Driving delay: separate events can share a completion effect

One delay ends on day 60; the other starts on day 62. Their effects still share 15 days beyond planned completion.

Live result

In this fixed network with 15 days of fabrication float, event-calendar overlap is 0 days and shared completion effect is 15 days of the 20 days overrun.

Two delays with no calendar day in common cannot be concurrent.

That is a reasonable reading of a bar chart: two bars either meet or they do not. It answers a question about the events. A programme answers a second question about when their affected branches would finish. Those questions can return different counts, even when every date and duration is agreed.

Synthetic two-branch network: setup takes 10 days, approval 60 and installation 30. Fabrication takes 60 minus the typed float. Both branches must finish before installation starts. Each event adds its full length to its own branch; no recovery, progress update or resource constraint is introduced.

1. The event-calendar reading

The event-calendar reading counts time inside both event intervals, including the start and excluding the end. The existing Concurrency lesson uses this reading. Here the contractor event starts on day 30. At the opening values its 30 days end on day 60, before the employer's 20 days begin on day 62. The calendar answer is therefore zero. Event dates alone supply everything this count needs.

Measure	Days
Event-calendar overlap	0
Shared completion effect	15
Combined overrun	20

Day from programme start

Contractor: Day 30 included to day 60 excluded
Employer: Day 62 included to day 82 excluded

Your fixed-network case

Fabrication floatdays

Whole days from 0 to 40. The fabrication duration changes with this value.

Contractor event lengthdays

Whole days from 0 to 120, inserted as extra duration on this branch.

Employer event start day

Whole day from 0 to 120. This changes calendar overlap only.

Employer event lengthdays

Whole days from 0 to 120, inserted as extra duration on this branch.

2. The completion-effect reading

The completion-effect reading compares two separate insertions into the same baseline. First insert only the contractor event; then insert only the employer event. The shared effect is the overlap between the completion extensions each insertion would produce. This is a comparison of hypothetical finishes, not a claim that both parties were delaying an activity on each plotted day. Installation waits for the slower branch in every run.

Day from programme start

The baseline behind the count

Activity	Start day	Finish day
Setup	0	10
Fabrication	10	55
Approval	10	70
Installation	70	100

Setup precedes both fabrication and approval. Installation follows both. The baseline forward pass puts approval's finish on day 70 and completion on day 100. Fabrication can finish earlier by the typed float. This declared connection, rather than the position of the event bar, is what carries the extra duration to completion.

Completion receipt

1. Baseline finish 100; contractor-only insertion finish 115; employer-only insertion finish 120.
2. The smaller separate finish is 115; its extension beyond day 100 is 15 days.
3. Shared effect 15, contractor-only tail 0 and employer-only tail 5 total 20 days; combined finish 120.

3. Move the event, keep the effect

All four cases below keep the opening durations and 15 days of float. Only the employer event's start moves. The event-calendar count changes from 0 to 20 days, while the shared completion effect stays at 15. Calendar overlap cannot recover the missing information about float. Agreement between the two counts in the third row is a coincidence of these inputs, not an identity between the readings.

Event-calendar overlap
Shared completion effect

Overlap, days

Employer start day	Calendar days	Shared effect days
62	0	15
55	5	15
45	15	15
30	20	15

The two counts share the event lengths. Each ignores a different input: calendar overlap ignores float; shared completion effect ignores event start dates.

4. Add float, remove the shared effect

Now keep the opening dates and durations and vary only fabrication float. The calendar count stays at zero. Shared completion effect disappears exactly when float reaches the contractor event's 30 days, provided the employer event remains positive. Below that point, only the part of the contractor event beyond float can join the employer's completion extension. The flat section at the left comes from the employer event's 20-day ceiling.

Event-calendar overlap
Shared completion effect

Overlap, days

Fabrication float, days

Float days	Calendar days	Shared effect days
0	0	20
10	0	20
15	0	15
20	0	10
25	0	5
29	0	1
30	0	0
40	0	0

For positive employer delay, shared effect is positive exactly while contractor delay exceeds fabrication float. Equality already gives zero.

