Analyze as Thin shell, Plate or membrane
Analyze as Thin shell, Plate or membrane
(OP)
I have a 0.06 inch thick plate 44 inch x 24 inch. It is supported at perimeter with HSS 2x2. I have to check how much UDL this plate can take. I am using older version of SAP. Shall I assign it as thin shell, plate or membrane or shall I analyze it manually as a thin strip (beam)






RE: Analyze as Thin shell, Plate or membrane
RE: Analyze as Thin shell, Plate or membrane
as a membrane, i'd avoid any linear FEM, due to large displacements. alternatively, model the flat panel as a spherical surface. it's easy to see deformed shape for the included circle (the circle tangent to the side) ... tricky to picture the shape near the corners (outside of the circle) ... but you can fudge something.
alternatively, as you say, you can analyze the central strips ... i expect this'll be conservative since there is some support at the sides that you're aren't accounting for.
best is a non-linear FEM ... all depends on how accurate you want to be.
RE: Analyze as Thin shell, Plate or membrane
Dik
RE: Analyze as Thin shell, Plate or membrane
if discretely, they'll behave as beams loaded obliquely by the membrane loading ( or in two directions (from the applied pressure) and from the membrane tension (the in-plane component).
RE: Analyze as Thin shell, Plate or membrane
I do not know so let's take worse case and assume connection to perimeter HSS random.
RE: Analyze as Thin shell, Plate or membrane
if the problem is (how do we say) alittle less real ... i'd assume it's supported on the corners, that'd be more conservative than say at 8 points on the perimeter (say the 1/3 and 2/3 points on each side).
RE: Analyze as Thin shell, Plate or membrane
I have looked at the as-built dwg. Looks like it is welded at four corners only.
RE: Analyze as Thin shell, Plate or membrane
RE: Analyze as Thin shell, Plate or membrane
What sort of loading? a few psi?
Dik
RE: Analyze as Thin shell, Plate or membrane
I have a 0.06 inch thick plate 44 inch x 24 inch. It is supported at perimeter with HSS 2x2.There are four wheel at four corners welded to u/s HSS
RE: Analyze as Thin shell, Plate or membrane
i think these welds will be the limiting feature of your design (0.06" sheet can handle a bunch of pressure).
how will you maintain a pressure seal around the perimeter of the sheet (0.06" really isn't plate) ? as the pressure is applied the sheet will deflect and open up gaps to the perimeter tube and vent the pressure.
again, what is stopping the tubes from flying off into space ? the pressure is applied to the sheet, the sheet is welded to the tubes, the tubes are attached to "the rest of the world" by ...
RE: Analyze as Thin shell, Plate or membrane
There are four wheels at four corners welded to u/s HSS.
It is just a kind of trolley. All I need to see how much weight we can place on this trolley?
RE: Analyze as Thin shell, Plate or membrane
so you've got a distributed load on the sheet, the sheet is tack welded in the corners to a tube frame, and the tube frame is supported by 4 wheels, also in the corners.
will the load in sheet put the welds (between the sheet and the tube frame) into tension (like is the sheet attached to the lower face of the tube frame) or into shear (the sheet has flanges onto the sides of the tube frame and the welds are running up/down ... i guess that's the clue about the loading on the welds ... if they're up/down they'll be in shear and pretty effective, if they're side-to-side then they'll be in tension and pretty inefficient.
what's the load capability of the wheels (and their axles) ?
RE: Analyze as Thin shell, Plate or membrane
I can take care of weld and HSS and Wheels. Pleaee guide me on plate only. It is tack welded on top of HSS at all corners. I just need the UDL whcih this plate can bear safely. Rest I can do thru welds, HSS and wheels and to ground. Thanks.
RE: Analyze as Thin shell, Plate or membrane
y(center) = 0.308*(1-v^2)* (p*a^4) / (E*t^3)
s(max) = 0.9152*(p*a^2)/t^2.
max stress is near enough W/t^2 (!) ... this is at the edge of the sheet. to understand the loading onto the welds, i'd draw an arc with y deflection in the middle (and zero deflection at the corners) ... you're looking at a section through the diagonal of the sheet. this'll show you the slope at the corners. this'll show you how much tension is being applied to your welds (as well as their shear, =W/4)
i'd start with W = 2*shear strength of a weld (i know there are 4, but you need to leave some capability for the tension loads). if that works out well, you can increase it (but you won't exceed 4*weld shear).
RE: Analyze as Thin shell, Plate or membrane
What are the criteria to define whether it is a thin shell, membrane or plate?
RE: Analyze as Thin shell, Plate or membrane
No such thing as a free lunch I am afraid.
RE: Analyze as Thin shell, Plate or membrane
RE: Analyze as Thin shell, Plate or membrane
I think you have to consider it as a membrane only... the spans are too great l/d = 400... There will be negligible flexural component.
RE: Analyze as Thin shell, Plate or membrane
RE: Analyze as Thin shell, Plate or membrane
Using Roarks 6th edition. I get 35 PSF or q (load per unit area). Keeping sigma ( bending stress in plate 21 ksi).
What will be the tension on ends of plate which weld and faring HSS needs to beasr.
RE: Analyze as Thin shell, Plate or membrane
Where
Beta is .57 (for a = 44 and b= 24 inch)
q = load per unit area
b = short side = 24 inch
t = thickness of plate = 0.06 inch.
So i kept sigma as 21 ksi and got q
RE: Analyze as Thin shell, Plate or membrane
shear on the welds looks like 60 lbs each. the tension on the welds is due to the sheet reacting the pressure with in-plane tension stresses, and these would collect at the corner welds (being the only thing connecting the sheet to the rest of the world.
check with roark about the deflection in the middle of the panel, i think it'll be more than 2*thk (so i wouldn't want to apply a plate formulae, which is what i'm assuming you've pulled from roark, as you talk about bending stresses) ...
but if 257 lbs does it for you (and how you want to use this thing), it doesn't sound unreasonable ... can you imagine one or two people standing on it ?
RE: Analyze as Thin shell, Plate or membrane
"from roark 4th ed pg239 (flat plate solution 81), a square plate a (= sqrt(b*d)?) loaded by pressure (uniformly distributed weight) = W/bd."
RE: Analyze as Thin shell, Plate or membrane
i'd suggest taking a strip along the diagonal deflect it in the middle by say 0.5" and see what the tension is to support a distributed load ... this'll also show you the (tension) loading on the welds. I suspect in your case this might be alittle high.
have you built one of these things ? why not test it ? (stand on it, with a friend if you're feeling lucky!)
RE: Analyze as Thin shell, Plate or membrane
Please see page 245 and Table on Page 246.
This table is when using ordinary elastic equations; deflection is more than half the thickness of plate. But I am having hard time to use the table. You have any wisdom for this?