construction management

4120 – Scheduling A Crash

What does crashing a schedule refer to? It has nothing to do with scheduling while texting, but rather is the process of reducing project duration by shortening the total time needed for a task or series of tasks in a project. Logically, only critical tasks should be considered for ‘crashing’ since shortening a non- critical item will only lengthen its float in the schedule. The crashing is often done by working overtime, adding personnel, doing additional shifts or sometimes just stacking two trades on top of each other is done. This, however may be closer to a crash that crashing - Imagine trying to hang drywall in the same room at the same time as the ceramic tile being placed on the floor. Other logical options for crashing may be pre-fabrication of components, getting equipment that will speed the process or ordering materials that are faster to install. Crashing can involve breaking tasks down into smaller sections, such as by area or by crews within a trade.

However it is done, crashing typically (but not always) requires extra money. Overtime costs more, working people extra hours lowers productivity rates, thus increasing costs per unit, stacking people on top of each other makes them lest efficient as well. Extra equipment or better materials will cost more money in most cases. Shortening a schedule should be considered so that you do not spend more money saving time that the value of the time that you saved. If you have a liquidated damage of $500 a day, it would not make fiscal sense to spend $800 a day to save time.

Consider the following example and determine the most reasonable way to save time on the schedule. Cutting time across the board on every trade is generally wrong. Reasoning should be employed when determining the most cost effective way to shaving time from a schedule. Assume here that you have asked the trades to tell you what it would take to cut time from their schedules:

You have three subs that can help you make up time. Sub A charges $300/day to take time off their 2 week schedule for up to 3 days and then $600/day for the next two days. They can save no time more than that. Sub B charges $500/day to save time on their three weeks of work for up to one week beyond that they can save no time. Sub C charges $400/day to take up to two days off their 7 day duration and $900/day for two more days. The three are each on the critical path there are no near critical tasks except for Sub D which has the same predecessor as Sub A. Sub D and Sub A have Sub C as a successor. Sub D has 4 days of float in the base schedule. You need to save two weeks on your schedule. What do you do? (A week is 5 days.)

Solution: Pred Sub A Sub B Sub C end 10 d 15 d 7 d Sub D 6d

Sub A Sub B Sub C Time & Cost Savings 3 days @ $300/day 5 days @ $500/day 2 days @ $400/day 2 days @ $600/day 2 days @ $900/day

4120 – Scheduling A Crash

Sub D has a 6 day duration because it has the same predecessor and successor as Sub A but also has a 4 day float. The table shows the ways in which we can save time. We first find the cheapest way to save time. The $300/day with Sub A is the cheapest, so we first take all those days we can – in this case, three. The next cheapest is $400 using Sub C. This cost only applies to the first 2 days with that sub. Now we have 5 days saved but need 5 more. The next cheapest option is $500/day with Sub B. Since they can do 5 days and we need five days, that will finish the problem. Thus:

3days @ $300/day with Sub A = $900 2days @ 400/day with Sub C = $800 5 days @ 500/day with Sub B = $2,500

For a total of $4,200 to save 10 days. Any other combination would cost more money to save the time needed.

Let us assume that Sub A changed their mind and said that after the 3 days at $300/day was good, but that they could do 2 days @ $400/day instead of the $600/day. Would that change your solution?

By changing the chart, you see that the $400/day from Sub A would be preferred to the $500/day from Sub B and at least equal to the 2 days from Sub C. Thus, we would take those days and give up two of the days from Sub B at $500 per day – saving $200 on this crash. However, we would be stupid because we did not consider that Sub D becomes critical after 4 days are saved by Sub A. Sub D’s float of 4 days means that they are near critical as Sub A shortens their duration. Once we do 4 days with Sub A, there is no advantage to doing additional days because Sub B will still be held up starting until Sub D is done. We would be able to only take one of the days at $400. Therefore the new solution would be:

3days @ $300/day with Sub A = $900 1 day @ $400/day with Sub A = $400 2days @ 400/day with Sub C = $800 4 days @ 500/day with Sub B = $2,000

For a total of $4,100 to save 10 days. A hundred dollars buy quite a few gallons of ice cream so we would appreciate this savings.

For practice:

1. Go back to the original solution with Sub A having the 2 days at $600/day. What would happen if a Sub E had Sub A as a predecessor and Sub C as a successor and its duration was 12 days?

2. What if Sub E now said that they could shave 3 days off their schedule for $300/day?