[TheBusDriver]

Hello. I am a software engineer by trade and a diesel mechanic by fate. As such, I apologize for asking what may likely be answerable by a first-year mechanical engineering student.

I have a bus. The bus has support ribs:



I wish to attach a second story to these support ribs, above the roof:



Unlike the bare bolts in the photograph above, support columns I add for my second story will have between them and the bus wall (the support ribs) spacer columns. I am unable to determine the best fastening scheme involving these columns, with concern to strength and resilience to road conditions. Will you please help me? I propose two fastening schemes below. Scheme A uses two long bolts through concentric holes, while Scheme B uses four short bolts. Which scheme is stronger and more resilient?

Attachment 366018

Thank you for whatever thoughts you share.

[kawazuki]

Both achieve the same result although with considerable differences! The first is quick, easy and less critical on accuracy also simple fabrication although weather penetration could be an issue!
The second is far more complicated requiring accurate drilling and tapping of both legs / body and spacer but could have decent weather proofing!
Personally i'd go with the first!

[TheBusDriver]

Thank you for the reply. You answer interests me, as it is not what I was expecting.

I do not plan on tapping, as I plan on using bolts (with non-threaded body sections bearing tube wall loads, likely with spacers on nut ends). In terms of structural integrity, however, would you say that Schemes A and B are more or less equal? Roads can be harsh. I expect a good deal of vertical shear.

[kawazuki]

Thank you for the reply. You answer interests me, as it is not what I was expecting.

I do not plan on tapping, as I plan on using bolts (with non-threaded body sections bearing tube wall loads, likely with spacers on nut ends). In terms of structural integrity, however, would you say that Schemes A and B are more or less equal? Roads can be harsh. I expect a good deal of vertical shear.

If you are concerned with rough roads / vertical shear then clamped surface area is your friend! In the bus pic above the small dowel like spacers provide little clamping area but if you used, say, a 3" sq plate welded to the leg and bolted through that and the body at 4 corners you have considerable loading capacity compared!