finish date or the early start date from the late start
date. The numbers will be the same. If not, you made
a math error.
Total Float = Late Start Early Start
(or Late Finish - Early Finish)
Free float is the number of days an activity can
be delayed without taking float away from the next
activity. Another way of saying the same thing is that
free float is the number of days an activity can
be delayed without delaying the early start date of
the next activity. To calculate the free float for an
activity, you subtract any lag and the early finish for
the activity from the early start for the next activity.
To calculate the free float for activity 1020 in figure
2-13 you would take the early start for activity 1050,
subtract any lag between 1020 and 1050 (zero in this
case), and subtract the early finish for activity 1020
(11 - 0 - 10= 1). Free float for activity 1020 is 1 day.
You can see that delaying activity 1020 by 1 day will
not delay activity 1050 from its early start date.
Delaying activity 1020 by 2 days will delay the start
of activity 1050 until day 12 and will reduce the float
for activity 1050 by 1 day (to zero, in this case).
Delaying activity 1020 by more than 2 days will delay
the project completion date because 1020 has only 2
days of total float.
Free Float = Early Start (next activity) Lag (if any)
Looking at activity 1020 in figure 2-13 you see
you could start that activity as early as day 3 or as late
as day 5. Now subtract 3 from 5 and enter 2 days as
the total float. Where the early start and late start are
the same there is no float. No float means you have to
start that activity on its early start date. It cannot be
delayed without delaying the project completion.
Activities with no float are said to be critical. The
first and last activities will always be critical and there
will be a critical path of activities between them. The
critical path in figure 2-13 is 1010-1040-1060-1070.
The critical path allows management to focus
attention on those activities that cannot slip.
DIFFERENT LOGIC TYPES
All examples shown so far have used finish-to-
start logic. This logic type requires an activity to
finish before the next one can start. There are two
other types of logic relationships that are frequently
encountered. They are the start-to-start (S/S) and
finish-to-finish (F/F). S/S is where the start of the
second activity is dependent on the start of the first
activity. F/F is where the finish of the second activity
is dependent on the finish of the first activity.
Finish-to-start logic will give you the longest total
project duration and is the most common logic
type used in the NCF. The S/S and F/F logic can be
used to compress (shorten) the schedule. This
compression is often used in the execution phase of
the project to catch up. These logic relationships also
can be used to plan repetitive work such as roadways
or sewer lines. For a sewer line you wouldnt want to
excavate the entire ditch before starting to lay pipe.
NOTE: Equations marked with an (*) are changed
with different types of logic (S/S or F/F).
Forward Pass: Early start + Lag = Early start (next
Backward Pass: Late start Lag = Late start
Free Float: Early start (next activity) Lag Early
Forward Pass: Early finish + Lag = Early finish
Backwwd Pass: Late finish Lag = Late finish
Free Float: Early finish (next activity) Lag Early
The general rule to follow with different types of logic
is to always follow your logic connectors.
Figure 2-14 is an example of a network with lag
times (between activities B and F, C and D, C and E).
Figure 2-15 is an example of logic relationships. Using
the formulas, work through the calculations.
LEVEL III BARCHARTS
Having determined the construction schedule on
the precedence network, you must now transfer that