The key to a successful productivity loss disruption claim is an accurate report and graphic representation of a meticulous Time Impact Analysis (TIA). In Primavera 6, the TIA may be represented with the baseline bar, and actual-bar, on the same chart. Another way of representing a productivity loss is using the Measured Mile. However; these charts are only window dressing to complement a well researched and documented productivity claim.
But even with the Measured Mile approach, productivity loss is a slippery-slope. The reason why, is the same reason that stymies researchers of productivity loss: the approach depends on the industry, or context. For example, an assembly line process and individual workers can be easily monitored – even for things like downtime. If productivity loss becomes evident, it will be easier to isolate by separating its constituents and measuring their productivity rates against past performance.
The same can hardly be said for the building industry. That’s not so much owing to a paucity of available trade specific research, as it is to difficulty in obtaining valid data. When estimating productivity for a resource driven CPM schedule, rates of production are entered into the resource activity. Most of that resource is on-task, but the rest of the time is found in preparatory or downtime activities. When actual resources perform worse than the baseline assignment – you get slip, or productivity loss.
Construction estimators and contractors allow for mobilization, and demobilization of their crews in their schedules. If a contractor has to demob/remob several times, it can be said his productivity overall was affected by the constant moving around. Yet, the workers on-taskproductivity rate remains pretty much the same. In other words, if workers could put down and work continuously and contiguously, production rates would soar.
But that’s not the building industry. Estimators and schedulers try to estimate the effect of things like access, constraints, shift-work, and other known disruptors, before a project mobilizes. They think through the specific logistics of the site, and shake out any encumbrances or constraints on the ground, or going up with cranes and hoists.
For example, if two hoists service a workforce bringing up a block and plank tower, they will eventually have to come down. By such time, the project may be pushing the fit out envelope, with a full workforce of 400 tradespeople. If there is only a single hoist, wait times could possibly eat into as much as 30% of their work-day. If the overload is a consequence of unanticipated disruptions or delays, it could not have been foreseen.
Cold weather can affect productivity adversely. Placing concrete, digging, jumping a crane, are all activities subject to take longer, as do other trades, as shown in the chart, below
In gauging accurate productivity rates, it’s necessary to consider support resources and management and supervision costs. Without factoring those soft costs and overhead, it’s not possible to determine an accurate overall productivity rate because those resources are seldom part of the equation – despite being part of the cost. These are known once the trades rates are known.
A common mistake some schedulers make is to bundle activities in a way that makes actual duration longer than it really is: if light-gauge framing is used, a scheduler might allow a day or two for axes and layout lines, a few more days for plate, and a few more days for door king studs, corners, and openings. At this point, the sheet metal (duct) goes in high and tight, perhaps two-weeks from delivery. The framers return for infill framing, and demob, to allow for the electricians and plumbers. They return one more time to drop ceiling framing. In this typical scenario, there are at least two comebacks. If there are hoist leave-outs to infill, that will be another trip.
Thus, the typical ten-days per/floor cycle might start with 50% completion, with the three remaining 50% remob-comebacks enfolding over a year or more. Otherwise, Actual Duration appears skewed. Light Gauge Framing: 10d OD 160 AD. Remob and demob affect MEP trades adversely when workers are bounced around a building, working wherever they can, because they can. Sequence of operations become deprecated as forces permeate disparate area of a building, further gumming up project logic.
Therein a big incentive to break out in the schedule the remobs into separate tasks. It’s best practice, as well, to network the broken out tasks with the other trades that are active in between the framers work. In this way, it becomes easier to isolate separate float paths, while at the same time maintaining project logic. Without project logic in tact, delay and EOT claims will founder.
