Resource loading and levelling

A critical path calculation assumes that every activity can start the moment its logic allows, with whatever crews and equipment it needs. Real projects do not work that way. A contractor has a limited number of carpenters, one tower crane, two concrete pumps booked on alternate days, and a site with room for a certain number of trades at once. Resource loading attaches the crews, equipment and other resources to each activity, so the schedule can show how many of each are needed day by day. Resource levelling then adjusts the timing of activities, usually within their float, to keep resource demand within limits or to smooth out peaks and valleys. This lesson covers both, with the hand methods every scheduler should understand.

What resources are

In scheduling, a resource is anything with limited availability that an activity needs. Construction schedules typically load:

• Labour, by trade or crew: carpenters, ironworkers, electricians, labourers.
• Major equipment: tower cranes, mobile cranes, hoists, concrete pumps, excavators.
• Space and access: work areas, laydown, hoist time, road closures.
• Materials with constrained supply, such as precast deliveries per day.
• Money, as a cost-loaded schedule that shows expenditure over time.

Not every resource needs to be loaded. Loading every labourer and every pickup truck makes the schedule complicated without improving decisions. Good practice is to load the resources that are scarce, expensive or shared between activities: the ones whose limits could change the plan.

The resource histogram

Once resources are attached to activities, the schedule can total the demand for each resource in each time period. Plotted as a bar chart against time, the result is a resource histogram. It shows at a glance when the project needs the most workers or equipment, how fast it builds up and runs down, and whether demand exceeds what is available. A cumulative curve of the same data, usually S-shaped, shows total labour-hours or cost over time and becomes the baseline for measuring progress.

Histograms are drawn from the early-start schedule unless stated otherwise. Because non-critical activities can move within their float, the same network can produce very different histograms depending on when those activities are scheduled. That flexibility is what levelling uses.

Two kinds of levelling problem

Resource levelling covers two related problems.

• Resource smoothing, sometimes called time-constrained levelling: the project finish date is fixed, and activities are moved only within their float to make demand as even as possible. The duration does not change.
• Resource-constrained scheduling, or resource-limited levelling: there is a hard limit on a resource, such as one crane or eight electricians, and activities are delayed as needed to stay within it, even if that extends the project.

Smoothing improves efficiency: a steady crew is cheaper and more productive than one that is hired, laid off and rehired. Resource-constrained scheduling is about feasibility: a schedule that needs two tower cranes on a site with room for one is not a schedule at all.

Smoothing within float

A simple manual approach to smoothing is to start with the early-start schedule, then try moving non-critical activities later within their float, one at a time, choosing moves that reduce the peak or make the profile flatter. A useful measure of smoothness is the sum of the squares of daily demand: for the same total, a flatter profile has a smaller sum of squares.

Software uses rules of the same kind, often called heuristics, to decide which activity to delay when resources conflict: activities with the least float first, activities with the earliest late start, or activities that use the most of the scarce resource. Different rules give different results, so the scheduler must check the outcome rather than accept it blindly.

Resource-constrained scheduling

When a resource has a hard limit, levelling may have to extend the project. The usual heuristic, in hand calculations and many software tools, works period by period: at each time, list the activities whose logic allows them to start, and give the scarce resource to the activities with the least total float (or another priority rule), delaying the rest until resources free up.

The example shows a key point: once resources are levelled, the critical path is no longer just a matter of logic. Activities linked by nothing but a shared crane or crew can become critical together. Schedulers sometimes add resource logic explicitly, linking the activities in the order the crew will do them, so that the dependency is visible to everyone who reads the schedule.

Crane and hoist planning

On multi-storey building sites, the tower crane is often the controlling resource. A crane can make only so many lifts per hour, and every trade wants it at the start of the day. Planners estimate crane hours from the number of picks and the cycle time of each, and draw up a crane schedule by day or half-day. Personnel and material hoists on taller buildings raise the same issue later in the job, when hundreds of workers and tonnes of material must move vertically every morning.

Cost loading and cash flow

When each activity carries its budgeted cost, the schedule can produce a cost curve: the planned cumulative cost against time. Owners use it to plan funding and draws on construction loans; contractors use it to forecast billings and working capital; and it becomes the baseline for earned value analysis. The shape is usually an S-curve: slow at the start, steep in the middle when many trades are working at once, and flattening at the end during finishes and commissioning.

The early-start schedule gives the steepest curve and the late-start schedule the flattest, and the area between them shows the range of feasible spending patterns. A contractor wanting to delay costs might favour late starts; an owner wanting early progress might press for early starts. Most real projects fall somewhere between.

Practical levelling

Automatic levelling in scheduling software is powerful but can produce strange results: activities split into fragments, critical work delayed in favour of non-critical work, or durations stretched far beyond what anyone expected. Good practice is to level selectively: level only the genuinely scarce resources, review every change the software makes, and record the results in the schedule as logic or as documented decisions so the reasons are visible. Superintendents often level by experience, adjusting crew sizes and moving trades between areas each week. The schedule should reflect those decisions rather than fight them.

Levelling also interacts with productivity. A trade that can work continuously, moving from area to area without waiting, works more efficiently than one that starts, stops and remobilizes. Schedules that keep key trades working continuously, a principle developed further in location-based scheduling in Lesson 9, often finish sooner and cost less than schedules that start every activity as early as possible.