Negative Float, an Owner Milestone, and a Recovery Plan
A 42-week contract for a compressor station expansion. At the third monthly update the CPM schedule shows a 44-week critical path and several activities carrying −2 weeks of total float. The negative float traces to a client-imposed turnover milestone that was inserted into the schedule without a change order. Activity A (site electrical) is on the critical path and can be crashed 2 weeks at $8,000/week. Activity B (foundations) can be crashed 3 weeks at $15,000/week. Activity C (offsite pipe fabrication) can be crashed 2 weeks at $22,000/week. A secondary path through the control building has 1 week of total float; its cheapest crash option is $12,000/week for up to 2 weeks. The project manager proposes to level the electrical crew's resources to 'find' the time.
1. What does −2 weeks of total float actually mean here, and what are the legitimate ways to remove it? Who should bear the cost?
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What it means. Total float is measured against the schedule's constraints. Negative float is not a property of the activity - it is a message that the logic cannot satisfy a constraint. Here the constraint is a client-imposed turnover milestone layered onto a 42-week contract without a change order. The schedule is telling you the 42-week obligation and the new milestone cannot both be met.
Legitimate exits, in order of preference:
- Challenge the constraint. If the client added the milestone without scope or time relief, this is a client-caused event. Request an extension or a formal change order. This is the correct first move and it costs nothing except a difficult meeting.
- Change the logic. Re-examine relationships, lags, calendar and any hard constraints in the schedule that are not real (a common source of phantom negative float). Only accept logic changes that reflect how the work will genuinely be executed.
- Accelerate. Crash or fast-track to buy back the two weeks.
Cost responsibility. The milestone is client-imposed and was not accompanied by compensation. Absent a contract clause to the contrary, the exposure is the client's, and you should submit a notice of delay and a costed impact before spending money. Accelerating first and claiming later converts an entitlement into an argument about whether you mitigated.
2. Assume the milestone will be enforced and no relief will come. Develop the least-cost recovery plan and compute the cost. Watch the secondary path.
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Requirement. Critical path 44 weeks, target 42: crash the critical path by 2 weeks. The secondary path is 43 weeks (1 week float), so once the critical path drops it becomes the driver unless it is also compressed.
Step 1 - crash the critical path cheapest first.
- Activity A: $8,000/wk, up to 2 weeks → take both weeks, $16,000, CP to 42.
- B at $15,000/wk and C at $22,000/wk are more expensive and A covers the whole requirement.
Step 2 - re-run CPM. With the critical path at 42, the control-building path (43 weeks) is now critical and the project still finishes at 43. Crash it 1 week at $12,000.
Step 3 - re-run again. Check for any new near-critical path exposed by the second reduction; a path that had 2 weeks of float against the original 44-week completion is now at zero.
Least-cost total: $16,000 + $12,000 = $28,000, giving a 42-week completion.
Sanity checks. Confirm each activity has the 2 and 1 weeks of crashability left after prior updates, that crashing electrical does not require overtime that erodes productivity and pushes duration back, and that no vendor lead time silently governs C. A crash plan that ignores resource and procurement limits is just an arithmetic exercise.
3. Critique the project manager's resource-leveling proposal.
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Resource leveling does not shorten a schedule. It moves work within available float to smooth peaks. The best case is unchanged duration; the normal case is longer duration, because leveling consumes float and can push activities past their late dates. Proposing leveling as a recovery method confuses two different techniques:
- Leveling - resource limits are fixed, duration flexes. Used when the crew size is capped.
- Crashing/compression - duration is fixed, resources flex (more people, overtime, second shift). Used to recover time.
The specific risk here. The electrical crew is already on the critical path. Leveling it will flatten peaks by delaying some electrical work, which by definition extends the critical path. The PM would spend the schedule's remaining float and deliver a later completion while reporting that action was taken.
What to say instead. If the crew genuinely cannot be expanded, say so and put the constraint on the table as a reason the milestone cannot be met - that is a legitimate input to the time-extension discussion. If it can be expanded, that is crashing, and it should be priced and approved as such, not disguised as leveling.