Aluminum Cantilever for Truck Camper

[plainairpainter]

TL;DR Summary

I am building a cab over slide in truck camper out of closed cell foam, fiberglass and an aluminum skeleton. The sleeping area in the cab overhang will be extended 4 1/2 feet over the cab beyond the vertical support behind the cab. The aluminum supports will run along each side of the overhang (7 1/2 feet wide) all the way to the back of the truck. The weight will also be suspended from the roof which runs from front to back. I’m trying to determine correct gauge and shape of aluminum supports.

(Continuing from the summary...)
I want to build this safely, so I am putting this out here for expert advice. My choice for the shape for the aluminum cantilever is angle, T, square tubing or round. The foam is 2” thick, so I’d like to keep the supports at 2” in width and height. I need to support the weight of 2 people sleeping as well as the portion of the camper above the cab. I figure 450lbs. for 2 adults max and 50 pounds for the weight of that portion of the camper. I assume a little bit of the weight will be suspended from the 1/8” aluminum angles running from front to back at the 2 edges of the roof and walls front to back.
So to support 500 lbs. in a cantilevered sleeping zone with decreasing weight as it moves forward from the cantilever starting point, would 2” x 2” aluminum square tubing on each edge of the cantilever front to back be enough to support the weight? My choices are 6061 alloy T6 temper or 6063 alloy T5 temper. The thickness of the aluminum is 1/8”, 3/16” or 1/4”. I want to be safe for sure, but don’t want to overkill as each pound is accounted for in this ultra lite camper. Thanks for any advice.

 

[anorlunda]

Cool project. I'm pleased to see your interest in safety.

My guess for max instantaneous load would not be the case of two people sleeping. It would be the case where heavy gear or provisions are stored on that sleeping spot, and the truck is driven too fast on a very bumpy road.

The sleeping area in the cab overhang will be extended 4 1/2 feet over the cab beyond the vertical support behind the cab.

You could learn a lot by inspecting other commercial cab over slides. Look at their dimensions. If all the others stop short of 4 1/2 feet, there must be a reason why. Use that as a caution.

 

[plainairpainter]

Cool project. I'm pleased to see your interest in safety.

My guess for max instantaneous load would not be the case of two people sleeping. It would be the case where heavy gear or provisions are stored on that sleeping spot, and the truck is driven too fast on a very bumpy road.
You could learn a lot by inspecting other commercial cab over slides. Look at their dimensions. If all the others stop short of 4 1/2 feet, there must be a reason why. Use that as a caution.

Thank you for the response. No gear will be stowed up there as I am keeping any moveable weight as low as possible, and then as forward as possible. Many use 4 1/2 feet because that is the width of a double bed. Some use more than 4 1/2 feet. Unfortunately, I have no way to find the gauge of aluminum they are using, hence my question. Most are so heavy to begin with, they use up all the available payload just with the camper and don’t leave any allowance for gear or people, not safe at all. So do you think 1/8” gauge is strong enough to support the weight only when not in motion?

 

[berkeman]

I am building a cab over slide in truck camper out of closed cell foam, fiberglass and an aluminum skeleton. The sleeping area in the cab overhang will be extended 4 1/2 feet over the cab beyond the vertical support

I'm not familiar with cab-over camper construction, but I was surprised by your cantilever comments. Is that common or unusual? For general structural reasons, I would expect that most cab-over campers would rest some of the weight of the overhang on the cab rooftop.

The cab rooftop is built pretty strong after all by the pickup truck manufacturers in order to meet roll-over safety tests, so it should be fine to use to support the cab-over sleeping area. You'd want to make the contact areas easy on the paint on the roof, but that problem must have been solved already, no?

EDIT/ADD -- Although looking at the image that I found, maybe a cantilever design for the overhang is common? I don't see any contact with the roof of the cab in the picture...

https://i.ytimg.com/vi/QKfkR_E1MnU/maxresdefault.jpg

 

[anorlunda]

The photos on this website may give good clues for the construction. Looking at it, I get the idea that structural strength is all in the wood, and that the aluminum is just a skin.

https://www.glen-l.com/campers/fairhaven-constr-3.html

But I also found this. You can almost get dimensions from the picture.

Aluminum tends to be lighter weight and the fiberglass sidewall is more durable, easier to keep clean and less susceptible to dents.

And this site has a discussion wood versus aluminum.
https://www.truckcamperadventure.com/why-wood-frame-construction/

 

[plainairpainter]

The photos on this website may give good clues for the construction. Looking at it, I get the idea that structural strength is all in the wood, and that the aluminum is just a skin.

https://www.glen-l.com/campers/fairhaven-constr-3.html

But I also found this. You can almost get dimensions from the picture.
And this site has a discussion wood versus aluminum.
https://www.truckcamperadventure.com/why-wood-frame-construction/

I have the dimensions, all I really need is the specific answers to my questions. I just need to know the sufficient gauge and shape of aluminum as per my very specific question. Thank you.

 

[Baluncore]

I have the dimensions, all I really need is the specific answers to my questions. I just need to know the sufficient gauge and shape of aluminum as per my very specific question. Thank you.

We do not have the information required.
The frame or truss structure must be defined before a stress analysis can be done. We do not know what style of space frame you will use, so we can't guess at the material requirements.

The load points on the bed of the vehicle need to transfer forces to the beams that carry those forces forwards and upwards, to be distributed, providing support under the cantilevered load.

For maximum strength for weight that structure should be diagonal and triangulated, not orthogonal as shown in post #5 image.

Can you post your drawings?

 

[256bits]

Many use 4 1/2 feet because that is the width of a double bed

The extension is determined by the rule of no obstruction of visibility from the cab.
One really does want to see the traffic lights and overhead road signage while driving.

