need for an oem bus crush w/out crushing your own. Extensive crash testing is already done to verify meeting federal spec...

I just saved you the cost of one bus. You're welcome!

As to a roof raise -- strength there, lost or gained is all in the quality of how the raise is done.
One could do a roof raise with nothing more than furring strips wedged into the cut ends of the hat channel held with a couple deck screws at each end. After skinning the gap with sheetmetal, that would more than hold the weight of the roof up but it would also introduce that achilles heel you're looking for -- the roof cap might very well break off in a hard roll but more likely just collapse inward held by the sheet metal...

 

[Sehnsucht-SKO]

Well the one factor I think might be applicable for the OEM bus is age. Crash tests and crush tests are conducted on new buses but those we deal with are usually 15+ years old and with age comes fatigue. I think it might be more apples-to-apples to crush a 15 year old OEM bus than rely on the tests of a new bus. But appreciate you looking out for my bottom line!! And it's all moot anyways unless I find the funds to buy a few used buses expressly for the purpose of crushing them.

 

[JackE]

Well the one factor I think might be applicable for the OEM bus is age. Crash tests and crush tests are conducted on new buses but those we deal with are usually 15+ years old and with age comes fatigue. I think it might be more apples-to-apples to crush a 15 year old OEM bus than rely on the tests of a new bus. But appreciate you looking out for my bottom line!! And it's all moot anyways unless I find the funds to buy a few used buses expressly for the purpose of crushing them.

My local school district has a couple non-running front engine gassers for sale for $500. It would be a lot of fun to see them get banged up.