## General Mechanical

Topics relate to Mechanical Enterprise, Motion, Additive Print and more

#### Apply rotational stiffness to a base

• Moustafa El-Sawy
Subscriber

I have a concrete block made up of SOLID elements. I am currently applying a spring at the base at each mesh element but I found I also want to apply rotation stiffness to the entire foundation. I have a single value of the stiffness of the system in each direction including rotation stiffness. Before knowing I need to apply rotational stiffness, I took the total value of stiffness in each translation direction and applied a portion of this to each node based on its tributary area. However I also now need to apply rotation stiffness to the entire system so I think this approach will not work.

SOLID nodes dont have rotation DOF, so I was thinking I would create a master node and assign it a single spring with x,y,z,rotx,roty,rotz and then connect this node with all my base nodes. This would distribute the stiffness to the nodes and allow the system to rotate with a certain stiffness.

• Can I perform this without tying all base nodes (X,Y,Z)? My behavior is most likely rigid but I was wondering if its possible. I dont think its possible but please correct me if im wrong
• If its not possible, can I make my base like a rigid plate (X,Y) direction but also able to rotate around the master node with a certain stiffness (around x,y).
• peteroznewman
Subscriber

Select the bottom face of your block and insert a Spring in the Connections folder. In the Details window, change the Scope to a Body-Ground spring and change the Behavior of the Mobile side (bottom face) to be Deformable.  Deformable means that the individual nodes in the face will have the freedom to move according to the loads distrubuted to them by the Deformable connection, but that the average motion will be controlled by the spring’s master node.

The spring created in Mechanical is a Longitudinal spring, COMBIN14, which is not what you want, but you can override that spring type with another spring element called a COMBI250 by using a Command Object under the Spring object in the model Outline.

A COMBI250 allows you to define six spring constants for the 3 translational and 3 rotational stiffness values.

Look up the COMBI250 element in the Mechanical APDL, Element Library section of the Help system. One line of the APDL code will switch the element type to COMBIN250 and other lines of code will provide the Real Constants for the spring stiffness values.

Here is one example of the needed code.  Here is another example.

• Moustafa El-Sawy
Subscriber

Thanks Peter. I am using APDL. With the COMBI250, I am able to assign springs in all directions at the nodes at the bottom of my solid elements even though the solid element nodes dont have DOF for rotation?

• peteroznewman
Subscriber

No, a rotational spring attached to a node on a solid element will not be able to apply any moment to that node. A pilot node that distributes forces and moments to a set of nodes can have a moment load applied to the pilot noe, which will be resolved as forces on the set of nodes it is connected to.

• Moustafa El-Sawy
Subscriber

Hi Peter, ive been trying to apply some rotation stiffness to my whole base but im unsure if my approach is correct can you take a look at the code below:

/CLEAR               ! Clear all previous data and start fresh
/PREP7               ! Enter preprocessor phase

! Element Definitions
ET, 1, SOLID185      ! Define SOLID186 element type for solid modeling
ET, 2, COMBIN14      ! Define COMBIN14 element type for springs
KEYOPT, 2, 2, 0
KEYOPT, 2, 3, 0

ET, 3, COMBIN14      ! Define COMBIN14 element type for springs
KEYOPT, 3, 2, 0
KEYOPT, 3, 3, 1

ET, 999, SHELL181    ! Define SHELL181 element type with a placeholder ID of 999
SECTYPE, 999, SHELL  ! Specify section type for SHELL181 element
SECDATA, 10        ! Define the thickness of the shell as 100 units

! Material Properties for the Solid
MP, EX, 1, (32836.56803/1000)/0.83   ! Define Young's Modulus for solid after adjusting value
MP, PRXY, 1, 0.2     ! Define Poisson's Ratio for the solid
MP, DENS, 1, (23.544/9810.0)/10**9   ! Define Density for the solid after adjusting value
MP, ALPX, 1, 10E-006 ! Define Coefficient of Thermal Expansion for the solid

! Material Properties for Fake/Rigid Areas
MAT, 999              ! Activate material with ID 999 for fake areas
MP, EX, 999, 1E+13    ! Define a very high Young's Modulus to simulate rigidity
MP, PRXY, 999, 0.2    ! Define Poisson's Ratio for the fake areas
MP, DENS, 999, 0.0    ! Define Density as 0, implying it's a "rigid" area
MP, ALPX, 999, 10E-006! Define Coefficient of Thermal Expansion for the fake areas

! Geometry Creation
BLC5, 0, 0, 40000, 40000, 50000  ! Create a block starting from origin with the given dimensions

! Meshing Settings for the Solid Block
TYPE, 1               ! Set the element type to SOLID186
MAT, 1                ! Set the material type to the first defined material (for solid)
ESIZE, (5000)         ! Set the mesh size
VMESH, ALL            ! Mesh the created volume

! Meshing Settings for the Base Area (Shell)
MAT, 999              ! Switch to the material ID 999 (for fake areas)
TYPE, 999             ! Switch to the SHELL181 element type
SECNUM, 999           ! Use the section definition with ID 999
ND_TOL = 10           ! Define a tolerance for node selection (helpful in selecting nodes near z=0)
ALLSEL, ALL           ! Select everything in the workspace
ASEL, S, LOC, Z, 0-ND_TOL, 0+ND_TOL  ! Select areas near the base within the defined tolerance
AMESH, ALL            ! Mesh the selected areas (i.e., the base) using SHELL181 element type