From what you wrote, it seems as if the skin will not be taking any of the load - ie foam insulation and fiberglass shell. In which case I would doubt that a 2-inch aluminium extension of 4 feet is adequate.
You would want to make the whole construction of a truss design, and that means also for the body of the camper, with strong main beams. Bridges are strong because of that.

Or, you can have the shell impart rigidity to the structure and take up some of the load. House construction is of that sort.

So, it seems you have to either beef up your framing, which will add weight.
Or add a rigid shell, which will add weight.

The overhang can be supported by a beam running along the top of the camper, with the floor attached to vertical hangers.

I have the dimensions, all I really need is the specific answers to my questions. I just need to know the sufficient gauge and shape of aluminum as per my very specific question. Thank you.

Unable to comply, as if we give you a size without knowing more details of your design, and even then; and if it fails, then who should take the blame. Nothing worse than having your camper blow apart from the wind load and vibration ( already mentioned ) while driving down the highway.

It's not an unsafe project, you just have to do some stress analysis on the joints and framing to be sure it will stand up under continued seasonal use.

 

[plainairpainter]

The extension is determined by the rule of no obstruction of visibility from the cab.
One really does want to see the traffic lights and overhead road signage while driving.

From what you wrote, it seems as if the skin will not be taking any of the load - ie foam insulation and fiberglass shell. In which case I would doubt that a 2-inch aluminium extension of 4 feet is adequate.
You would want to make the whole construction of a truss design, and that means also for the body of the camper, with strong main beams. Bridges are strong because of that.

Or, you can have the shell impart rigidity to the structure and take up some of the load. House construction is of that sort.

So, it seems you have to either beef up your framing, which will add weight.
Or add a rigid shell, which will add weight.

The overhang can be supported by a beam running along the top of the camper, with the floor attached to vertical hangers.Unable to comply, as if we give you a size without knowing more details of your design, and even then; and if it fails, then who should take the blame. Nothing worse than having your camper blow apart from the wind load and vibration ( already mentioned ) while driving down the highway.

It's not an unsafe project, you just have to do some stress analysis on the joints and framing to be sure it will stand up under continued seasonal use.

Thank you for that information. I might abandon the idea of a cabover altogether, and do a Murphy bed concept in a smaller camper. To be continued after further research. I definitely want it to be safe.

 

[plainairpainter]

The photos on this website may give good clues for the construction. Looking at it, I get the idea that structural strength is all in the wood, and that the aluminum is just a skin.

https://www.glen-l.com/campers/fairhaven-constr-3.html

But I also found this. You can almost get dimensions from the picture.
And this site has a discussion wood versus aluminum.
https://www.truckcamperadventure.com/why-wood-frame-construction/

Thank you

 

[256bits]

Thank you for that information. I might abandon the idea of a cabover altogether, and do a Murphy bed concept in a smaller camper. To be continued after further research. I definitely want it to be safe.

That is a lot of space to lose out on, while building to satisfaction a dream camper.
,
here is a calculator to find deflection and whatever.
with a 54-inch 2 inch square beam, 0.125 wall thickness.
If I have done it correctly, then
I get near a 1.5 inch deflection at the tip with a load at the 44 inch spot.

You can try it, or another,
Cantilever Beam Calculator | calcresource

 

[Lnewqban]

Thank you for that information. I might abandon the idea of a cabover altogether, and do a Murphy bed concept in a smaller camper. To be continued after further research. I definitely want it to be safe.

Do you have a basic diagram of your idea, regarding the structure?
Some general dimmensions would be helpful as well.
The means (welding, bolts, rivets, etc.) that you have available to joint the different elements is also important to know.

Regarding the material, go with square or rectangular, round and Tee are not strong enough for this project.

Consider diagonal bracing on the vertical and horizontal planes or sides.
If the points of connections to the structure are strong and durable and can stand vibration, steel cables of small diameter can help much to save weight (think of ultralight airplanes).
The fiberglass skin will not be strong enough for efective bracing.

 

[plainairpainter]

Do you have a basic diagram of your idea, regarding the structure?
Some general dimmensions would be helpful as well.
The means (welding, bolts, rivets, etc.) that you have available to joint the different elements is also important to know.

Regarding the material, go with square or rectangular, round and Tee are not strong enough for this project.

Consider diagonal bracing on the vertical and horizontal planes or sides.
If the points of connections to the structure are strong and durable and can stand vibration, steel cables of small diameter can help much to save weight (think of ultralight airplanes).
The fiberglass skin will not be strong enough for efective bracing.

Thanks. I’m going back to the drawing board and starting from scratch.

 

[onatirec]

I generally agree with the sentiments so far in the thread: it's necessary to have a more complete picture of the loading scenarios. I imagine even wind could stress your structure a fair amount when traveling at highway speeds. You'd have to think about your joints (bolted? welded? - both introduce stress concentrations / heat affect zones), vibrations, fatigue life (aluminum doesn't exactly have an endurance strength like steel)...

Doom and gloom aside. I think your question is reasonably concise, so I've worked a bit of it out for you below. Assuming a linearly decreasing load on a single cantilevered beam (which is more or less equivalent to a point load @ a distance of 1/3 L from cantilever), 2" square tubing (and the more calc-conservative thickness of 1/8"), I find a bending stress of 16.3ksi and shear stress of 1.2ksi. These maximums both occur at the fixed end, though the bending stress is highest at the top and bottom, whereas the shear is highest in the center of the cross-section. Probably you would want to combine these into an equivalent von mises stress...

McMaster lists the yield strength of 6061 @ 35ksi and 6063 @ 16ksi... ~25% higher cost for twice the strength at the same weight is a no brainer for me.

You'll see the corners behave as stress concentrations, egged on by artifacts of the boundary conditions.