floor beams load due to fuselage pressurization load

[ABV97]

Hi All,
I am analyzing a floor beam modification and was wondering how can I incorporate fuselage pressurization load on top of 6g down load, no FEM model available? Conservatively stress within the floor beam due to pressurization load would be around 10 ksi, but I need to validate it. Your thoughts will be appreciated.

 

[rb1957]

6g down isn't an awful lot ... ultimate ?? (I'd be happier if it were limit.)

load due to pressure would depend a lot on the fuselage ... round or double lobe ?

it should be an easy model it investigate the load due to pressure.

why do you think the floor beam stress would be 10ksi ? hoop stress ??

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

Use a free body diagram of the floor loads applied to the beam??

 

[ABV97]

...well I thought floor beam would react the pressurization load, as a balloon expanding and creating mostly axial tensile loads on the beams. It is B737, with 20' beams pitch, so I assumed a area of A =20 x 40 = 800 in^2, delta p = 9 psi(assumed), P = 800 x 9 = 7,200 lbf. Beam cross section area is 1.13 in^2, so f = 7,200/1.13 = 6372 psi (tension). Not sure if this approach is sound, or I should go to the skin long and hoop stress and somehow transverse it to the beam?

 

[rb1957]

I believe a B737 is a double lobe fuselage, so the underfloor beams act like a tie-rod between the fuselage sides.

it hard to do a "back of a fag packet" calc ... the fuselage frame messes up the hoop stress calc significantly. there's some logic to assuming that the stress is equal to hoop stress (to have the same durability as the skin) ... this should be conservative.

I guess the problem is that you're splicing onto the beam and need to "show it good". the problem is that the splice increases the Kt (at the load transfer rivets) in the beam at the start of the splice (where you have added rivet loads to the unmodified beam). I'd also expect the beam to be in bending (not pure tension), so there's a 50% chance you're on the compression side ... but you'd need to figure that out (with some FE, which should be a simple model to build)

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

I thought the sizing load cases for floor beams were depressurization and crash loads. The tensile load due to reacting the double bubble pressurization is probably not that critical.

 

[ABV97]

Thank you SWComposite, I think, on top of 6g down ultimate flight load, load of the floor beams due to pressurization should be included as well!

 

[Sparweb]

SW,
You're right, as far as ultimate loads go, but in the floor beam area it seems like the fatigue load cycle will consume 90% of the number-crunching time. Especially if the modification changes the stress ratio or the ground-air-ground cycle, as RB1957 has pointed out.

ABV97,
Back to the "back of the envelope" calcs. I believe it's possible to build a system of equations that represent the pressure loads on the frames, their consequent stresses, and resulting expansion of the frames as they "inflate", and then solve for the tensile stress in the floor beams that apply an equal pinch load on the frame, and stretch the beam at the same time. This would be a system of equations to be solved in a matrix, and require a detailed free-body diagram of each node.

I've seen this done as "hand calculations" by an engineer who worked for Shorts and later Lockheed, working it out for a cargo door mod (on what aircraft, I can't remember). He did not use FEA, but it did make for a thick report. Sorry, I haven't done it myself, to know if it really is "as easy as it sounds".

Good luck (sounds like fun to me).


STF

 

[rb1957]

"so I assumed a area of A =20 x 40 = 800 in^2, delta p = 9 psi(assumed), P = 800 x 9 = 7,200 lbf" ... well, that's an assumption and if you like it, great; who's going to look over your calcs ? someone who'll question the details ? or someone who'll accept 7ksi in the underfloor beam due to 9psi pressure ? For me there's a story to assume hoop stress (durability of the u/floor beam should be like the durability of the cabin shell).

it's a reasonable number, with a somewhat reasonable story, untill someone says "show me". you might boost your story (based on pressure only loads) by using the 2deltap case as your ultimate design. something to remember is cabin payload, bending the beam ?

another day in paradise, or is paradise one day closer ?

 

[ABV97]

...we did go to an FEM model, will see how it goes. Thanks all for your thoughts!

 

[stressebookllc]

You will have interiors structure loading the floor beams in the 6.0DWN case, this can be a flight load case, sometimes limit and I have also seen integrators define it as ultimate for interiors structures.

Regarding load on the floor beam from cabin pressure, there should be a running load along the width applied on the floor beams. This comes from the pressure on the floor panels above the floor beams. Same would apply in case of a rapid decompression case, UP or DOWN. My 2 cents.

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