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Discussion Starter #1 (Edited)
Howdy,

I'm looking for a little help determining how many beams and what gauge steel I need for my bed framing. Bed dimensions will be 80" long by 73" wide with no vertical supports in the middle. Storage and head space is a premium so I was hoping to get away with 1"x1" square steel tubing instead of rectangular. Can someone tell me the formula for calculating the required gauge and number of beams with this setup? Also, what's a standard deflection for this application, type of steel that is fatigue resistant? I have a local quote for A500/A513.

Is 80/20 (although much more expensive) feasible for this application in terms of strength? I imagine it will be much much lighter.
 

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Equations depend on how the many beams will be loaded, but that's the easy part. Estimating the load correctly is far more difficult in my opinion.

I'm assuming you will span 73 inches clear from side to side with no middle support, but how will the beams tie together (if at all) to distribute the load between adjacent beams? How about the end connections? That makes a big difference also, although I expect beams will not have a way to resist rotation at ends.

And what is load exactly? Sleeping in middle of span won't be as bad as sitting in very middle of bed. Beams at front and rear of bed may also see much higher loads unless there is a bulkhead supporting weight, so climbing in and out of bed may be worse-case load. A couple "moving up and down" could also increase load significantly.


Regarding steel, the higher the strength the greater the resistance to fatigue at greater stresses. However, in this case since span is 73 times greater than beam depth, it's very possible your design will be limited by deflection more so than stress or fatigue. You'd first have to define how much deflection is acceptable.
 

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https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html

To do the moment of inertia of a hollow tube, calculate the moment of inertia of a solid with the same outer dimensions then subtract the moment of inertia of a solid with the same inner dimensions ... assuming it's symmetrical, which it is, in your case.

With 1/16" wall, I = ( 1.000^4 - 0.875^4)/12 = 0.0344

Don't change the E (modulus of elasticity), all steels are in very close to the same range.

Hint: it's going to be marginal ... and wobbly.

If you can do 1.5" square tube, even at 1/16" wall thickness, it's going to be a whole lot better.
 

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IKEA's Skorva bed rails are about $20 each. They will run crosswise in the van with no center support. Lots of us have used them. Search "Skorva" on this forum for details.
 

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Discussion Starter #7
My plan was to copy watercamper’s build. 2x6’s bolted to the walls with slots cut in them for the steel stringers. My buddy seemed to think 6 1” x 1” x 1/8” x 73” would be more then strong enough? I’m really pressed for head room and the ikea pieces are thicker then 2”. I’m trying to build the bed platform under 1.5”

Admittedly, math was never my favorite subject...
 

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Hi,

Here is a pretty easy to use steel tubing calculator.
http://www.atc-mechanical.com/calculators/tube-size-using-structural-properties/

How many tubes you use probably has more to do with how you support the mattress between tubes -- eg plywood?
It only does loads at the midpoint of the span, but that's probably a good way to go as you might end up putting most of your weight in the middle at times (eg getting in).

The calculator gives you a max load for failure and a max load for keeping the deflection down to 1/360th of the span.

1/360th of span is usually used for deflection critical applications and might be a little conservative for a bed, but it would probably be a good way to go.

Just as an example, a 1.5 in square tube with 0.12 inch thick walls has a failure load of 846 lbs applied in the middle of an 80 inch span, and the 1/360 deflection load is 208 lbs.

Remember that you need to consider all loadings -- for example, two heavy people might choose to sit on the edge of the bed and put all of their weight on the first tube.

If you used aluminium tubing, the failure strengh is not that much different than steel depending on which alloys of each is used, but the stiffness of alum is 1/3rd of steel so it may take a heavier secion of alum to control deflection.


Gary
 

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Discussion Starter #10
Thanks a lot for the link, it looks like I won't be able to build it as planned. I think I'm going to redesign the platform to have a vertical support in the middle, effectively reducing the span to 36". With good quality 15/32" plywood I think I can get away with 1" square tubing then and still keep my overall height less then 1.5"

I guess I should first plan the bike setup (with or without slides) and see how much vertical room they actually take up. Hopefully I can get this done tomorrow.
 

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Hi,

Here is a pretty easy to use steel tubing calculator.
http://www.atc-mechanical.com/calculators/tube-size-using-structural-properties/

How many tubes you use probably has more to do with how you support the mattress between tubes -- eg plywood?
It only does loads at the midpoint of the span, but that's probably a good way to go as you might end up putting most of your weight in the middle at times (eg getting in).

