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Iowa Demonstration Project: Accelerated Bridge Construction on US 6 over Keg Creek
A key aspect of HfL demonstration projects is quantifying, as much as possible, the value of the innovations deployed. This involves comparing the benefits and costs associated with the innovative project delivery approach adopted on an HfL project with those from a more traditional delivery approach on a project of similar size and scope. The latter type of project is referred to as a baseline case and is an important component of the economic analysis.
For this economic analysis, the Iowa DOT supplied the cost figures for the as-built project and baseline construction. Traditional methods would have involved the use of cast-in-place construction coupled with standard pre-tension precast concrete bridge beams.
The baseline scenario would have closed the bridge for at least 6 months to accommodate traditional cast-in-place construction methods. The ABC approach allowed the contractor to fabricate the bridge components ahead of time and then assemble the bridge during a minimal 16-day bridge closure.
The project was awarded based on lowest bid. There were 7 qualified bids received of which Godbersen-Smith Construction Company was the lowest bid at $ 2,658,823.35. Table 2 lists the results of the 7 bids.
Table 3 presents the 2011 construction costs of the baseline and the as-built alternatives based on the awarded as-built contract and Iowa DOT estimates. Assumptions regarding the baseline case include:
The comparison shows the as-built total cost was $2,680,823 compared to $1,511,530 for the baseline total cost. Considering only the bridge portion of the contract, the as-built costs were nearly double the baseline bridge costs. Contract items such as roadway improvements, traffic control, and the reinforced concrete flume were assumed to cost the same in either case.
Generally, three categories of user costs are used in an economic/life cycle cost analysis: vehicle operating costs (VOC), delay costs, and crash- and safety-related costs. Because the bridge would have been closed to traffic under both the baseline and as-built case, the possible safety hazard to the traveling public from a work zone was eliminated, so safety-related costs were not evaluated. However, VOC and delay costs were compared and are discussed in the following subsections.
The savings in VOC from using ABC is essentially the difference between the mileage-related VOC applied to the 6 months (183 days) of detour time for the baseline case and the 16 days for the as-built case applied to the detour distance of 21 miles. In the absence of actual vehicle count data, the VOC can be estimated conservatively by considering only commercial vehicles (light and heavy trucks) traveling the designated route due to weight restrictions on other county roads in the area. Passenger vehicles are discounted because there are numerous county roads which could serve as non-planned detour routes preventing an accurate traffic estimation based solely on the AADT on US 6.
Assuming an average unit cost of $0.81 per mile7 for commercial vehicles for the variable operating costs (including costs for fuel, maintenance and repair, tires, and depreciation) based on highway travel and given the 2009 AADT of 3,890 with 9 percent commercial vehicles, the following VOC is computed:
Baseline CaseVOCcommercial = 3,890 (AADT) * 0.09 (percent commercial vehicles) * 21 (mi) * $0.81 (per mi) * 183(days) = $1,089,802
As-Built CaseVOCcommercial = 3,890 (AADT) * 0.09 (percent commercial vehicles) * 21 (mi) * $0.81 (per mi) * 16 (days) = $95,283
The total saving in VOC because of the detour differential between the baseline and as-built scenarios is as follows:VOCDifferential = $1,089,802baseline – $95,283As-built = $994,519
As with the VOC calculation, only delay costs occurred by commercial vehicles were considered since passenger vehicles could have chosen an alternate detour route. The effect of reducing the duration of the bridge closure saved $615,267. The following provides a basis for this conclusion:
Using these assumptions and cost figures, the saving in delay cost is as follows:
Baseline CaseDelaycommercial = 3,890 (AADT) * 0.09 (percent commercial vehicles) * 0.41(hrs/vehicle) * $25.67 (per hour) * 183 (days) = $674,215
As-Built CaseDelaycommercial = 3,890 (AADT) * 0.09 (percent commercial vehicles) * 0.41(hrs/vehicle) * $25.67 (per hour) * 16 (days) = $58,948
The total saving in delay costs between baseline and as-built scenarios is as follows: DelayDifferential = $674,215Baseline – $58,948As-built = $615,267
From a construction cost standpoint, the ABC delivery approach cost the Iowa DOT $1,169,293 more than traditional construction but saved time users would have otherwise been detoured. Considering the savings in user costs of $1,609,785 from combined VOC and delay costs ($994,519 + $615,267), the cost differential is $440,492 or 29 percent less than traditional construction for a project of this size and scope.
7Barnes and Langworthy, The Per-Mile Costs of Operating Automobiles and Trucks, 2003. Report No. MN/RC 2003-19, Minnesota Department of Transportation. Adjusted for fuel price increase and inflation in 2011.
8Mallela and Sadasivam, Work Zone Road User Costs Concepts and Applications, 2011. Report No. FHWA-HOP-12-005, Federal Highway Administration.