Which Fastening Scheme Is Stronger?

[Marceps]

The attached image shows square tubing bolted to support ribs from the outside of a bus, designed to support a second floor. Each scheme has a longer section of square tubing with a shorter section (spacer) underneath. Scheme A uses two long bolts through concentric holes. Scheme B uses four short bolts. Which one is better suited to rough the road for years to come?

[Marceps]

Did I ask the question wrong?

I have an answer of my own but didn't want to influence feedback.

[Rameses]

In scenario B, how will you get the bolts in place and how will you tighten them?

[Robin97396]

There's nothing wrong with attaching your raised floor to the side walls, although we normally try to avoid making holes in the outer skin of the bus.

The side walls themselves won't adequately support a bus width floor without leaving you some bounce, so you'll need support from the original bus floor itself as well as partitions in the storage areas under your new floor so things don't slide around when you stop quickly.

I bet a floor like that would drive customs agents crazy when crossing the borders.

[Marceps]

Thanks for the replies. This is the kind of "second floor" I'm going for, but I want it to last a lifetime. (The bus pictured was probably built to keep beer cold and make nightly whoopee for a few seasons at RAGBRAI.)



I agree about avoiding holes in bus exteriors. Still, I don't see any other sound way to do it--and holes in the wall are definitely better than holes in the roof.

Originally Posted by Rameses

In scenario B, how will you get the bolts in place and how will you tighten them?

There will be bolt holes in the spacer/support columns. Schemes A and B have the same number of holes (as per the updated drawing). For re-tightening, support-column holes will be close enough to openings in the square tubing for access with combination wrenches.

[Iceni John]

Bolts are designed to hold loads only in tension, not in shear or bending. Shear loads should be minimal on a bolt, or a separate method of location (e.g. dowel pins) must be used for only those loads. Don't expect bolts to do everything! Also use the appropriate grade of fasteners (bolts, nuts, washers, lock washers) depending on the load - tables showing proof load strengths are available online. Grade 2 (general-purpose hardware store grade) as far as I'm concerned has no use on a bus for any structural loads. At the very least use Grade 5, and use Grade 8 for non-bending high-tensile loads only because their heat treatment makes them slightly more brittle than surface-hardened bolts like Grade 5. Stainless steel has about the same strength as Grade 5, but you MUST use anti-seize or lubricant on the threads otherwise they WILL seize together. If a fastener need a specific torque, then use a torque wrench, every time! Use NyLok nuts if you cannot tighten the bolt enough to slightly stretch it. Do NOT have any shear load on the threaded section of a bolt, only on the plain unthreaded shaft. Size the holes exactly right to the bolts - ideally there should be only a few thous clearance, but in reality a 64th or even a 32nd of an inch will work OK most of the time. Oh yes, one more thing, make friends with a good industrial hardware supplier that sells only good quality fasteners, and ask them for advice if you're not sure. Don't buy crap fasteners from Home Despot, and definitely not from Harbor Fright! Done right, a mechanically-fastened connection is perfectly good for a vehicle, sometimes even better than a weld, but if not done right it's an accident waiting to happen.

John

[jazty]

I'm 100% certain that scheme B is the stronger solution. You can't appropriately torque the long bolts through square tubing. The square tubing wall acts as a spring and will deflect and eventually start crushing when the bolts are tightened (assuming you're using a sensible wall thickness). It's the easier method of the two schemes and that's why you see it a lot for light duty stuff (yard equipment and so on).
It _can_ be made more rigid by filling in the tube before bolting. You could insert a piece of well fitting square stock into the tube then drill through the whole (hole) shebang.

[Marceps]

Thank you for the advice concerning fastener grades, stainless, and load-bearing lag sections. I take good advice to heart.

Originally Posted by jazty

It's the easier method of the two schemes and that's why you see it a lot for light duty stuff (yard equipment and so on).

This is what threw me off. I've yet to see a skoolie with a second floor attached with Scheme B but can provide dozens of examples using Scheme A. The more I thought about it, the more B made sense--but the more confusing it was that everyone used A.

Then again, we've all shared the road with plenty of crazier hacks that still manage to function.

[jazty]

Yeah, scheme A can be sized to be acceptable, but for otherwise identical set ups scheme B is always going to be stronger.

[DoubleO7]

If you add a piece of round tube cut to the inside size of the support tube on the bolts as sleeves, you eliminate the bolt crushing the sq. tubing.
And thus can properly torque the long bolts.
Which ever is easier and/or less expensive either scheme would work.
If it were me I would spend the extra $$ and start the square tubing at the existing floor inside.
The resulting exo-skeleton might come in handy for attaching other things.

[Marceps]

Originally Posted by DoubleO7

If it were me I would spend the extra $$ and start the square tubing at the existing floor inside.

I am not sure I follow. Wouldn't the square tubing have to go straight through the roof?