Scheduling

Three weeks after the contract is signed, Fairweather sends the contractor's baseline schedule for review: 214 activities, a start date in late March, a date for substantial completion eighteen months later and a red line through the activities that drive it. Thibodeau asks Ingrid to lead the review. She expects it to be a formality, and finds instead that the schedule shows the roofing finishing in late November, the envelope closed in December, and the interior trades working in an unheated building through January. 'A schedule is a promise and a plan,' Thibodeau says, 'and your job is to read it as both. Is it realistic, and does it tell you when the contractor will need things from you?'

Why schedules matter to the architect

The contractor owns the schedule. The general conditions require it to prepare a construction schedule, keep it up to date and complete the work within the contract time, and they leave the planning of the work to it as part of its means and methods. The architect does not approve the schedule in the sense of accepting responsibility for it. But the architect needs to understand it for three reasons. It shows when the contractor expects submittals to be reviewed, questions to be answered and decisions to be made, which sets the architect's own workload. It is the basis on which claims for extra time will later be judged. And it shows whether the owner's dates, for moving in furniture, training staff or opening to the public, are realistic.

A schedule is reviewed, then, for completeness, logic and realism, and for what it asks of the design team. Ingrid's review of the baseline produces a list: the submittal dates for the glulam and CLT shop drawings allow ten working days for review, while the specification allows fifteen; the curtain wall at the commons has no activity for its mock-up; the commissioning period before substantial completion is two weeks, too short for a building with ground-source heat pumps; and the roof finishing in November in Minneapolis puts the most weather-sensitive work at the worst time of year. She sends the comments to the contractor through the chain, worded as observations rather than instructions, because the sequence is the contractor's to set.

Bar charts and networks

The simplest schedule is a bar chart, sometimes called a Gantt chart after the engineer who popularized it in the early twentieth century. Each activity is a bar along a time scale. Bar charts are easy to read and good for communicating, but they do not show why one activity follows another, so they cannot show what happens when something is late. The critical path method, developed in the late 1950s at the chemical company DuPont and the computer company Remington Rand, adds the logic. Each activity has a duration and a set of predecessors that must finish before it can start. From the logic and the durations, a calculation finds the earliest and latest dates for every activity and the longest chain of dependent activities through the project, the critical path. A delay to any activity on that path delays the whole project.

The calculation runs in two passes. The forward pass starts at the beginning and finds, for each activity, the earliest it can start, which is the latest of the early finishes of its predecessors, and the earliest it can finish. The backward pass starts at the end, from the project finish, and finds the latest each activity can finish and start without delaying the project. The difference between an activity's late start and its early start is its total float, the time it can slip without moving the finish date. Activities with zero float are on the critical path. The code below runs both passes for a simplified version of the contractor's schedule.

# Critical path method for a simplified Rail Yard Commons schedule (weeks).
acts = {  # activity: (duration, predecessors)
    "mobilize":              (2,  []),
    "piles and grade beams": (9,  ["mobilize"]),
    "concrete cores":        (8,  ["piles and grade beams"]),
    "timber erection":       (10, ["concrete cores"]),
    "roofing":               (7,  ["timber erection"]),
    "envelope panels":       (10, ["timber erection"]),
    "windows":               (6,  ["envelope panels"]),
    "services rough-in":     (14, ["roofing", "windows"]),
    "elevators":             (10, ["concrete cores"]),
    "interior finishes":     (14, ["services rough-in"]),
    "site and landscape":    (10, ["roofing"]),
    "commissioning":         (5,  ["interior finishes", "elevators"]),
    "substantial completion": (0, ["commissioning", "site and landscape"]),
}
es, ef = {}, {}
for a in acts:                                   # listed in a valid order
    d, pre = acts[a]
    es[a] = max((ef[p] for p in pre), default=0); ef[a] = es[a] + d
finish = max(ef.values())
lf, ls = {}, {}
for a in reversed(list(acts)):
    succ = [b for b in acts if a in acts[b][1]]
    lf[a] = min((ls[b] for b in succ), default=finish); ls[a] = lf[a] - acts[a][0]
print(f"project duration: {finish} weeks")
for a in acts:
    tf = ls[a] - es[a]
    print(f"{a:23s} ES {es[a]:3d}  EF {ef[a]:3d}  LS {ls[a]:3d}  float {tf:3d}{'  critical' if tf == 0 else ''}")

project duration: 78 weeks
mobilize                ES   0  EF   2  LS   0  float   0  critical
piles and grade beams   ES   2  EF  11  LS   2  float   0  critical
concrete cores          ES  11  EF  19  LS  11  float   0  critical
timber erection         ES  19  EF  29  LS  19  float   0  critical
roofing                 ES  29  EF  36  LS  38  float   9
envelope panels         ES  29  EF  39  LS  29  float   0  critical
windows                 ES  39  EF  45  LS  39  float   0  critical
services rough-in       ES  45  EF  59  LS  45  float   0  critical
elevators               ES  19  EF  29  LS  63  float  44
interior finishes       ES  59  EF  73  LS  59  float   0  critical
site and landscape      ES  36  EF  46  LS  68  float  32
commissioning           ES  73  EF  78  LS  73  float   0  critical
substantial completion  ES  78  EF  78  LS  78  float   0  critical

