statically determinate and indeterminate aircraft structure

[bykncn]

Hello everyone,

I'm currently working on a structural analysis for an aircraft cabin interior furnishing. In my research, I came across guidance in the Boeing D6-55441 document that discusses test fixture validation and the determination of whether a component should be modeled/tested using a rigid support or if its actual stiffness values need to be explicitly simulated. However, I'm having trouble interpreting the exact criteria or thresholds regarding when to use a rigid support assumption versus when to incorporate specific stiffness values into the analysis/test for structure/unit. Are there specific thresholds or guidelines or related industry standards that indicate when the structure/unit should be considered statically indeterminate rather than determinate? Any practical examples or case studies that illustrate this decision-making process for cabin interior furnishings with attachment(hardpoint or seat track, etc.) or any type of structure?

 

[dcascap]

Hi,
I don't see the relation between fixed support and statically indeterminate. As you could have statically determinate structures that use rigid supports...
Also is this a FE model are you trying to build? Could you provide a bit more info in the furnishing and attachment points?
I will try to give a global reasoning on your question based on the info you already provided.
Coming back to the question of when or when not to sue stiffness values, I would say if you can be conservative in your assumption then it's fine.
If you have the stiffnesses of your attachment points I would always include them but I can imagine that is not the case...
To know if it is conservative you need to check what is the real load path of expected load path of the structure and build your model depending on it. You could also use sensitivity analysis in your fixtures to get a feeling and see what is conservative, in the case it is too difficult to know what is happening.

Cheers,
Diego

 

[bykncn]

It's not just about creating tests or analytics, it's about both at the same time because both need to have the same assumptions as much as possible. Boeing D6-55441 document says that if your structure is determinate then rigid fixture is enough for both testing analyzing but otherwise you need to have correct stiffness value for your support point of your structure. It can be any interior furnishing(galley, lavatory, oven, partition/divider).

 

[Ng2020]

Have a read of this.
Might provide a bit more friendly guidance than the d6 doc.

https://gama.aero/documents/gama-publication-13-acceptable-practices-document-cabin-interior-monument-structural-substantiation-method-version-1-0/

 

[davidparker]

Hello everyone,

I'm currently working on a structural analysis for an aircraft cabin interior furnishing. In my research, I came across guidance in the Boeing D6-55441 document that discusses test fixture validation and the determination of whether a component should be modeled/tested using a rigid support or if its actual stiffness values need to be explicitly simulated. However, I'm having trouble interpreting the exact criteria or thresholds regarding when to use a rigid support assumption versus when to incorporate specific stiffness values into the analysis/test for structure/unit. Are there specific thresholds or guidelines or related industry standards that indicate when the structure/unit should be considered statically indeterminate rather than determinate? Any practical examples or case studies that illustrate this decision-making process for cabin interior furnishings with attachment(hardpoint or seat track, etc.) or any type of structure?

I've worked with D6-55441 for cabin structures before, and from my experience, the decision depends on load path complexity and attachment flexibility. Generally, if the mounting points are significantly stiff relative to the component, a rigid support assumption is reasonable. However, if there's notable compliance (like seat tracks or hardpoints with some flexibility), modeling actual stiffness is better for accuracy. No strict thresholds, but FEM validation with correlation to test data helps. A good rule of thumb—if deflections or load redistributions meaningfully affect stress results, go with explicit stiffness modeling. Hope that helps!

 

[bykncn]

It says same things "For multi-load path, indeterminate structures, the airplane support structure stiffness can have astrong influence and therefore, the stiffness as well as the geometry should be simulated." but why is it because of deflections? why does stiffness strong influence when structure is indeterminate?

 

[rb1957]

"why does stiffness strong influence when structure is indeterminate?" ... seriously ? I think you need to review statically indeterminate structures. Stiffness is the whole point ! Statically Determinate structures are solved using the equations of equilibrium, because they have fewer than (or exactly) 6 independent reactions. Statically Indeterminate structures can't be solved by equation of equilibrium alone as they have more reactions than the number of available equations, an example is a propped cantilever. To solve these we use deflections and stiffness ... as an example, remove the prop and solve the cantilever, now determine how far the unsupported tip deflects; now assume there is a restoring load at the tip, how much load is required to cancel the deflection ? then combine these two and voila !

IMHO, you should never over constrain a model (unless you are being very careful). People often over constrain interior monuments ('cause it's easy, and they are "lazy"). It is just as easy to use finite stiffness to support the monument and to get reasonable results.

It is also "easy" to test a monument (obviously some are easier to test than others). I'd say that a couple of tests should be enough to validate a modelling approach.

 

[SWComposites]

The stiffness of the supporting aircraft structure can significantly affect the load distributions in the monuments; the airplane is NOT rigid. That is why the D6 document exists and why the GAMA document exists and probably several FAA advisory circulars and policy memos (hat I don't want to bother to look up) also exist. In general your monument FEM should not assume rigid supports.

 

[WKTaylor]

/WARNING1/ Be extremely careful citing proprietary company documents... such as the Boeing D6-55xxx document in Your post #1 in this thread. Also, NEVER-EVER actually post or copy data technical data from any proprietary document.

WHAT Kinds of furnishings... galleys, cart stowage, toilets, cabinets, stowage bins/clothing racks, PAX seats, crew seats, palletized, etc? Anchor mounted, track/rail mounted, webbing restrained , etc. composition: 100% metallic, 100% composite, mixed materials... ETC.
Related? Smaller Acft...
GAMA PUBLICATION NO. 13 ACCEPTABLE PRACTICES DOCUMENT, CABIN INTERIOR MONUMENT STRUCTURAL SUBSTANTIATION METHODS
/NOTE1/
/GAMA document has many important/related references/
/See other documents/
FAA... AC, STC, Cert, ACs, FO's, FARs, documents, etc.
SAE... AIR/ARP/AS on specific items, such as seats, toilets, galleys, cargo, etc

 

[GregLocock]

It's about an hours work to measure the stiffness of say 4 seat mounting brackets in situ with a hammer and an accelerometer in 3 translational directions, rotational stiffness is far more difficult, but is not usually necessary unless you are cantilevering from a single point, which would normally be a rotten design.

 

[rb1957]

it is 10^-4 to 10^-6, give or take. the precise stiffness doesn't significantly impact the results.

with infinite stiffness large redundant forces develop as the structure will try to deflect ever so slightly (but is prevented from doing so, hence the large force).
any sort of finite stiffness allows the small deflection and the results are much more reasonable.

to my mind it is "lazy" to bang on redundant constraints and say "it's conservative" which I'm sure it is but ...

 

[GregLocock]

My rule of thumb for cars is that with special attention I can get 50 kN/mm at a mounting point in the important direction, but generally 5 kN/mm is more typical. Honda had a fad for testing every bracket and component for a while, it is good, you get to develop a useful level of gut feel for what typical features will give you.

 

[rb1957]

I agree, we have developed our own "mark 1 eyeballs", what is typical for our industries. But calculating a stiffness can be "daunting", so trying out some analytically can show how significant the value is (or isn't) on the results. The OP should develop his own experience, rather than relying random folks on the interweb.

 

[bykncn]

Thank you all for answers.