Critical Path Method Practice Problems

Whereas general contractor Todd Lubash would organise more than 250 distinct activities with correspondingly complex precedence constraints for the construction of the Jacobson house, let us consider a representative subset of these activities.

Exhibit 1 lists the activities for both the traditional wood-framed house and the Bone structure build. The precedence constraints with traditional wood framing are included in column three and estimates of time durations in column four. **The BONE structure alters the precedence relationships between activities. Estimates of activity time durations for the BONE structure are included in column 5.

Lubash was confident in the duration estimates with a wood-frame building because he was very experienced in wood-frame construction with his crews and subcontractors. He was less confident with the duration estimates with a BONE structure because Jacobsons home would be his first build of this kind.

Exhibit 1: Activities, durations and precedence constraints

Activity		Wood Frame		Bone Structure
		Immediate Predecessor	Duration (days)	Duration (days)
	City Government
1.	Permitting		13	13
	Procurement
2.	Order/delivery lead time: doors and windows	1	57	57
3.	Order/delivery lead time: Interior finishes (countertops)	6	48	48
	Construction
4.	Site work and foundation	1	45	45
5.	Slab	4	8	8
6.	Framing and roof trusses*	5	70	3*
7.	Installation: doors and windows	2,6	9	7
8.	Roofing	6	25	5
9.	Rough-in	6	14	7
10.	Insulation	9	11	*
11.	Drywall	10	11	11
12.	Interior finishes	3,11	101	101
13.	Exterior	7,8	85	85
14.	Landscaping	13	42	42

Notes:

* A BONE structure includes frame, roof panels, and insulation and can be assembled on site in an estimated 3 days.

At the rough-in stage, the basic electrical, plumbing and HVAC lines are laid in without making final connections to interior fixtures.

** Using a BONE structure changes the precedence constraints as well as the activity durations:

• The dimensional accuracy of the BONE structure eliminates the precedence constraint that interior finishes (countertops) must be ordered after framing; instead, interior finishes could be ordered immediately after obtaining a building permit.
• Also, with BONE, the slab is poured after installation of the BONE structure, i.e., the order of activities 5 and 6 are reversed.
• Lastly, the slab must be completed before any of the following activities can occur: installation of doors, windows, plumbing, electricals, HVAC, and roofing.