A strip of length 20 metres and width 0.3 metres
            is subjected to uniform loading. 
            The STxx and FIxx  defines/marks the start
            end points of the loaded region (the wheel tire).

            ST1R - STart position of Rear tire of vehicle 1.
            FI1R - FInisihing position of Rear tire of vehicle 1.
            ST1F - STart position of Front tire of vehicle 1
            and so on.  2 is for vehicle 2.

            By adding TIME(1) * multiplied by (RDL-1) allows the
            loading to move from the left to the right for the
            full span of the road.
   
            This models a moving load region of length 1 metre.

            THe expression for F can be generalised to specify
            a mon-uniform loading across the loaded region.

      THe dimensions used are NOT based on REAL roads/vehicles.

       2 parallel roads are considered.
       tire tracks are 0.3 m wide and are spaced 1 m apart.
      Only 2 set of tires (say front and back pair) are modelled.

      Similarly another 2 set of tires in the second roadway
      is modelled with a time shift.

      This can be extended to model 3 or more sets of wheels/tires
      and more vehicles.
C
      To start with the whole length of the tire tracks 
      are subjected to non-uniform pressure loading.

      Note : It is not necessary to apply the non-uniform loading
             to the tire tracts alone. It would be simpler and easier 
             to specify this loading to the entire surface area.
             This would not result in any error as the subroutine
             checks that the loading is restricted to the tracks by
             checking  the Z co-ordinates. The Load Facor F is set to
             zero outside of the tracks. 

      Also note that the magnitude of the pressure loading antered in 
      the dialog box should be set to 1 to avoid any confusion. Earlier
      it was assumed that the magnitude was used to scale up the F value 
      specified in the subroutine. The results indicate otherwise and the
      loading is solely determined by the F value and the load magnitude
      specified in CAE is ignored and has no effect on the loading. 
      The input file has been modofoed accordingly.

      Then ST and FI defines the position of the tire
      (a loaded region of length 1 m) and for this length
      the load is set to be uniform. Beyond this region the load is
      set to zero. By adding the time into the expression
      the region loaded is moved along modelling the
      movement of the wheel/tire.
C
      IROAD = 1 refers to  the first road. IROAD = 2
      is for other road. Remember the subroutine is only
      called when the co-ordinates falls on the 4 tire tracks.
      (strips). The Z co-ordinate is used to check which
      tire strip is referred to in the current call.
   
       Z = 2.0 to 2.3 road 1 tire track 1
       Z = 3.3 to 3.6 road 1 tire track 2
C
       Z = 8.6 to  8.9 road 2 tire track 1
       Z = 9.9 to 10.2 road 2 tire track 2
C
       More vehicles on both roads can be modelled by
       working out which position and which region of the track
       is loaded at any given time and then adjusting the scale
       factor for the time the speed of the movement can be
       modelled for each vehicle.
C
       Use the DIRECT method  to give constant increment size in
       the ABAQUS step. If using CAE in the STEP module toggle 
       the FIXED  button instead of AUTOMATIC.
C
       Automatic time stepping should be avoided.
C
       Since time step = 1. I have used a factor  of (RDL-1) * TIME
       which takes the moving load right across the length of
       20 metres in the given time of 1 second. Speed of 20 m/s. 
C
C----------------------ROAD LENGTH
       RDL=20.
C
C----------------------WHEEL LOADING Vehicle : 1 or 2. F - front R - Rear
       WLD1F=200.
       WLD1R=200.
       WLD2F=200.
       WLD2R=200.
C
C----------------------FRONT to REAR WHEEL SPACING for the 2 vehicles
       WLSP1=5.
       WLSP2=5.
C
C-----------This determines which strip/track loading  is dealt with
C           in the current subrotuine call.
       if(coords(3).gt.2.0.and.coords(3).lt.2.3) then
          IROAD=1
       else if(coords(3).gt.3.3.and.coords(3).lt.3.6) then
          IROAD=1
       else if(coords(3).gt.8.6.and.coords(3).lt.8.9) then
          IROAD=2
       else if(coords(3).gt.9.9.and.coords(3).lt.10.2) then
          IROAD=2
       endif
C
      IF(IROAD.EQ.1) THEN
      ST1R=0.+TIME(1)*(RDL-1.)
      FI1R=1.+TIME(1)*(RDL-1.)
C
      ST1F=ST1R+WLSP1
      FI1F=FI1R+WLSP1
C
      ELSE IF(IROAD.EQ.2) THEN
      ST2R=3.+TIME(1)*(RDL-1.)
      FI2R=4.+TIME(1)*(RDL-1.)
C
      ST2F=ST2R+WLSP2
      FI2F=FI2R+WLSP2
      ENDIF
C
       F=0.
C
C--------------- VEHICLE 1 REAR wheel load
       if(COORDS(1).GE.ST1R.AND.COORDS(1).LE.FI1R)THEN
         F=WLD1R
         ST=ST1R
         FI=FI1R
C 
        Note that ST and FI set below are purely for printing
        these values out for checking purposes.
C
C---------------- VEHICLE 1 FRONT wheel load
      ELSE IF(COORDS(1).ge.ST1F.AND.COORDS(1).LE.FI1F) THEN
C
C-------------FOR UNIFORM LOADING over the loaded region
         F=WLD1F
         ST=ST1F
         FI=FI1F
C
C---------------- VEHICLE 2 REAR wheel load
       else if(COORDS(1).GE.ST2R.AND.COORDS(1).LE.FI2R)THEN
         F=WLD2R
         ST=ST2R
         FI=FI2R
C
C--------------- VEHICLE 2 FRONT wheel load
      ELSE IF(COORDS(1).ge.ST2F.AND.COORDS(1).LE.FI2F) THEN
C
C-------------FOR UNIFORM LOADING.
         F=WLD2F
         ST=ST2F
         FI=FI2F
C
C------------ FOR PARABOLIC DISTRIBUTION OVER THE LOADED REGION
             WITH A MAXIMUM VALUE OF 1 for the bracketed term.
             This expression should be scaled with the wheel load. 
C
        F=((COORDS(1) - ST) * (FI - COORDS(1) / 0.25) 
        F=WLD2F * F
C
      ENDIF 
         write(6,910)COORDS(1),ST,FI,F
C 910     FORMAT(1X,'COORDS(1), ST, FI, F =',4E15.5)
C
      RETURN
      END