Effect of Longitudinal Floor Beams on Fuselage Neutral Axis

[SAITAETGrad]

Can anyone shed some light on the effect of longitudinal floor beams on the fuselage neutral axis due to vertical load bending?

Bruhn and the Boeing FEA guidelines suggest that the analysis only include the basic skin and stringers.

But Howe's Aircraft Loading and Structural Layout points out that longitudinal floor beams added below the neutral axis (such as CRJs, Gulfstreams, Lears, etc.) can have a negative effect on crown skin stress.

Since the longitudinal floor beams are attached at relatively unstiff lateral floor beams and not directly tied to the hull, how effective are these members in fuselage bending relative to the longerons, stringers, and skin? What effect does the length of these longitudinal floor beams have in their bending effectivity?

What pitfalls exist for aircraft modifiers?

Are there any references (OEM or otherwise) that covers this issue?

 

[rb1957]

one question, how weel connected are the floor beams to the fuselage ? how can the bending loads get into them ? alot of times the floors are not very structural (they seem to be there to stop the pax from interferring with the flt cntl cables) so assuming them to be ineffective would be reasonable. some OEMs include the floors in the static test articles which makes them primary structure ... to your 2nd question, how would a modifier ever know ?? to your 1st question, whether the beams are effective or not, their effect on the crown skins has got to be about 1 "RCH". sure in smaller planes the floor is (proportionally) a lot lower than on a larger one, but i'd return to the original observation, rephrasing ... can these beams deflect independently of the fuselage skin, or do deflections onthe fuselage drive deflections in the beams ?

on the other hand, your basic premise is an excellent one ... if i'm modifying something, what else could possibly be affected ? if only everyone was as thorough.

 

[Dan320]

Seems unreasonable to assume the floor beams to remain straight when the fuselage deflects.
If it was my problem I would make up a cross section in Auto-Cad and let the sofware calculate the I values with and without the floor beams. Just to get a feel for the size of it. This model probably doesn't have to be dimensionally correct to the last decimal.

 

[SAITAETGrad]

Thank you, RB (yet again). I am starting to think my question is an "academic" one. My simplified calc shows a 5% life reduction due to the addition of a number of longitudinal floor beams with the assumption that these are 100% effective in fuselage bending. Obviously, they are not.

The next step would be a highly simplified FEA model of constant section and fixed joints on the floor support structure to the frames (conservative). I suspect the relative effect of the addition of multiple longitudinal floor beams across many frames will be quite low.

 

[SAITAETGrad]

Hi Dan, thank you for your response. I did exactly what you suggested prior to posting the thread. The effect of this sectional approach(5% crown fatigue life reduction) was enough to warrant a closer look.

I'm sort of polling others out there that have maybe done this before for what design feel they have for the problem. Obviously, the number of longitudinal floor beams, their sections, and attachment are all relevant. I'm just interested in whatever thoughts exist in general terms. Is this a potential show-stopper for a mod or is this an academic question...