Schedule compression: crashing and fast-tracking

Sooner or later every project manager is asked to finish earlier: the owner wants to open before the holiday season, a tenant's lease starts on a fixed date, or the project has fallen behind and liquidated damages loom. Shortening a schedule without changing the scope is called schedule compression. There are two main techniques. Crashing adds resources or money to shorten critical activities. Fast-tracking overlaps activities that would normally be done in sequence. Both cost something, in money, risk or both, and both work only on the critical path. This lesson shows how to compress a schedule systematically, how to find the cheapest days to buy, and how to judge whether compression is worth it.

Only the critical path matters

The first rule of compression is that shortening a non-critical activity does nothing for the finish date. If an activity has 10 days of float, cutting its duration by 5 days simply increases its float to 15. Compression must target the critical path, and as soon as one path is shortened, another path may become critical. Effective compression therefore proceeds one step at a time, recalculating the critical path after each step.

Direct and indirect costs

Project costs split into two kinds that behave in opposite ways as the duration changes.

• Direct costs are the costs of performing the activities: labour, materials and equipment. Shortening an activity usually increases its direct cost, because of overtime premiums, extra shifts, extra equipment, less efficient crews or premium delivery charges.
• Indirect costs are time-related project costs: site supervision, temporary facilities, equipment on standby, insurance, financing, and the contractor's general conditions. They fall as the duration falls.

Liquidated damages, or a bonus for early completion, act like indirect costs: each day saved is worth the daily rate. Total cost is the sum of direct and indirect costs, and there is usually a duration at which it is lowest. Compressing beyond that point costs more than it saves, unless there is a separate reason, such as a hard opening date, to finish earlier.

Normal and crash points

For each activity that can be shortened, the planner estimates two points.

• The normal point: the duration and direct cost under the planned, efficient method.
• The crash point: the shortest practical duration and its direct cost, with all reasonable measures applied: overtime, extra crews, additional equipment, faster methods.

Assuming cost rises roughly evenly between the two points, the cost slope is the extra direct cost per day saved.

The cost slope is the price of buying a day on that activity. The cheapest days to buy are on critical activities with the lowest cost slopes.

The crashing procedure

The classic procedure is a simple loop.

• Calculate the critical path and the project duration with normal durations.
• Identify the critical activities that can still be shortened, and their cost slopes.
• Shorten the critical activity, or the combination of activities when there are several critical paths, with the lowest total cost slope, by one day or by as many days as are possible before another path becomes critical.
• Recalculate the critical paths and costs.
• Repeat until the target duration is reached, no further crashing is possible, or the cost of the next day exceeds the indirect cost saved.

When two parallel paths are both critical, shortening the project by a day requires shortening both, so the relevant cost is the sum of one activity's slope on each path, unless an activity common to both paths can be shortened instead.

A worked crashing example

The example also shows why compression gets more expensive as it goes. The cheapest days are bought first. Each later day costs more, both because the cheaper activities are used up and because more paths become critical and must be shortened together.

Practical ways to crash activities

In the field, crashing means choosing specific measures, each with its own cost and risk.

Overtime deserves special caution. Research and industry studies, including work summarized by trade associations and the construction research community, have repeatedly found that sustained scheduled overtime reduces productivity, so that after several weeks of 50- or 60-hour weeks crews produce little more than they did in 40 hours, at much higher cost and with higher accident rates. Short bursts of overtime to recover a specific critical activity work; months of overtime to recover a late project often do not.

Fast-tracking

Fast-tracking shortens the schedule by overlapping activities or phases that would normally be done one after another, without necessarily adding resources. Common examples include starting construction before design is complete, so that early packages such as excavation and foundations are tendered and built while later packages are still being designed; starting interior work on lower floors while upper floors are still being framed; and starting procurement of long-lead equipment before the design is final.

Fast-tracking changes finish-to-start logic into start-to-start or finish-to-finish logic with lags. It can save a great deal of time at little direct cost, but it adds risk: work may be built on design information that later changes, causing rework; more trades work in the same space at once; and coordination becomes harder. Contract strategies such as construction management at risk and design-build are often chosen partly to make fast-tracking practical.

Choosing between crashing and fast-tracking

Crashing buys time with money; fast-tracking buys time with risk. The best approach often combines both, choosing for each critical activity the cheapest and least risky way to save a day. Some useful questions are: Which critical activities have the lowest cost slopes? Which finish-to-start links could safely become overlaps? Can the critical path be changed altogether, by resequencing work or using a different method, such as prefabrication? And is the time really worth what it costs? An owner who needs to open for a school year may value a week saved at hundreds of thousands of dollars; an owner with no hard date may value it at the cost of a week's financing.

Compression and safety

Compressed schedules put more people in less space for longer hours, with more pressure to cut corners. Incident rates tend to rise with fatigue, congestion and haste. A compression plan should include its safety implications: limits on consecutive days and hours, extra supervision on second shifts, lighting and traffic plans for night work, and coordination of overlapping trades. Saving a week is never worth a serious injury, and an injury will often cost more time than the compression saved.