Linear and location-based scheduling

Much construction work is repetitive. A high-rise building repeats the same sequence of trades floor after floor; a highway, pipeline or rail line repeats the same operations kilometer after kilometer; a housing development repeats the same house dozens of times. Critical path networks can model repetitive work, but they do it clumsily, with hundreds of near-identical activities and a bar chart that hides the most important question: are the crews moving through the locations at rates that keep them working continuously without running into each other? Linear and location-based scheduling methods answer that question directly by plotting work against both time and location. This lesson introduces line of balance, flowline and time-location charts, and takt planning, which the lean construction course (CON-410) develops further.

The problem with networks for repetitive work

Imagine scheduling eight stories of interior work with four trades. A CPM schedule needs 32 activities and dozens of links, and the bar chart becomes a staircase of short bars that is hard to read. More importantly, the CPM calculation starts every activity as early as logic allows. If a faster trade follows a slower one, it catches up, waits, works, waits again: the crew starts and stops, remobilizes, and loses productivity. The network shows no problem, because waiting time is invisible in it.

Location-based methods treat the rate of progress, called the production rate, as the central variable, and aim for continuous work by each crew through the locations. The goal is a schedule in which each trade starts once, works steadily through all the locations, and leaves, with the trades spaced so that they never interfere.

Time-location charts

In a time-location chart, also called a flowline chart or line-of-balance chart, the horizontal axis is time and the vertical axis is location: floors of a building, chainage along a road, or units in a development. Each trade's work is drawn as a line or band from its start location and time to its finish location and time. The slope of the line is the production rate: steeper lines are faster trades. When two lines converge, the following trade is catching up with the one ahead; when they cross, the schedule is impossible, because the second trade would be working in a location before the first has finished there.

Time-location charts make problems visible at a glance: a trade that is too slow, a following trade that will catch up, a location where many trades are crammed at once, or a gap where nothing is happening. They are widely used for highways, railways, pipelines, tunnels and high-rise buildings, and many scheduling programs can now draw them from the same data as the CPM network.

Production rates and continuous work

The production rate of a trade is the number of locations, or the length or quantity, it completes per unit of time. It depends on crew size: two crews will usually progress about twice as fast as one, up to the limits of space. The key planning rule is that when a faster trade follows a slower one, the faster trade must start later than it would in a CPM schedule, so that it reaches the last location just as the slower trade finishes there. When a slower trade follows a faster one, the gap between them grows over time, and the slower trade can start as soon as the first location is free.

Compare that with a CPM approach in which each trade starts each floor the moment the previous trade is done. Drywall would start floor 1 on day 4, finish it on day 7, then wait a day for framing to free floor 2, work 3 days, wait another day, and so on. It would finish floor 8 on day 35, exactly as before, but with seven separate pauses. The finish date is the same; the difference is that the continuous plan keeps the drywall crew working without interruption, which in practice means better productivity, fewer remobilizations and a more reliable schedule.

Balancing rates

The most powerful lever in location-based planning is to make the production rates of successive trades similar, by changing crew sizes. When rates match, the lines on the chart are parallel, trades follow each other at a steady distance, and the total duration shrinks because no trade has to wait for a slower one to clear the last location.

In the formula, \(n\) is the number of locations, \(m\) the number of trades or work steps, and \(t\) the time each trade spends in each location. Reducing the time per location shortens the whole sequence, and adding locations, by splitting large floors into zones, also helps, because smaller locations let more trades work at once in different places.

Buffers

A perfectly tight location-based schedule is fragile: if one trade has a bad day, every following trade is delayed. Planners therefore insert buffers between trades, either as time buffers (a gap of some days between one trade leaving a location and the next arriving) or as location buffers (keeping at least one empty location between trades). Buffers absorb variation and protect the flow, at the cost of a slightly longer total duration.

Line of balance for linear projects

On highways, pipelines and railways, location is a continuous distance, often given as chainage in kilometers or stations in feet. The time-location chart works the same way, with each operation drawn as a line whose slope is its rate in meters or feet per day. Operations that must stay a minimum distance apart, such as excavation ahead of pipe laying or paving ahead of line painting, are shown with a minimum spacing, and the chart reveals where they would come too close.

US road builders usually express location as stations, where one station is 100 ft, so station 123+50 means 12,350 ft from the origin. The time-location chart's vertical axis uses the same station numbers.

Takt planning

Takt planning, borrowed from manufacturing and widely used in lean construction, sets a single rhythm, the takt time, for the whole sequence. The building is divided into zones of roughly equal work content, and every trade is sized to complete its work in each zone within the takt time, then move to the next zone, like cars moving along an assembly line. A typical building takt might be one zone per week or per three days. Trades move together in a train of wagons, each wagon a work step, and the whole train advances one zone each takt period.

Takt planning makes the schedule highly visible and predictable, exposes bottlenecks immediately and encourages trades to plan their crews to the rhythm. It requires careful zoning, balanced work content, and trades willing to adjust crew sizes. It also depends on reliable short-term planning, such as the Last Planner System, so that each zone is truly ready when its takt period begins. The lean construction course covers these methods in depth.

Combining methods

Location-based methods do not replace the critical path method; they complement it. Most building projects use a CPM network for the overall schedule, including procurement, approvals and non-repetitive work, and a location-based plan for repetitive interior or structural work. Some scheduling software integrates the two, letting the same activities be viewed as a network, a bar chart or a flowline chart. Contracts and owners usually still require a CPM schedule for submittals and delay analysis, so the location-based plan must be consistent with it.