How to size stiffeners?
How to size stiffeners?
(OP)
Let's say I have an aluminum skin, carrying a compressive load over distance "a" from frame to frame, and supported by longitudinal stiffeners equally spaced at width "b". I am sizing everything, having the required loading in lb/in. After selecting a skin thickness I can get the buckling stress of the skin based on the dimensions "a" and "b", assuming that all sides are simply supported (Bruhn chapter C5). How do I size the stiffeners? I know to check for crippling, but there must also be some minimum moment of inertia for the stiffener to provide a simple support, below which it is no longer effective at restraining the skin. Bruhn offers such an equation for stiffening a shear web in equation C10.8, but I can find nothing in Bruhn or our OEM manuals on sizing a stiffener for a panel under compression, or combined shear/compression. Can anyone offer help or point me to a reference?
RE: How to size stiffeners?
RE: How to size stiffeners?
remember the skin can buckle in shear (diagonal tension) and this adds load into the stringer.
you are correct … there should be some design guidance about stiffener size verses skin thickness … a small stiffener on a heavy skin won't support the skin (this may be in the diagonal tension section (since it relates to the ability of the stiffener to resist the skin shear buckle.
another day in paradise, or is paradise one day closer ?
RE: How to size stiffeners?
NACA TN 455
Comparison of Three Methods for Calculating the Compressive Strength of Flat and Slightly Curved Sheet and Stiffener Combinations
https://ntrs.nasa.gov/search.jsp?R=19930081295&...
NASA-TM-108399
Preliminary analysis techniques for ring and stringer stiffened cylindrical shells
https://ntrs.nasa.gov/search.jsp?R=19930013915&...
NACA TN 3781, 3782, 3783, 3784, 3785. & 3786
Handbook of Structural Stability (inc. Compressive Strength of Flat Stiffened Panels)
https://ntrs.nasa.gov/search.jsp?R=19930084529&...
There is also a very nice treatment in Theory and Analysis of Flight Structures Robert Rivello
Pages 479-500
I am assuming since it seems like the driving factor for your design is compressive strength, you are not in an area of appreciable tensile stress and so you are not concerned as much with things like residual strength of a cracked panel ie load shedding to substructure. However, this is simply an assumption - there are areas of a fuselage which see both tension and compression during a GAG cycle and crack buckling can be a real concern. So if you need to look at multi-element damage this would be a different ball game.
Keep em' Flying
//Fight Corrosion!
RE: How to size stiffeners?
Are you looking at wing or fuselage stiffeners ?
another day in paradise, or is paradise one day closer ?
RE: How to size stiffeners?
So the best I can tell, the process would be:
- Determine target design stress and sketch initial geometry to match this
- Calculate section properties to include the stiffener plus the effective skin width at the design stress (or 1/2 distance to next stiffener if skin is fully effective). Use this section for the following two calculations.
- Column buckling analysis, with length being from frame to frame, pin-ended (or whatever restraint I feel best), and considering eccentric loading if the stiffener runs out rather than attaching to the frame
- Crippling analysis
- Skin buckling analysis if no buckling is allowed (for us it is generally not allowed at limit load), typically assuming simply supported at frames and stiffeners
- Repeat process until everything has suitable margin of safety
Is this correct? Am I missing anything?
RE: How to size stiffeners?
Your basic problem is that you don't know any detail stresses. The best we can do (IMHO) is
1) calc the allowable stress for the pre-mod panel,
2) assume the load is the pre-mod allowable, (this isn't as easy as it sounds !?),
3) calc load redistribution caused by your mod,
4) reinforce the structure to handle these loads (lick o'paint).
another day in paradise, or is paradise one day closer ?
RE: How to size stiffeners?
Niu Chap. 14 "Compression Panels" will be your go to reference for sizing panels (skin as well as stiffeners).
>> considering eccentric loading if the stiffener runs out rather than attaching to the frame.
Could you elaborate on the above?
I'd think it would be wise to go through Flabel just to determine & be informed when effective width contribution from skin would be a valid consideration (or not). IIRC, it depends on the inter-rivet spacing and thus the buckling strength. Usually, the spacing is such that the effective skin area contribution would be effective, but from a knowledge perspective, it would be helpful to learn on how to determine the above.
Lastly, on your current mod issue, how are you sizing your stiffeners & skin if they are loaded in tension+shear? I have go through Bruhn or other reference books again, but I do not recall seeing any kind of MS interaction equation for shear & tension combined loading.
RE: How to size stiffeners?
RE: How to size stiffeners?
RE: How to size stiffeners?
And shear buckling. Bruhn C5, IIRC
If your cut-out is aft of the spar, there's always shear from torsion (the rudder).
Fuselage bending gives you shear on the sides (particularly around windows and doors).
If you track down a copy of Niu, you can look at Chapter 6 on "large cutouts", in addition to the chapters referenced already.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF
RE: How to size stiffeners?