The calculator gives you a max load for failure and a max load for keeping the deflection down to 1/360th of the span.

1/360th of span is usually used for deflection critical applications and might be a little conservative for a bed, but it would probably be a good way to go.

Just as an example, a 1.5 in square tube with 0.12 inch thick walls has a failure load of 846 lbs applied in the middle of an 80 inch span, and the 1/360 deflection load is 208 lbs.

Remember that you need to consider all loadings -- for example, two heavy people might choose to sit on the edge of the bed and put all of their weight on the first tube.
We'll, if it's a fixed bed platform and the top is attached to the rails, it's not going to be all carried by that first rail. If it was floating, yeah, that'd be a consideration. I wouldn't even vaguely consider the l/360 deflection when building a steel bed. It's something you're never going to notice, AND so far under failure. And failure on steel with something like this isn't going to be a catastrophic surprise either...

To op, steel is elastic. Get a piece, hang it on something, and see how it seems and bends back. Boom, good to go if it bends back with your weight (and some bouncing off you like).

Me, after popping a couple rivnuts into that thin steel tonight, I'm more concerned about the van itself.
 

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Thanks a lot for the link, it looks like I won't be able to build it as planned. I think I'm going to redesign the platform to have a vertical support in the middle, effectively reducing the span to 36". With good quality 15/32" plywood I think I can get away with 1" square tubing then and still keep my overall height less then 1.5"

I guess I should first plan the bike setup (with or without slides) and see how much vertical room they actually take up. Hopefully I can get this done tomorrow.
It's not all or nothing, and there are other options beyond giving up, although you have to really consider what a 1/2" or even 1" of extra headroom is worth to you, because as stated by others it makes a big difference.

One option is to use more beams spaced closer together. The plywood will then span between them without as much flex so that load will be carried by a few beams working together. You'd need to center the plywood for best result (don't place seam in middle).

Another option is to use 1X2 rectangular tubing on its side, which is much stiffer and stronger without adding height. Or 1X1-1/2 as numbers work out.

A third option is to place vertical supports under platform but not in middle where they affect storage underneath the most. Although mechanically more effective to place in middle of span, I'd first go to vertical supports closer to wheelwells at each side, leaving the center wide open. You could easily reduce span from 73" to 54", which reduces stresses some and deflection a lot more.

Just options to consider, which can be used in combination.
 

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A third option is to place vertical supports under platform but not in middle where they affect storage underneath the most. Although mechanically more effective to place in middle of span, I'd first go to vertical supports closer to wheelwells at each side, leaving the center wide open. You could easily reduce span from 73" to 54", which reduces stresses some and deflection a lot more.
Most people box in their wheel wells anyway. It's simple to extend them to bed height with storage above the wheel, which can be accessed from a hinged bedboard--hinge centered on top edge of box. Mine are wide enough to accommodate two Mountain Dew cartons side-by-side (we all have our priorities). This gave a full 1.5" thickness for Thinsulate over the wheel well. Mine don't go the full front-back depth of the bed, but they easily could.

 

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We'll, if it's a fixed bed platform and the top is attached to the rails, it's not going to be all carried by that first rail. If it was floating, yeah, that'd be a consideration. I wouldn't even vaguely consider the l/360 deflection when building a steel bed. It's something you're never going to notice, AND so far under failure. And failure on steel with something like this isn't going to be a catastrophic surprise either...

To op, steel is elastic. Hey a piece, hang it on something, and see how it seems and bends back. Boom, good to go if it bends back with your weight (and some bouncing off you like).

Me, after popping a couple rivnuts into that thin steel tonight, I'm more concerned about the van itself.
Hi,
Not sure what you mean -- it the bed spans across the van width and has multiple steel rails to support the mattress, then if a couple people sit on the edge of the bed facing forward, it seems to me that essentially all of their weight will be on the first rail?

It seems like two people sitting on the edge of the bed to eat a meal or work on something is going to happen for sure during the life of the van.

I guess if it were me, I'd want to have a factor of safety of 2 on a couple 200 lb people sitting on the edge of the bed. Even airplanes (where weight is critical) have a factor of safety of 1.5 on failure with the most extreme service loads.

Maybe the most forward tube and the most aft tube (which will see heavier loads) should be thicker?

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Bear in mind that the for a rectangular tube section, the bending stress goes down with the square of the height, and deflection goes down as the cube of height, so small changes in height make a lot of difference. Take a 2 by 4 supported at each end, put it on its side and step on the middle and bounce a bit then turn it the other way and bounce on it -- big difference in stiffness.

