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DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

(OP)
when viewing the mode shapes of a 3d building with a symmetrical rectangular (LX=LY) plan view at all stories (stiffness and mass symmetry)something strange is happening. The deformed shape of X and Y displacement Natural Modes (1st, 4th e.t.c. modes) is displaced diagonally. The same is illustrating at TABLE: Modal Participating Mass Ratios. But in symmetrical plan view 3D buildings i'm waiting to see only X or Y displacements at X or Y displ.Natural Modes.
I'm modelling the X and Y Mass and Moment Mass Inertia about Z at master Joints at CM of stories with constrained nodes with Diaphragm constrain.
This is happening only at RECTANGULAR plan view symmetrical 3D Buildings. The mode shape results of Orthogonal plan view 3D Building (LX>LY or LY>LX) are OK and physsicaly expected.
Why is this happening ?

TABLE: Modal Participating Mass Ratios
OutputCase StepType StepNum Period UX UY
Text Text Unitless Sec Unitless Unitless
MODAL Mode 1 1.255363 0.43213 0.43032
MODAL Mode 2 1.253714 0.4314 0.43322
MODAL Mode 3 0.876538 0.00108 0.00108
MODAL Mode 4 0.414052 0.04878 0.04817
MODAL Mode 5 0.413505 0.0483 0.0489
MODAL Mode 6 0.289555 0.00012 0.00012
MODAL Mode 7 0.248978 0.01521 0.0152
MODAL Mode 8 0.248646 0.01524 0.01525
MODAL Mode 9 0.188519 0.00384 0.00384
MODAL Mode 10 0.188266 0.00385 0.00385
MODAL Mode 11 0.174579 0.0000399 0.00003991
MODAL Mode 12 0.132425 0.00001008 0.00001008

RE: DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

There are plenty of cases where the natural mode shapes will deflect diagonally. One classic example is a structure with 4 walls/frames making a + in plan. Since the plan X and Y stiffnesses are greater than the plan 45 degree or 135 degree stiffnesses, those mode shapes will have longer periods and be the principal cases in most finite elements solutions.

RE: DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

(OP)
Thank's for your answer. I'll make you clear, my analysis program is sap2000 v.16 and my 3d rectangular plan view symmetrical building (stiffness and mass symmetry) is conformed from 3 identical frames per direction with two equal spans per frame and my material is concrete. The displacement mode shape response in either dir is diagonal (not the diagonal of plan view) without rotation of stories, AND the angle of diagonal dispacement of stories is increcemental from 1st disp. mode to upper displ. modes response.
So from your answer i understand that this diagonal displacement response of displ. modes (without rotation of stories) can happen. If you want to make a comment about this please do it!

RE: DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

If you have some screen shots of your deflected shape that would help. Attach them please.

Not sure about your question, but for a symmetrical building with the center of mass on top of the center of rigidity, your primary Modes (1 and 2) should be displacement in each X and Z direction. The third mode should be torsional.

You should be able to see a plot of your center of mass and center of rigidity. Check your mass layout first and make sure the mass if where it should be and with the correct magnitude. With the CSI products, I believe you input everything as a mass rather than force, so keep that in mind with your units!

If your primary mode is torsional, something is wrong as you suspect. (unless of course, the building is torsional irregular!)

RE: DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

Maybe your frame stiffness is very equivalent in each direction, so you don't have a dominant mode in either X or Z direction...

Maybe try using a much stiffer frame in one direction, and see if your mode shapes stops dancing sideways.

RE: DISPLAYING MODE SHAPES AT A RECTANGULAR PLANVIEW 3D SYMMETRICAL BUILDING

(OP)
I attach the SAP2000 file (with v.15.2). Can you examine it and tell me your conclusions ?
This is a 4 story symmetrical frame concrete building with rectangular plan view (CR coincides CM) and equal stiffness at each Dir. Please take a look to Displacement Natural Mode Shapes. It seems that X or Y displ.modes have Y or X parts.
If i will run a response spectrum analysis, the superposition mode results will be accurate ?
Thanks.

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