How Early and Late date are calculated in Start to Start relationship in Primavera P6

In Finish to Start relationship, Primavera calculate Early and Late date base on 2 process

Forward Pass (to calculate Early Start and Early Finish)

how-early-and-late-date-are-calculated-in-start-to-start-relationship-in-primavera-p6-1

Backward Pass (to calculate Late Start and Late Finish)

how-early-and-late-date-are-calculated-in-start-to-start-relationship-in-primavera-p6-2

In Backward Pass, the Late Start of successor (Activity C) will be the base to calculate Late Finish of predecessor (Activity A and B)

How about in Start to Start relationship.

In follow picture, the relationship between Activity B and Activity C is Start to Start.

how-early-and-late-date-are-calculated-in-start-to-start-relationship-in-primavera-p6-3

There is no activity standing behind activity B.

So how can P6 calculate Late Finish for activity B.

In Start to Start relationship Primavera will do it in reverse direction. It will calculate Late Start first, then Late Finish = Late Start + Duration.

First Late Start of B = Late Start of C = 25/Sep

Then Late Finish of B = Late Start of B + Duration of B = 25/Sep +5 = 29/Sep

Bonus :

In Finish to Finish relationship, the finish dates will be calculated first.

In the forward pass, EF = predecessor's EF
In the backward pass, LF = successor's LF

Then, the start dates get calculated from finish dates.

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