! Select nodes at the base
ND_TOL = 10
NSEL, S, LOC, Z, 0-ND_TOL, 0+ND_TOL

! Create a component for the selected nodes
CM, BASE_NDS, NODE

! Get essential parameters for node looping
*GET, NMAX, NODE, 0, NUM, MAX
*GET, NODE_NMIN, NODE, 0, NUM, MIN
*GET, NODE_NCOUNT, NODE, 0, COUNT

KX = 1000  ! Spring constant in X direction
KY = 500  ! Spring constant in Y direction
KZ = 2000  ! Spring constant in Z direction

R, 1001, KX
R, 2001, KY
R, 3001, KZ

CMSEL,S, BASE_NDS ! Select component of nodes
L_SPRING = 100

! Loop over each node in the created component
NODE_NNUM = NODE_NMIN
*DO, i, 1, NODE_NCOUNT
! Get coordinates of the current node
*GET, X_COORD, NODE, NODE_NNUM, LOC, X
*GET, Y_COORD, NODE, NODE_NNUM, LOC, Y
*GET, Z_COORD, NODE, NODE_NNUM, LOC, Z

! Create spring in X direction
N, , X_COORD-L_SPRING, Y_COORD, Z_COORD   ! Generate a new node
*GET, newNode, NODE, 0, NUM, MAX ! Get the ID of the newly created node
D, newNode, ALL             ! Fix the ground node for X direction
TYPE, 2
REAL, 1001                     ! Define spring constant for X
E, NODE_NNUM, newNode

! Create spring in Y direction
N, , X_COORD, Y_COORD-L_SPRING, Z_COORD   ! Generate a new node
*GET, newNode, NODE, 0, NUM, MAX ! Get the ID of the newly created node
D, newNode, ALL             ! Fix the ground node for Y direction
TYPE, 2
REAL, 2001                     ! Define spring constant for Y
E, NODE_NNUM, newNode

! Create spring in Z direction
N, , X_COORD, Y_COORD, Z_COORD-L_SPRING   ! Generate a new node
*GET, newNode, NODE, 0, NUM, MAX ! Get the ID of the newly created node
D, newNode, ALL             ! Fix the ground node for Z direction
TYPE, 2
REAL, 3001                     ! Define spring constant for Z
E, NODE_NNUM, newNode

! Move to the next node in the component
NODE_NNUM = NDNEXT(NODE_NNUM)
*ENDDO

! add node at center of block and assign rotational torsion springs to this node (master node)
! Create a master node at the center of the block
N, , 0, 0, -500
*GET, masterNode, NODE, 0, NUM, MAX ! Get the ID of the master node
*GET, X_COORD, NODE, masterNode, LOC, X
*GET, Y_COORD, NODE, masterNode, LOC, Y
*GET, Z_COORD, NODE, masterNode, LOC, Z

! Assign rotational torsion springs to this node (master node)
KRX = 500  ! Spring constant for rotation about X axis (Torsional Spring)
KRY = 500  ! Spring constant for rotation about Y axis (Torsional Spring)
KRZ = 500  ! Spring constant for rotation about Z axis (Torsional Spring)

R, 4001, KRX
R, 5001, KRY
R, 6001, KRZ

! Create Torsion springs at the master node
N, , X_COORD-L_SPRING, Y_COORD, Z_COORD   ! Generate a new node offset in X for ROTX spring
*GET, newXNode, NODE, 0, NUM, MAX
D, newXNode, ALL
TYPE, 3
REAL, 4001
E, masterNode, newXNode

N, , X_COORD, Y_COORD-L_SPRING, Z_COORD   ! Generate a new node offset in Y for ROTY spring
*GET, newYNode, NODE, 0, NUM, MAX
D, newYNode, ALL
TYPE, 3
REAL, 5001
E, masterNode, newYNode

N, , X_COORD, Y_COORD, Z_COORD-L_SPRING   ! Generate a new node offset in Z for ROTZ spring
*GET, newZNode, NODE, 0, NUM, MAX
D, newZNode, ALL
TYPE, 3
REAL, 6001
E, masterNode, newZNode

! Transfer rotational stiffness from the master node to the slave nodes (base nodes)
CMSEL, S, BASE_NDS   ! Select nodes in BASE_NDS component
NSEL,A,NODE,,masterNode
CERIG,masterNode,ALL,RXYZ

! Re-select everything for subsequent commands
ALLSEL, ALL

/SOLU                           ! Enters the solution processor

DAMPING_RATIO = 0.00            ! Modal Damping Ratio
N_MODES = 25                    ! Number of Modes, When Method = LANPCG, NMODE should be less than 100 to be computationally efficient.

ANTYPE, MODAL                   ! Perform a modal analysis
MXPAND, N_MODES                 ! Number of modes
DMPRAT, DAMPING_RATIO           ! Modal damping ratio
MODOPT, LANB, N_MODES

ALLSEL, ALL
/MKDIR, 'OUTPUT'
/OUTPUT, 'OUTPUT/Modal_Analysis', 'OUT'
SOLVE

FINISH                          ! Exits solution processor /SOLU

• Moustafa El-Sawy
Subscriber

If I need to apply springs in all 6 directions to the base of my solid elements, is there any way to accomplish this (individual nodes not the entire base with a single value).

I was thinking of adding shell elements to the base that share the same nodes as the solid elements and this will enable those nodes to take moment? Do you think this will work? In this case I was planning on assigning these shells as massless and with small stiffness to not affect the dynamic behavior of the structure.

• peteroznewman
Subscriber

I am not an expert at APDL code. I use Workbench and only use a few lines of APDL occasionally. Maybe someone else can comment.