Add a New Element Fortran

to be completed

To add a new Element module using the Fortran language, the developer must provide a new Fortran routine for the Element.

Some information about the state of the model is passed as arguments to the element methods. The input arguments are:

• the element object, eleObj
• the model state, modl
• the isw switch, isw, which indicates what action is needed for each invocation of the procedure.

The output arguments are:

• the tangent stiffness matrix, tang
• the residual vector, resid
• the error code, error

The name of the routine is important for the OpenSees interpreter: when it encounters a new element type it will look for a library with the same name of the element.

NOTE: This document assumes the reader is familiar with the Fortran programming language.

Element Routine

It should be noted the code contains an export directive, which is necessary if you compile it as external library. Please note that, before calling memory allocated for the pointers, the Fortran code must make a call to the Fortran routine c f pointer().

Example - Truss2D

In the following section we will provide all necessary code to add a new 2d planar truss element into an OpenSees interpreter. The stress-strain relationship will be provided by a UniaxialMaterial object. Please refer to the comments inserted in the code for further explanations.

Please note that the following example has been corrected and expanded by the author of this page. The following code may not match the actual version in the OpenSees repository.

      SUBROUTINE trussf(eleObj,modl,tang,resid,isw,error) 
      
!DEC$ IF DEFINED (_DLL)
!DEC$ ATTRIBUTES DLLEXPORT :: TRUSSF
!DEC$ END IF

      use elementTypes
      use elementAPI
      implicit none
      
      type(eleObject)::eleObj
      type(modelState)::modl
      double precision tang(4, *)
      double precision resid(4)
      integer::isw;
      integer::error;

      integer :: tag, nd1, nd2, matTag, numCrd, i, j, numDOF
      real *8, pointer::theParam(:)
      integer, pointer::theNodes(:)

      double precision A, dx, dy, L, cs, sn
      double precision dLength, force, k

      integer :: iData(3);
      integer :: matTags(2);
      
      type(c_ptr) :: theCMatPtr
      type(c_ptr), pointer :: theCMatPtrPtr(:)
      type(matObject), pointer :: theMat

      double precision dData(1), nd1Crd(2), nd2Crd(2)
      double precision d1(2), d2(2), tran(4)
      double precision strs(1), strn(1), tng(1)
      
      integer numData, err, matType

The main IF/THEN structure of the routine begins here; it is needed to select the proper code depending on what the flag isw is requesting.

      IF (isw.eq.ISW_INIT) THEN
         
c     get the input data  - tag? nd1? nd2? A? matTag?

         numData = 3
         err = OPS_GetIntInput(numData, iData)
         tag = iData(1);
         nd1 = iData(2);
         nd2 = iData(3);

         numData = 1
         err = OPS_GetDoubleInput(numData, dData)
         A = dData(1);

         numData = 1
         err = OPS_GetIntInput(numData, iData)
         matTag = iData(1);

c     Allocate the element state 

         eleObj%tag = tag
         eleObj%nnode = 2
         eleObj%ndof = 4
         eleObj%nparam = 4
         eleObj%nstate = 0  
         eleObj%nmat = 1

         matTags(1) = matTag;
         matType = OPS_UNIAXIAL_MATERIAL_TYPE;
         err = OPS_AllocateElement(eleObj, matTags, matType)

c     Initialize the element properties

         call c_f_pointer(eleObj%param, theParam, [4]);
         call c_f_pointer(eleObj%node, theNodes, [2]);

         numCrd = 2;
         err = OPS_GetNodeCrd(nd1, numCrd, nd1Crd);
         err = OPS_GetNodeCrd(nd2, numCrd, nd2Crd);

         dx = nd2Crd(1)-nd1Crd(1);
         dy = nd2Crd(2)-nd1Crd(2);

         L = sqrt(dx*dx + dy*dy);

         if (L == 0.0) then
c            OPS_Error("Warning - truss element has zero length\n", 1);
            return;
         end if

         cs = dx/L;
         sn = dy/L;

         theParam(1) = A;
         theParam(2) = L;
         theParam(3) = cs;
         theParam(4) = sn;

         theNodes(1) = nd1;
         theNodes(2) = nd2;

      ELSE

         IF (isw == ISW_COMMIT) THEN

In ISW_COMMIT, the state of the model is saved. If your element uses state variables, save them here.

            call c_f_pointer(eleObj%mats, theCMatPtrPtr, [1]);
            theCMatPtr = theCMatPtrPtr(1); 

            j=OPS_InvokeMaterialDirectly(theCMatPtr, modl, strn, strs,
     +       tng, isw)
            
         ELSE IF (isw == ISW_REVERT_TO_START) THEN

            call c_f_pointer(eleObj%mats, theCMatPtrPtr, [1]);
            theCMatPtr = theCMatPtrPtr(1); 

            j=OPS_InvokeMaterialDirectly(theCMatPtr, modl, strn, strs,
     +       tng, isw)

         ELSE IF (isw == ISW_FORM_MASS) THEN

In ISW_FORM_MASS, the mass matrix (if given by the element) must be returned in TANG. IMPORTANT: if your element returns no mass, remember to initialize TANG to zero! If not initialized, Fortran will return a mass matrix with random values. Also, do not overwrite RESID!

         ELSE IF (isw == ISW_FORM_TANG_AND_RESID) THEN

In ISW_FORM_TANG_AND_RESID, all the trials during a non-linear analysis are performed. DO NOT save state variables here.

            call c_f_pointer(eleObj%param, theParam, [4]);
            call c_f_pointer(eleObj%node, theNodes, [2]);
            call c_f_pointer(eleObj%mats, theCMatPtrPtr, [1]);
            theCMatPtr = theCMatPtrPtr(1); 

            A = theParam(1);
            L = theParam(2);
            cs = theParam(3);
            sn = theParam(4);
            nd1 = theNodes(1);
            nd2 = theNodes(2);

            numDOF = 2;
            err = OPS_GetNodeDisp(nd1, numDOF, d1);
            err = OPS_GetNodeDisp(nd2, numDOF, d2);    

            tran(1) = -cs;
            tran(2) = -sn;
            tran(3) = cs;
            tran(4) = sn;
            
            dLength = 0.0;
            do i = 1,2
               dLength = dLength - (d2(i)-d1(i)) * tran(i);
            continue

            strn(1) = dLength/L;

c            i = 0
c            i=OPS_InvokeMaterial(eleObj, i, modl, strn, strs, tng, isw)
            j=OPS_InvokeMaterialDirectly(theCMatPtr, modl, strn, strs,
     +       tng, isw)

            force = A*strs(1);
            k = A*tng(1)/L;

            do i =1,4
               resid(i) = force * tran(i);
               do j = 1,4
                  tang(i,j) = k * tran(i)*tran(j);
               continue
            continue

         END IF

      END IF

c     return error code
      error = 0

      END SUBROUTINE trussf

Compilation Instructions for Visual Studio on Windows

The compilation can be carried out with Visual Studio, if Windows machines are used. Intel Visual Fortran can be used, it integrates itself with Visual Studio IDE during installation.