Simple Macro for Imposing Zero Free Float Constraints in Microsoft Project

Here I outline a method – including the ImposeZFF macro – to provide “just-in-time” scheduling of flagged tasks in MSP.  I also include a separate macro, RemoveZFF, to restore the tasks to as-soon-as-possible scheduling.

Another missing feature for anyone coming to MSP from Primavera’s planning tools is the “Zero Free Float (ZFF)” constraint.  (Primavera now calls this the “As Late as Possible (ALAP)” constraint – NEVER to be confused with MSP’s identically-named version, which is pretty useless for forward scheduling.)  The ZFF constraint delays an activity as much as possible without impacting any successor activities.  It is useful for scheduling just-in-time works and deliveries while preserving the correct logic flow through the project schedule.

Consider the simple project shown here.  The overall project completion is limited by a constraint on task B.  (I never use such mandatory constraints in practice but use it here to avoid having to add other extraneous logic to the example.)  Task A must be completed before B, but for maximum efficiency it is desired to perform this work just-in-time.  Because MSP has no other obvious feature to arrive at the desired schedule outcome, the scheduler is tempted to make task A a Start-to-Finish successor of B.  Such an approach is unsound and is to be avoided for a number of reasons.  All links in a logic-driven project schedule must reflect real logical constraints, and real Start-to-Finish constraints are extremely rare.

So what is the scheduler to do?  Most situations are easily solved by adding logical milestones to the project schedule.  For example, a logic-driven “Site Ready for Delivery” milestone as a Finish-to-Finish predecessor of “Eqpt Delivery” would be quite normal.  In the very few occasions when suitable logical workarounds cannot be devised, then it may be useful to impose early-start constraints to achieve the zero-free-float objectives.  For good control over the process, I use a custom flag field to designate specific tasks requiring ZFF constraints.  Then I run a macro that automatically computes and assigns the appropriate constraint to all the ZFF tasks in the project at once.  This makes ZFF constraints easy to audit, review, and justify.

For our simple project example, I inserted the Flag20 local field into the Gantt Chart entry table and called it ZFF.  Then I marked task A with the ZFF flag and ran the macro.  The macro automatically imposed a new early-start constraint on the task.  As a result, the 12 days of Total Slack (like the 12 days of Free Slack) that previously existed have been consumed.  Task A is now correctly shown as Critical.

If your project has a logical chain of tasks that are all ZFF-flagged, then you’ll have to re-run the macro several times (or add a simple iteration loop to the code.  I have one but didn’t include it here for simplicity.)

[May 2019 Edit: Though I don’t use this macro, one user has reminded me that when used to delay a string of connected tasks, selecting the tasks in reverse order may be more efficient than the forward order first included in this article.  I.e. use the “For i…” block shown here in place of the “For each t” block originally included.  I might get around to modifying that code eventually.]

    Dim i As Long
    For i = ActiveProject.Tasks.Count To 1 Step -1
        Set t = ActiveProject.Tasks(i)
        If Not t Is Nothing Then

            'If t.GetField(Application.FieldNameToFieldConstant(FieldName))....
               '...
               '...
             End If  
        End If
    Next i

The code for imposing the constraints is shown below.  Drop it into a VBA module in your Global.mpt file (or in a specific .mpp file if necessary), then look for and run macro “ZFFImpose”.  I normally use another function to find the ZFF flag field in a project, but that function was too big to include here.  So if you want to use a custom flag field other than Flag20, just change the “FieldName = “Flag20″” statement in the code.

Sub ZFFImpose()
' Macro Coded 31-08-16 by Thomas Boyle PE PSP PMP.
On Error GoTo 0
    
    Dim i As Integer
    Dim t As Task
    Dim L1 As String
    Dim L2 As String
    Dim SW1 As Boolean
    Dim SW2 As Boolean
    Dim SW3 As Boolean
    Dim FieldName As String
    
    'FieldName = FindField_Flag("ZFF") 'Function not included here
    Fieldname="Flag20"
    If FieldName = "" Then
        MsgBox Prompt:="No ZFF Flag Field Found", Title:="ZeroFreeFloat Constraints"
        Exit Sub
    End If
                
    CalculateProject

        For Each t In ActiveProject.Tasks
            If Not t Is Nothing Then
                If t.GetField(Application.FieldNameToFieldConstant(FieldName)) = "Yes" Then
                    If t.FreeSlack > 0 Then
                        t.ConstraintType = pjSNET
                        If t.Calendar = "None" Then
                            t.ConstraintDate = Application.DateAdd(t.Start, t.FreeSlack)
                        Else
                            t.ConstraintDate = Application.DateAdd(t.Start, t.FreeSlack, t.CalendarObject)
                        End If
                        CalculateProject
                        L1 = L1 & t.ID & " " & t.Name & " " & t.ConstraintDate & vbCrLf
                        SW1 = True
                    Else
                        L2 = L2 & t.ID & " " & t.Name & vbCrLf
                        SW2 = True
                    End If
                End If
            End If
        Next t
    GoTo Finish

tEHandler:
    MsgBox ("Failed to impose Zero Free Float Constraints")
    Exit Sub
    
Finish:
    CalculateProject
    If SW1 = True Then MsgBox Prompt:="Imposed Early Start Constraints to remove free float:" & vbCrLf & L1, Title:="ZeroFreeFloat Constraints"
    If SW2 = True Then MsgBox Prompt:="No free float to remove on tasks: " & vbCrLf & L2, Title:="ZeroFreeFloat Constraints"
    If SW1 = False And SW2 = False Then MsgBox Prompt:="No tasks marked for ZFF", Title:="ZeroFreeFloat Constraints"
    
End Sub

What is done with the “ZFFImpose” macro is easily undone with the “ZFFRemove” macro shown below.  This macro returns all ZFF-flagged tasks to “As Early As Possible” scheduling.

Sub ZFFRemove()
' Macro Coded 31-08-16 10:30 by Thomas Boyle PE PSP PMP.
On Error GoTo 0
    
    Dim t As Task
    Dim L1 As String
    Dim SW1 As Boolean
    Dim FieldName As String
    
    FieldName = "Flag20"
    'FieldName = FindField_Flag("ZFF")
    If FieldName = "" Then
        MsgBox Prompt:="No ZFF Flag Field Found", Title:="ZeroFreeFloat Constraints"
        Exit Sub
    End If
                
    For Each t In ActiveProject.Tasks
        If Not t Is Nothing Then
            If t.GetField(Application.FieldNameToFieldConstant(FieldName)) = "Yes" Then
                If t.ConstraintType = pjSNET Then
                    t.ConstraintType = pjASAP
                    CalculateProject
                    L1 = L1 & t.ID & " " & t.Name & vbCrLf
                    SW1 = True
                End If
            End If
        End If
    Next t
    GoTo Finish

tEHandler:
    MsgBox ("Failed to remove Zero Free Float Constraints")
    Exit Sub
    
Finish:
    CalculateAll
    If SW1 = True Then MsgBox Prompt:="Removed Early Start Constraints from designated ZFF tasks:" & vbCrLf & L1, Title:="ZeroFreeFloat Constraints"
    If SW1 = False Then MsgBox Prompt:="No tasks marked for ZFF", Title:="ZeroFreeFloat Constraints"
End Sub