Learning Forum

[Dan Woods]

Hi,

I'm building a cable stayed bridge in ANSYS mechanical and struggling to get LINK180 to act in tension only.

I've used SECCONTROL,,1 after defining the link element with SECTYPE but based on my output it doesn't seem to have worked.

When I plot FXI and FXJ the longest link element still seems to be taking a compressive load as its colour is within the -1966 to -9485 kN range. Am I using SECCONTROL incorrectly or is there some issue with my plot? All I did was make an etable with FXI and FXJ and then plot it with PLLS. The compressed cables are definitely the correct section since I can select them with the section ID.

For more context this is my definition of the link section:

SECTYPE,4,LINK,,Cable
SECDATA,0.12566370614359174
SECCONTROL,,1

Any help would be appreciated thanks.

[wrbulat]

Hi Dan,

I think you found a problem with the SECONTROL command. I could be wrong but after a fair bit of testing it appears that the command fails to establish tension/compression link settings for both LINK180 and CABLE280. The default for LINK180 is that it carries both tension and compression, and it seems SECONTROL,,1 is NOT changing this (consistent with your observation). The default for CABLE280 is the ratio of compressive stiffness to tensile stiffness is 1e-5. SECONTROL,,,,,CV3 appears NOT to change this either, but for CABLE280, the default behavior is to only very slightly carry tension - probably a pretty reasonable representation of reality So for now this is the best option I can recommend.

 

I created two test cases, each a vertical "tower" that is laterally supported by cables (think of a radio transmitting tower).

My LINK180 test case follows:

fini

/cle

 

/vup,1,z

/vie,1,3,2,1

/esha,1

 

 

C********************************************

C*** PARAMETERS

C********************************************

pi=acos(-1)

 

h_tower=25 ! TOWER HEIGHT

a_tower=0.1 ! TOWER SQUARE CROSS SECTION EDGE LENGTH

E_tower=2e11 ! TOWER ELASTIC MODULUS

nu_tower=0.3 ! TOWER POISSON'S

 

n_cables=3 ! # OF CABLES

h_cable=15 ! HEIGHT OF CABEL ATTACHMENT TO TOWER

r_cable=10 ! RADIUS OF CABLE ANCHORS TO GROUND

rcross_cable=0.01 ! RADIUS OF CABLE CROSS SECTION

E_cable=2e11 ! CABLE ELASTIC MODULUS

nu_cable=0.3 ! CABLE POISSON'S

 

esz=1 ! MESH SIZE

 

FX_top=1000 ! X COMPONENT FORCE AT TOP OF TOWER

 

 

C********************************************

C*** MODEL

C********************************************

/prep7

 

et,1,188 ! TOWER ATTRIBUTES

mp,ex,1,E_tower

mp,nuxy,1,nu_tower

sect,1,beam,rect

secd,a_tower,a_tower

 

et,2,180 ! CABLE ATTRIBUTES

mp,ex,2,E_cable

mp,nuxy,2,nu_cable

sect,2,link

secd,pi*rcross_cable**2

secontrol,,1 ! TENSION-ONLY

 

lsel,none ! TOWER GEOMETRY

k,1

k,2,,,h_cable

k,3,,,h_tower

l,1,2

l,2,3

dk,1,ux

dk,1,uy

dk,1,uz

dk,1,rotz

fk,3,fx,FX_top

latt,1,1,1,,,,1

 

lsel,none ! CABLE GEOMETRY

csys,1

*do,i,1,n_cables

 k,3+i,r_cable,(i-1)*(360/n_cables)

 l,3+i,2

 dk,3+i,ux

 dk,3+i,uy

 dk,3+i,uz

*enddo

 

latt,2,2,2,,,,2

 

alls ! MESH

esiz,esz

lmes,all

 

fini

 

 

C********************************************

C*** SOLVE

C********************************************

/solu

acel,,,9.81

nsub,5,25,5

outr,all,all

nlge,on

pivc,off

alls

save

solve

 

 

C********************************************

C*** POST PROCESS

C********************************************

/post1

/pbc,rfor,,1

/pbc,f,,1

/esha,10

/vsc,1,2.5

plns,u,x

 

/eof

 

 

 

My CABLE280 test case follows:

fini

/cle

 

/vup,1,z

/vie,1,3,2,1

/esha,1

 

 

C********************************************

C*** PARAMETERS

C********************************************

pi=acos(-1)

 

h_tower=25 ! TOWER HEIGHT

a_tower=0.1 ! TOWER SQUARE CROSS SECTION EDGE LENGTH

E_tower=2e11 ! TOWER ELASTIC MODULUS

nu_tower=0.3 ! TOWER POISSON'S

 

n_cables=3 ! # OF CABLES

h_cable=15 ! HEIGHT OF CABEL ATTACHMENT TO TOWER

r_cable=10 ! RADIUS OF CABLE ANCHORS TO GROUND

rcross_cable=0.01 ! RADIUS OF CABLE CROSS SECTION

E_cable=2e11 ! CABLE ELASTIC MODULUS

nu_cable=0.3 ! CABLE POISSON'S

cv3=1 ! STIFNESS RATIO (COMPRESSION TO TENSION)

 

esz=1 ! MESH SIZE

 

FX_top=1000 ! X COMPONENT FORCE AT TOP OF TOWER

 

 

C********************************************

C*** MODEL

C********************************************

/prep7

 

et,1,188 ! TOWER ATTRIBUTES

mp,ex,1,E_tower

mp,nuxy,1,nu_tower

sect,1,beam,rect

secd,a_tower,a_tower

 

et,2,280 ! CABLE ATTRIBUTES

mp,ex,2,E_cable

mp,nuxy,2,nu_cable

sect,2,link

secontrol,,,,,cv3

secd,pi*rcross_cable**2

secontrol,,,,,cv3 ! STIFNESS RATIO (COMPRESSION TO TENSION)

 

lsel,none ! TOWER GEOMETRY

k,1

k,2,,,h_cable

k,3,,,h_tower

l,1,2

l,2,3

dk,1,ux

dk,1,uy

dk,1,uz

dk,1,rotz

fk,3,fx,FX_top

latt,1,1,1,,,,1

 

lsel,none ! CABLE GEOMETRY

csys,1

*do,i,1,n_cables

 k,3+i,r_cable,(i-1)*(360/n_cables)

 l,3+i,2

 dk,3+i,ux

 dk,3+i,uy

 dk,3+i,uz

*enddo

 

latt,2,2,2,,,,2

 

alls ! MESH

esiz,esz

lmes,all

 

fini

 

 

C********************************************

C*** SOLVE

C********************************************

/solu

acel,,,9.81

nsub,5,25,5

outr,all,all

nlge,on

pivc,off

alls

save

solve

 

 

C********************************************

C*** POST PROCESS

C********************************************

/post1

/pbc,rfor,,1

/pbc,f,,1

/esha,10

/vsc,1,2.5

plns,u,x

 

/eof

 

Copy these into two separate files and read them individually into MAPDL interactively. They solve instantly and leave you looking at a post processing plot including reaction force symbols from which you can infer whether or not the forward cable (the one extending in global x direction) is carrying tension in response to the x component force applied at the top of the tower. We of course expect it to go slack.

Thanks for keeping us honest,

Bill

[Dan Woods]

Thanks for the response, swapping to cable280 does seem to have solved my problem.

[Dan Woods]

Hi Bill,

Being forced to use nonlinear analysis as I'm forced to use cable280 is actually causing some problems. Are there any other alternative ways of modelling a tension only axial element? How did this bug even go unnoticed, does noone use link180?