Floor beam end fixity 2

[JJ22mW]

Hello,

I have a question concerning end fixity of floor beams. Am I correct in interpreting end fixity as the ratio of the end rotation of the beam when subject to a unit moment (theta1 = L / 3EI) to the sum of theta1 and the rotation of the end support when subject to a unit moment (theta2 = 1 / Ksupport)?

So,
fixity = theta1 / (theta1 + theta2)

Please clarify if I am incorrect. Also, how are end fixities typically modeled in FEM, using torsional springs? If so, how are the values of the torsional springs related to the fixity values. I have searched the popular aircraft analysis references (Niu, Bruhn) and have not been able to fully understand this subject. Any insight is much appreciated, thanks.

 

[SWComposites]

depends, depends, depends, ...

what type of aircraft?

some floor beam to frame connections are designed as pinned joints, some have moment continuity; what is your specific configuration?

how are you modelling the floor beams and frames in the FEM? beam elements, plates for webs + beams for chords, fine grid plates, solids, ??

do you need to use a torsional spring because its a beam FEM?

I don't think your fixity formula is correct or useful.

 

[JJ22mW]

Typical applications are for 737, 747, etc. My company does executive interiors and many times we have to model floor structures to obtain x,y,z stiffnesses at certain locations of interior monument attachments. Floor structure is typically modeled using bar elements with pinned/fixed constraints used at beam/frame connections when specified in Boeing docs. Sometimes the documents specify end fixities (50%, 55%, etc) at connections to frames. My main questions are what is the "definition" of end fixities for this application and how are the values incorporated/calculated for input into floor structure FEM models. Thanks for your input. PS, I got the definition for end fixity in the original post from a civil eng book on semi rigid frames, so it may or may not related to the aircraft industry concept/definition of fixity.

 

[ieaz123]

The Boeing meaning of floor beam end fixity is a percentage of the fully fixed end moment. For narrow body it is usually 33% for flight conditions and 50% for emergency landing conditions.

Make a pinned-pinned bendimg moment diagram and a fixed-fixed BMD. For 33% fixity your BMD is 33% of the way from PP to FF.

 

[JJ22mW]

Thanks for the reply. Are end fixities typically modeled using torsional springs when the beam is modeled in the FEM using beam elements? If so, how is the spring constant value determined? Since the end moments will be a function of both the beam itself and the support structure, can you point me in the direction of the applicable equation relating the stiffnesses of the beam/support and the end fixity values? Or is it typical practice to just massage the spring constant values until the desired percentage of the fixed-fixed condition end moments are achieved?

 

[RPstress]

End fixity is usually a hand calc issue. I've never seen an FE model with rotational springs at beam ends. With FE the assumption is usually that the stiffness of the assembly is correctly modelled by the stiffness of the structure at the beam ends. Can't speak for Boeing fuselage modelling practice.