Create rotational spring at beam ends

[ZegersSander]

Hi,

I want to model a beam and find it's natural frequency. At the end of the beam I want to add a rotational spring. Therefore I define a double node at the same coordinates and use Combin 14 (Also tried combin7). I used the D command on the node that only connects to the Combin element and coupled the UX and UY of both coincedent nodes.

This however doesn't seem to work. Using the Combin 7 element there is no influence of the rotational stiffness. When using combin 14 I need to use the D command on both nodes, otherwise it gives rigid body movement

Is this the correct element and am I using it correctly

Any help is greatly apreciated

Here the input file

FINISH
/CLEAR,START
/BATCH
! /COM,ANSYS RELEASE 8.1A1 UP20040609 15:01:53 10/27/2004
!*
/NOPR
/PMETH,OFF,0
KEYW,PR_SET,1
KEYW,PR_STRUC,1

/GO

/PREP7
!Parameters in in standaard eenheden
Blength=10 !in meter
Bmass=10
Bnodes=51
massa=100 !in kg
Emodulus=2.1e5 !in N/mm2
Akom=1000e4 !in mm4
Hoogte=200 !in mm
RotatiestijfheidLinks=1e10 !in Nm/rad
RotatiestijfheidRechts=1e10 !in Nm/rad

!Omrekenen naar SI-eenheden
rekEmodulus = Emodulus*1e6
rekAkom = Akom/1e12
rekHoogte =Hoogte/1000
rekOpp = 1

ET,1,BEAM3
ET,2,COMBIN7
ET,3,COMBIN14,0,6

KEYOPT,1,6,0
KEYOPT,1,9,9
KEYOPT,1,10,0


R,1,rekOpp,rekAkom,rekHoogte,0,0,massa
!R,2, , , ,RotatiestijfheidLinks
!R,3, , , ,RotatiestijfheidRechts
R,4, RotatiestijfheidLinks
R,5, RotatiestijfheidRechts
MP,EX,1,rekEmodulus
MP,PRXY,1,0.3

TYPE,1
MAT,1
REAL,1

!Creeer knopen
*DO,teller,1,Bnodes,1
N,teller,(teller-1)*Blength/(Bnodes-1),0
*ENDDO

!Dubbele knopen tbv veer
N,Bnodes+1,0,0
N,Bnodes+2,Blength,0

!Hulpknopen tbv combin7 element
!N,Bnodes+3,0,0,1
!N,Bnodes+4,Blength,0,1

!Balkelementen
*DO,ecurrent,1,Bnodes-1,1
EN,ecurrent,ecurrent,ecurrent+1
*ENDDO

!Combin14 elementen
TYPE,3
REAL,4
EN,Bnodes,Bnodes+1,1
REAL,5
EN,Bnodes+1,Bnodes+2,Bnodes

!Koppelen van translatie vrijheidsgraden in de dubbele knopen
CP,1,UX,Bnodes+1,1
CP,2,UY,Bnodes+1,1
CP,3,UX,Bnodes+2,Bnodes
CP,4,UY,Bnodes+2,Bnodes
!CP,5,ROTZ,Bnodes+1,1
!CP,6,ROTZ,Bnodes+2,Bnodes
!D, 1, UX, 0, , Bnodes, Bnodes-1, UY
D, Bnodes+1, UX, 0, , , , UY,ROTZ
D, Bnodes+2, UX, 0, , , , UY,ROTZ

!Rvw knopen aan balkelement. Het lijkt me niet dat deze vastgezet moeten worden, maar wanneer dit niet gebeurd is
!het resultaat een verticale translatie. Het vastzetten zou met het CP commando moeten zijn verzorgd.
!D, 1, UX, 0, , , , UY
!D, Bnodes, UX, 0, , , , UY

FINISH



/SOL
ANTYPE,2
MODOPT,LANB,5
EQSLV,SPAR
MXPAND,5, , ,0
LUMPM,0
PSTRES,0
SOLVE
FINISH


/POST1

SET,1,1

/DSCALE,ALL,AUTO
/VIEW,1,1,1,1
/ANG,1
/PNUM,NODE,1
/PSYMB,DOT,1
/PBC,ALL,,1
PLDI, ,
ANMODE,20,0.1, ,0

FINISH

 

[Drej]

What is a "rotational" spring?

 

[ZegersSander]

Probably bad english

I'll try and describe it.

At the beam ends I want to restrict in a certain amount the free rotation, so when the beam-end rotates, a moment, proportional to the rotation is created.

 

[Drej]

OK.

I think you need KEYOPT(3) = 1 for the combin14 (KEYOPT,3,3,1 in your input deck OR ET,3,COMBIN14,0,0,1), hopefully it will be clear (and a lot easier than me trying to explain) if you look at the help file for combin14. If KEYOPT(2) is zero, the length of the spring-damper element must not be zero, i.e., nodes I and J should not be coincident, since the node locations determine the spring orientation. Hence, go to a 3D beam but constrain all the Z-DOF and rotations, and make the spring nodes non-coincident. Don't forget to ground (apply constraint to) the spring nodes!

Good luck,

-- drej --

 

[ZegersSander]

Drej,

Thanx for your answer. Though it didn't prove to be giving the correct geometry it put me on the track of checking the DOF. I use COMBIN14 with KEYOPT(2)=6, which results in a spring with only the ROTZ DOF. It also requires the nodes to be at the same coordinates.

Because it only has the ROTZ DOF it can't use the info that the UX and UY DOF are set to be 0 (fixed), and so the coupling doesn't work.
By only fixing the ROTZ for node 1(only Combin14) and the UY and UX for node 2(Connection node between Combin14 and Beam) it works great.