I am trying to calibrate a 3D structure from a centrifuge test in FLAC2D using beam elements. To account for the plane strain condition, I calculated the cross sectional area as 1 m* thickness of the element into the page, and moment of inertia as 1m*thickness^3/12. My hand calculations give me my target natural period, but for some reason, the free vibration analysis in FLAC shows a different stiffness. I think my issue might be related to how I’m modeling the structure and using gravity. The structure has a 0.7 m embedment depth so I fixed those nodes in this code, but with my actual soil model, those nodes are slaved to the soil. I can’t figure out why I’m getting a Tn=0.7 seconds instead of 1.0 seconds so I would appreciate any help. I am attaching my fish code.
Main questions are:
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Should I solve for static equilibrium first and then apply the force when gravity is on, or should I do all in one step? If I do all in one step, I have a downward movement before a horizontal movement.
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Am I making wrong assumptions when converting 3D properties to plane strain? If not what am I doing wrong?
model new
model title “Centrifuge Test 1- Structure F1 (9.4 m x 9.4 m)- All Beams”
model configure dynamic
model large-strain off
model gravity 9.81
model dynamic active off
fish define _prop
; Foundation (Aluminum)
_foundation_dens = 3943 ;kg/m3
_foundation_E = 6.9e10 ;Pa
_foundation_poiss = 0.3
_foundation_a = 0.73
_foundation_I = 0.03
; Steel Columns
_col_E= 2.2e11
_col_pois= 0.3
_col_dens = 7800
_col_a= 0.14 ;m2
_col_I= 2.4e-4
_L = 6.1
_k_col = 212.0_col_E* _col_I/(_L*_L*_L)
; Top Mass (Lumped Mass)
_lumpedmass_E= 2.2e11
_lumpedmass_pois= 0.3
_lumpedmass_dens = 3724
_lumpedmass_a= 1.74 ;m2
_lumpedmass_I= 0.44
end
[_prop]
;create foundation
structure beam create by-line (0.0 0.0) (9.4 0.0) segments 10 id 1 …
element-type euler-bernoulli group ‘Foundation’
structure beam create by-line (0.0 0.0) (0.0 0.7) segments 1 id 2 …
element-type euler-bernoulli group ‘Column1_edge’
structure beam create by-line (9.4 0.0) (9.4 0.7) segments 1 id 3 …
element-type euler-bernoulli group ‘Column2_edge’
;create column 1
structure beam create by-line (0.0 0.7) (0.0 6.8) segments 3 id 4 …
element-type euler-bernoulli group ‘Column1’
;column 2
structure beam create by-line (9.4 0.7) (9.4 6.8) segments 3 id 5 …
element-type euler-bernoulli group ‘Column2’
;lumped mass
structure node create (0.0 6.8)
structure node create (9.4 6.8)
structure beam create by-nodeids 24 25 segments 1 element-type euler-bernoulli group ‘Mass’
structure node join
structure beam cmodel assign elastic
structure beam property young [_col_E] poisson [_col_pois] density [_col_dens] …
cross-sectional-area [_col_a] moi= [_col_I] range group ‘Column1’ or ‘Column2’ or ‘Column1_edge’ or ‘Column2_edge’
structure beam property young [_lumpedmass_E] poisson [_lumpedmass_pois] density [_lumpedmass_dens] …
cross-sectional-area [_lumpedmass_a] moi= [_lumpedmass_I] range group ‘Mass’
structure beam property young [_foundation_E] poisson [_foundation_poiss] density [_foundation_dens] …
cross-sectional-area [_foundation_a] moi= [_foundation_I] range group ‘Foundation’
; fix the bottom beam
;
structure node fix velocity range component-id 1
structure node fix rotation range component-id 1
structure node fix velocity range component-id 2
structure node fix rotation range component-id 2
structure node fix velocity range component-id 3
structure node fix rotation range component-id 3
structure node fix velocity range component-id 4
structure node fix rotation range component-id 4
structure node fix velocity range component-id 5
structure node fix rotation range component-id 5
structure node fix velocity range component-id 6
structure node fix rotation range component-id 6
structure node fix velocity range component-id 7
structure node fix rotation range component-id 7
structure node fix velocity range component-id 8
structure node fix rotation range component-id 8
structure node fix velocity range component-id 9
structure node fix rotation range component-id 9
structure node fix velocity range component-id 10
structure node fix rotation range component-id 10
structure node fix velocity range component-id 11
structure node fix rotation range component-id 11
structure node fix velocity range component-id 12
structure node fix rotation range component-id 12
structure node fix velocity range component-id 13
structure node fix rotation range component-id 13
structure node fix velocity range component-id 14
structure node fix rotation range component-id 14
structure node fix velocity range component-id 15
structure node fix rotation range component-id 15
structure node fix velocity range component-id 16
structure node fix rotation range component-id 16
structure node fix velocity range component-id 20
structure node fix rotation range component-id 20
;apply a static force
structure node history displacement-x position 0.0 6.8
structure node history velocity-x position 0.0 6.8
structure node history displacement-y position 0.0 6.8
model solve ratio 1e-6
;structure node initialize displacement 0.0 0.0
structure node apply force [0.0001*_k_col] 0 range position-x 0.0 position-y 6.8
model solve ratio 1e-6
model save ‘Structure1_beams_static’
model restore ‘Structure1_beams_static’
;remove force
structure node apply remove force
model dynamic active on
model dynamic time-total 0
model history dynamic time-total
model solve time-total 1.2
model save ‘Structureallbeams_dynamic’