The disagreement also reverses: with 40 days of float and employer start day 55, calendar overlap is 5 days but shared completion effect is 0. The contractor's entire 30-day addition fits inside float.

The calculation

Two counts from the same case

Calendar overlap intersects the event intervals. Shared effect intersects the two completion extensions. The closed form below belongs to this two-branch network with nonnegative fabrication float; it does not carry over automatically to a network with other merges or changing progress.

Intersect the event intervals, in days:

O_{\mathrm{cal}}=\max\left(0,\min(s_c+c,s_e+e)-\max(s_c,s_e)\right)

$O_{\mathrm{cal}}$ is event-calendar overlap and $s_c$ is the contractor event start and $c$ its length and $s_e$ the employer event start and $e$ its length.

Intersect the completion extensions, in days:

O_{\mathrm{drv}}=\max\left(0,\min(c-\Phi,e)\right)

$O_{\mathrm{drv}}$ is shared completion effect and $\Phi$ is fabrication float.

Calculate the separate insertion finishes, as programme days:

T_0=100,\quad T_C=100+\max(0,c-\Phi),\quad T_E=100+e

\begin{gathered}T_0=100, \\ T_C=100+\max(0,c-\Phi), \\ T_E=100+e\end{gathered}

$T_0$ is baseline completion and $T_C$ is the contractor-only insertion finish and $T_E$ the employer-only insertion finish.

Reconcile the overrun into its three parts, in days:

R=\max(T_C,T_E)-T_0=O_{\mathrm{drv}}+R_C+R_E

$R$ is the combined overrun and $R_C$ is the contractor-only tail and $R_E$ the employer-only tail.

O_{\mathrm{drv}}>0\iff (c>\Phi)\land(e>0)

Opening case, to the day

The event intervals do not intersect, giving calendar days:

\max(0,\min(60,82)-\max(30,62))=\max(0,60-62)=0

Only the contractor duration beyond float joins the employer extension, giving shared days:

\max(0,\min(30-15,20))=15

The three parts reconcile to the combined overrun, in days:

120-100=15+0+5=20

Source basis

An authored synthetic network, evaluated by the site's critical path forward and backward passes. Both interval intersections are derived here from their endpoints. The names distinguish the two readings on this page; no published protocol wording or standard table is reproduced.

Scale: Linear day axes, increasing left to right
Basis: One baseline, two separate duration insertions and one combined insertion
Assumptions: Four activities, finish-to-start links with zero lag, unchanged installation duration
Model version: 1.0.0

These are two named readings of a fixed network; they determine no entitlement, extension of time, compensation, responsibility or preferred concurrency convention.

Driving delay calculation record

Driving delay: separate events can share a completion effect

Inputs in days

Fabrication float: 15
Contractor event length: 30
Employer event start day: 62
Employer event length: 20
Fixed contractor start day: 30

Two readings and completion days

Event-calendar overlap: 0
Shared completion effect: 15
Combined overrun: 20
Planned finish: 100
Contractor-only insertion finish: 115
Employer-only insertion finish: 120
Combined finish: 120

Numbered receipt

1: Baseline finish 100; contractor-only insertion finish 115; employer-only insertion finish 120.
2: The smaller separate finish is 115; its extension beyond day 100 is 15 days.
3: Shared effect 15, contractor-only tail 0 and employer-only tail 5 total 20 days; combined finish 120.

Source basis

Source basis: An authored synthetic network, evaluated by the site's critical path forward and backward passes. Both interval intersections are derived here from their endpoints. The names distinguish the two readings on this page; no published protocol wording or standard table is reproduced.
Print date

1. The event-calendar reading

2. The completion-effect reading

The baseline behind the count

Completion receipt

3. Move the event, keep the effect

4. Add float, remove the shared effect

The calculation

Two counts from the same case

Opening case, to the day

Source basis

Related

Inputs in days

Two readings and completion days

Numbered receipt

Source basis