SUBROUTINE DLOAD(F,KSTEP,KINC,TIME,NOEL,NPT,LAYER,KSPT,
     1 COORDS,JLTYP,SNAME)
C
      INCLUDE 'ABA_PARAM.INC'
C
      DIMENSION TIME(2), COORDS (3)
      CHARACTER*80 SNAME
C
C-----------a region of length 20 metres is subjected to uniform
C           unit loading. The ST and FI defines/marks the start
C           end points of the loaded region.
C
C           ST1R - STart position of Rer tyre of vehicle 1.
C           FI1R - FInisihing position of Rear tyre of vehicle 1.
C           ST1F - STart position of Front tyre of vehicle 1
C           and so on.  2 is for vehicle 2.
C
C           By adding TIME(1) * multiplied by (RDL-1) allows the
C           loading to move from the left to the right of the
C           full span of the road.
C  
C           This models a moving load region of length 1 metre.
C
C           THe expression for F can be generalised to specify
C           a mon-uniform loading across the loaded region.
C
C      write(6,920)TIME(1),TIME(2)
C 920  format(1x,'time ',2f15.3)
C================================================================
C     THe dimensions used are NOT based on REAL roads/vehicles.
C
C      2 parallel roads are considered.
C      tyre tracks are 0.3 m wide and are spaced 1 m apart.
C      Only 2 set of tyres (say front and back pair) are modelled.
C
C      Similarly another 2 set of tyres in the second roadway
C      is modelled with a time shift.
C
C      This can be extended to model 3 sets of wheels/tyres
C      and more vehicles.
C
C      To start with the whole length of the tyre tracks 
C      are subjected to non-uniform pressure.
C      Then ST and FI defines the position of the tyre
C      (a loaded region of length 1 m) and for this length
C      the load is set to be uniform. Beyond this region the load is
C      set to zero. By adding the time into the expression
C      the region loaded is moved along modelling the
C      movement of the tyre.
C
C      IROAD = 1 means the first road. IROAD = 2
C      is for other road. Remember the subroutine is only
C      called when the co-ordinates falls on the 4 tyre
C      strips. The Z co-ordinate is used to check which
C      tyre strip is referred to in the current call.
C   
C       Z = 2.0 to 2.3 road 1 tyre track 1
C       Z = 3.3 to 3.6 road 1 tyre track 2
C
C       Z = 8.6 to  8.9 road 2 tyre track 1
C       Z = 9.9 to 10.2 road 2 tyre track 2
C
C       More vehicles on both roads can be modelled by
C       working out which position and which region of the track
C       is loaded at any given time and then adjusting the scale
C       factor for the time the speed of the movement can be
C       modelled for each vehicle.
C
C       Use the DIRECT method  to give constant increment size in
C       the step. If using CAE in the STEP module toggle the FIXED
C       button instead of AUTOMATIC.
C
C       Automatic time stepping should be avoided.
C
C       Since time step = 1. I have used a factor  of (RDL-1) * TIME
C       which takes the moving load right across the length of
C       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-REAR WHEEL SPACING
       WLSP1=5.
       WLSP2=5.
C
       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 
C        Note that ST and FI set below are purely for printing
C        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
C             WITH A MAXIMUM VALUE OF 1 for the bracketed term.
C             This expression should be scaled with the wheel load. 
C
C        F=((COORDS(1) - ST) * (FI - COORDS(1) / 0.25) 
C
      ENDIF 
C         write(6,910)COORDS(1),ST,FI,F
C 910     FORMAT(1X,'COORDS(1), ST, FI, F =',4E15.5)
C
      RETURN
      END