The critical path runs through the piles, the cores, the timber, the envelope panels and windows, the rough-in, the finishes and commissioning: 78 weeks in this simplified version. The roofing has nine weeks of float, because the services rough-in also waits for the windows; the elevators have a large float, because they can be installed while the envelope and interiors proceed. The site and landscape work also has a large float, more than thirty weeks, which tells the owner that it can safely be scheduled for the spring after the building is closed in. A real schedule has many more activities and relationships, including partial overlaps, but the arithmetic is the same.

Weather, seasons and the order of the work

In a northern city the season shapes the schedule more than any other factor. Piles and foundations are best done before the ground freezes or after it thaws; concrete placed in cold weather must be protected and heated to reach its strength, under requirements in ACI 306 and the concrete specification; roofing membranes and sealants have minimum application temperatures; and interior finishing needs a closed, heated building. Mass timber helps, because it can be erected in cold weather and closes in a building quickly, but it must be protected from wetting, and the specification for the Rail Yard Commons limits the moisture content of the CLT before it is covered.

Ingrid's comment about the roof in November is therefore not about the contractor's competence but about risk. If the roofing slips by a month, it falls into December, when the membrane cannot be applied; the interior trades lose a closed building and the critical path moves. The contractor's answer, after a site meeting, is to resequence: the roof teeth at the east end are prefabricated with their membranes applied in the factory, the main roof is dried in with a temporary membrane by late October, and the permanent roofing finishes in spring, off the critical path. The baseline is revised, and the commissioning period is extended to five weeks at the same time.

Delays, extensions and float

Delays are inevitable, and the contract decides who bears them. An excusable delay is one the contractor could not reasonably foresee or avoid, such as an owner change, an act of the owner or the architect, labor disputes beyond the contractor's control, unusually severe weather or a stop-work order not caused by the contractor. The contractor is entitled to an extension of the contract time for an excusable delay that affects the critical path, and to compensation if the owner caused it. A non-excusable delay, such as late delivery of materials the contractor ordered late or a shortage of its own crews, earns no extension; the contractor must recover the time or pay the consequences. Many contracts also provide for liquidated damages, a fixed amount per day of late completion agreed in advance as an estimate of the owner's loss.

Under A201 the contract time is extended by change order for delays caused by the owner or the architect, by changes in the work, labor disputes, fire, unusual delay in deliveries, unavoidable casualties and other causes beyond the contractor's control, and the contractor must make a claim within the time the contract sets. Whether the contractor also recovers costs depends on the cause and on the contract terms. The owner and the contractor waive certain consequential damages, but liquidated damages, where the agreement provides for them, are not affected by that waiver.

Two harder questions arise often enough that the students should recognize them. The first is concurrent delay, when an owner delay and a contractor delay affect the critical path at the same time; the usual result is an extension of time without compensation, though contracts and courts differ. The second is who owns the float. If the owner's late decision uses up the float in a non-critical activity, has the contractor lost something? Many contracts and specifications state that float belongs to the project, available to whoever needs it first; others give it to the contractor. The architect should know which rule applies before the first delay arises.

Updates and look-aheads

The baseline is only the first version. The specification usually requires the contractor to update the schedule monthly, showing actual progress, the remaining durations and any changes to the logic, and to submit a narrative explaining what has changed and why. The architect compares each update with the baseline and the last update. A critical path that changes often, durations that shrink each month without explanation, or float that disappears are signs of trouble worth raising at the site meeting. Most contractors also issue a short-term look-ahead schedule each week or two, which shows the work planned for the next three to six weeks in detail. The look-ahead is where the design team learns which questions need answers next week.

Exercise 6: reviewing the baseline

• Review the baseline schedule supplied by the instructor against the specification's submittal review times, mock-ups and commissioning requirements.
• Identify the critical path and the three activities with the least float that are not on it.
• Mark every activity that depends on a design team action, and list the dates by which each must happen.
• Write the review comments as observations, not instructions, and send them through the chain.

Self-check

• Have I left the sequence of the work to the contractor?
• Do I know which delays earn extensions and costs under this contract?
• Have I checked the season of the weather-sensitive work?
• Do I know when the contractor needs each of my answers?