This is an example following a case study at the end of the Start to Finish Relationship article.
It presents the following case:
Schedule the following activities to allow concrete to be poured, note we are assuming that we need to pour within 90 minutes from the time the concrete is mixed to pouring.
- Completed the form work (3 hrs)
- Obtained the approval for the delivery truck to enter the site (10 mins)
- Deliver Concrete (1 hr)
- Pour Concrete (10 mins)
What did you end up with?? Something like this?
Most schedules I’ve seen use a standard Finish to Start (FS) relationship and logically this makes sense; we can’t Start to pour the concrete (Activity 4) until it has been delivered (Activity 3). No problems in using this logic, but what issues can you spot in the above schedule?
Those with industry experience will quickly spot the fact that the concrete has been delivered to site 2 hours and 50 minutes prior to the start of the pour.
- Issue 1 there is a risk that the cement may dry up and fail a slump test.
- Issue 2 you will have a very angry truck driver instructed to wait for nearly 3 hours.
The issue is whilst yes the finish to start is logical, there are issues if the successor isn’t able to occur straight away.
You could “fix” the schedule still using the default Finish to Start relationship, by not starting the delivery until the form has been completed and the site permit issued.
Ok same game spot the issues, in this case you don’t need industry experience to note that you have delayed your project by 1 hour and 10 minutes from a duration of 3 hours 15 minutes to 4 hours 25 minutes.
After a couple of iterations you will probably come to the conclusion, “why don’t we finish 2/3rds of the form-work then have the concrete delivered, by the time the form-work is complete the concrete will have arrived, ready to pour. So far this is the best schedule. If there are delays in the form-work or the approvals we can delay the delivery. This will also bring your projects duration back to a respectable 3 hours and 15 minutes whilst reducing the risk of the concrete drying up. If you got to this point by yourself well done.
So let’s see what happens when we play this out, the concrete arrives on site just as the form-work is completed- perfect orchestration.
The concrete is ready to be poured but all of a sudden an electrical storm gets very close, the Work Health and Safety team issue a stop-work notice as the lighting is close, preventing the pour. The concrete dries up and has to be re-ordered. I used this scenario as it actually happened.
Could this have been avoided through scheduling, possibly not if it was a freak storm with lack of prior notice. But what if we had been given notice. So earlier this morning the scheduler received notice that the pouring of the concrete has to be delayed until the storm passes, effectively no work can occur from 10am to 12pm.
So he adjusts the schedule. In this case he has increased the effort to complete the form-work in 2 hours and pushes the other work- pour concrete till after the storm, as he assumes the concrete can still be delivered during a storm. Now we hit the same issues- the concrete is delivered too early relative to the pour- leading to quality issues.
This scenario isn’t uncommon, often the Project Manager (or industry expert) will work with the scheduler in the integration of technical requirements during the initial development of the schedule. During implementation the scheduler receives additional information but may not understand that the concrete can be delivered to site and sit there for hours until its ready to be poured.
Which brings us to the Start to Finish solution.
We want to link the activities so the concrete delivery doesn’t finish until the start of the pouring, this will help as when the scheduler postpones the pouring the deliver concrete task will also shift.
How this would look in a schedule
Now I hope this makes sense, best of luck with your schedules and remember don’t use the Start to Finish relationship if you don’t need to.
Happy to answer any questions as well as provide examples that may be of relevance to a particular industry.