Rib Design - Stress on a rectangular plate
Rib Design - Stress on a rectangular plate
(OP)
Hello,
I have been designing, in brief, a rectangular box that has to take relatively high pressures and have hit a rather big snag on the way.
I am using Aluminium LM6 and have 16mm wall thickness and have calculated the deflection (Roark's deflection on a rectangular plate fixed on all sides, Machinists Handbook 28) only to find that I will get 21mm deflection. (flat face is 540x340mm)
I would like to keep the wall thickness at 16mm but add ribs to prevent any permanent deformation at 50Bar.
I am having no end of trouble trying to calculate these ribs, is there a basic rule of thumb or a 'simple' rule to follow to give me width, depth, draft, radius etc.
To make it a bit more interesting, there can be a variety of holes machined into this face. These will be combinations of a window, 100x100 and numerous Ø20 holes. These are not in a fixed position and could in any place dissect a rib.
I have attached a couple of pictures showing the lid in its worse possible form.
Thanks in advance.
James
I have been designing, in brief, a rectangular box that has to take relatively high pressures and have hit a rather big snag on the way.
I am using Aluminium LM6 and have 16mm wall thickness and have calculated the deflection (Roark's deflection on a rectangular plate fixed on all sides, Machinists Handbook 28) only to find that I will get 21mm deflection. (flat face is 540x340mm)
I would like to keep the wall thickness at 16mm but add ribs to prevent any permanent deformation at 50Bar.
I am having no end of trouble trying to calculate these ribs, is there a basic rule of thumb or a 'simple' rule to follow to give me width, depth, draft, radius etc.
To make it a bit more interesting, there can be a variety of holes machined into this face. These will be combinations of a window, 100x100 and numerous Ø20 holes. These are not in a fixed position and could in any place dissect a rib.
I have attached a couple of pictures showing the lid in its worse possible form.
Thanks in advance.
James





RE: Rib Design - Stress on a rectangular plate
i'd assume pinned edges, unless you have a dble row of fasteners. then i'd assume the pressure is reacted by both bending and in-plane membrane load ...
assume a deflection of say 0.5*thickness. use Roark to find out how much pressure is required to get this deflection, this is the portion of pressure load reacted by bending. then make an arc of the deflected side. apply the remainng pressure and determine your hoop stress (pR/t). combine the stresses
RE: Rib Design - Stress on a rectangular plate
Steven Fahey, CET
RE: Rib Design - Stress on a rectangular plate
Certainly the holes cut into the plate are not amenable to any form of hand calc's. You are into nonlinear FEA territory - even if the material stays linear. Getting your product design right will be well worth a modest fee to an experienced and qualified consultant. Just do it. No, I don't moonlight - nothing in this for me.
jt
RE: Rib Design - Stress on a rectangular plate
What I'd do is spend a pleasant afternoon cutting card or kraftwood up and sticking a model together with a glue gun. Or even use Meccano like a real engineer. http://en.wikipedia.org/wiki/Meccano
Even if it is not wonderfully accurate I bet you could quickly generate some rules of thumb for a better hand analysis.
Cheers
Greg Locock
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RE: Rib Design - Stress on a rectangular plate
Anyway, coming back to your original question, as you don't seem to be able to accept the situation as is: to calculate the ribs you have two possible approaches.
1) The ribs are in one direction only (presumably along the short span): you take each rib with the contributing portion of the cover (deducting the holes) and treat it as a beam.
2) The ribs are closely spaced and in both directions (to be noted that it will be difficult to make the intersections strong enough): you derive an equivalent thickness by equating the stiffness of the ribbed plate to that of a flat one.
And of course if you dissect a rib, then that one is lost and of no use.
prex
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