Negative Float on a Constrained Milestone
During a schedule review of a hospital fit-out, you find that activity M-410 (mechanical rough-in, 12 working days) carries a finish-no-later-than constraint of 15 May, tied to the owner's equipment delivery milestone. The software reports total float of -8 days on M-410 and its predecessor B-210 (26 days). Every other path in the network has at least 5 days of float. The superintendent says, 'Negative float means we're late, so crash everything.' Two compression options exist: M-410 can be shortened by up to 4 days at $3,500/day; B-210 can be shortened by up to 6 days at $5,200/day. The contract carries no liquidated damages for this milestone.
1. What does the -8 days of float actually tell you, and what does it not tell you?
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It tells you the imposed date is 8 working days earlier than the logic of the network allows the path to finish. That is a statement about the constraint, not about performance. A finish-no-later-than date overrides logic and manufactures negative float; delete the constraint and the same path would show positive float.
It does not tell you the project is behind, and it does not tell you which activities are late. Before spending anything, verify:
- Is 15 May a contractual date, or an owner preference recorded as a constraint?
- Is the data date accurate - are actuals and remaining durations current?
- Is the predecessor logic valid, or is a soft logic tie masquerading as a hard one?
- Has 15 May already been missed, in which case crashing buys nothing on the constraint?
The superintendent's reading is the classic error: negative float is a scheduling diagnostic, not a progress measurement.
2. If recovery is required, which activities do you compress and how much does it cost?
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Recover the 8 days in cheapest-slope-first order, but only along the driving path through M-410:
- M-410: 4 days x $3,500 = $14,000 (exhausts its compressibility)
- B-210: 4 days x $5,200 = $20,800
- Total: 8 days recovered for $34,800
Caveats that belong in the answer, not the footnotes:
- Verify feasibility of the compression. Taking M-410 from 12 to 8 days is a 33% reduction - that is shift work or overtime, not 'work smarter.' Confirm the crew and the material delivery can support it before you book the saving.
- Watch for a shifting critical path. Once these 8 days come out, the next-longest path (previously 5 days of float) may become the driver and additional spend may be needed.
- Do not compress float-bearing activities. Every dollar spent off the driving path buys nothing against the constraint.
3. Before authorizing the $34,800, what non-cost actions would you take?
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Compression is the last resort, not the first. Sequence:
- Validate the date with the owner in writing - is it the equipment vendor's ship date, the owner's internal target, or a contract milestone? If it is soft, seek relief or a re-baseline rather than buying days.
- Check for resequencing. Can work be pulled forward, split, or overlapped (fast-tracking) at no premium? Overlap is usually cheaper than overtime.
- Confirm the equipment delivery date is real. If the milestone exists to receive equipment that will not arrive until June, recovering 8 days produces idle time, not value.
- Check the no-LD clause. With no liquidated damages attached to this milestone, the business case for $34,800 must rest on downstream float consumption or owner relationship - state which.
- Decide who pays. If the constraint is owner-imposed and the underlying delay is not the contractor's, an extension of time is the correct instrument, and the crash cost is a change, not overhead.
Only after 1-4 do you sign the crash authorization, and you document the basis.