--------

As mentioned, if you have access to some tubing and can just try a couple sizes, that's a perfectly reasonable way to get to the right size.


Gary
 

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Hi,
Not sure what you mean -- it the bed spans across the van width and has multiple steel rails to support the mattress, then if a couple people sit on the edge of the bed facing forward, it seems to me that essentially all of their weight will be on the first rail?

It seems like two people sitting on the edge of the bed to eat a meal or work on something is going to happen for sure during the life of the van.

....cut......

Gary
Gary, that's why I mentioned the "bulkhead" idea in my first post here. Not sure if that's the right term to use, but I expect most loft beds will be closed off at front, either partially or totally, which means there can be many vertical supports to address the front edge.

At the very rear of van I would span the width of the van just above the platform by turning a lip up. With 3 to 5 inches or so it would keep mattress from sliding back, and would reinforce the very back in case there was ever the need to crawl out the back. This would not reduce sleeping nor storage headroom.
 

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Most people box in their wheel wells anyway. It's simple to extend them to bed height with storage above the wheel, which can be accessed from a hinged bedboard--hinge centered on top edge of box. Mine are wide enough to accommodate two Mountain Dew cartons side-by-side (we all have our priorities). This gave a full 1.5" thickness for Thinsulate over the wheel well. Mine don't go the full front-back depth of the bed, but they easily could.

Yes, what you describe is similar framing to what Winnebago does with original Travato floorplan, where a cabinet supports part of bed on driver side. They fold part of the bed up for access, but it does pretty much what you describe. The Hymer Aktiv also has similar idea except cabinets on either side are much wider than wheelwell.

Instead of having part of bed flip up, the entire platform could be made fixed. Most likely there won't be much room in front or back of wheelwell to store anything large anyway, other than maybe a short kayak or tandem bike at an angle. Even then, I would frame like in your picture with vertical supports only at corners of wheelwell, and run a front-to-back member on each side.

Actually, with stiff front-to-back members it may not need vertical supports at all, which sounds even better.
 

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Hi,
Not sure what you mean -- it the bed spans across the van width and has multiple steel rails to support the mattress, then if a couple people sit on the edge of the bed facing forward, it seems to me that essentially all of their weight will be on the first rail?

It seems like two people sitting on the edge of the bed to eat a meal or work on something is going to happen for sure during the life of the van.

I guess if it were me, I'd want to have a factor of safety of 2 on a couple 200 lb people sitting on the edge of the bed. Even airplanes (where weight is critical) have a factor of safety of 1.5 on failure with the most extreme service loads.

Maybe the most forward tube and the most aft tube (which will see heavier loads) should be thicker?

---------
Bear in mind that the for a rectangular tube section, the bending stress goes down with the square of the height, and deflection goes down as the cube of height, so small changes in height make a lot of difference. Take a 2 by 4 supported at each end, put it on its side and step on the middle and bounce a bit then turn it the other way and bounce on it -- big difference in stiffness.

--------

As mentioned, if you have access to some tubing and can just try a couple sizes, that's a perfectly reasonable way to get to the right size.


Gary
I think it'd be quite unlikely that 2 people sitting would be applying all their weight on only one beam if the platform is attached to the beams (vs floating on them).

With regard to aircraft safety factor, yup, they use a 1.5x factor, when they're concerned about falling out of the sky and killing people- I'm pretty sure I'd be comfortable with a lower factor when the likely outcome of overloading is simply some plastic definition of the steel that wouldn't exactly spring out of nowhere. ;)
 

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Discussion Starter #18 (Edited)
Hi,


Maybe the most forward tube and the most aft tube (which will see heavier loads) should be thicker?

I can't believe I didn't think of this. My priority is MTB storage underneath and headroom. With the way I'm mounting the bikes, there will be a lot of dead space in the area of the most forward tube. It's a no brainer to make that one 1.5 x 1.5 or even 2 x 2. I might even be able to get away with make the middle support larger. Thanks!
 

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Discussion Starter #19 (Edited)
One last question on the topic. If I go back to my original plan to get the height I want I will need to use 2x10's instead of the setup Watercamper did that I'm trying to copy who used 2x6's.(below). It looks like the spacer he used on top is would be a 2x4 that I could run another bolt through into a rivnut in the recessed vertical supports, I would imagine that would prevent any twist and I don't think it would effect the walls. It looks like the weight difference is 8lb per side. Opinions?

Watercamper's setup reposted
 
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