Comprehensive Truck Size and Weight Limits Study - Bridge Structure Comparative Analysis Technical Report
Chapter 3: One-Time Bridge Rehabilitation/Replacement Costs
In order to estimate the probable one-time cost effect of employing alternative truck configurations in each scenario, the USDOT study team developed a methodology that estimates the relative increase in cost relative to the base vehicles. A rating factor (RF) of 1.0 was set as the acceptance criteria when determining bridge improvement needs. It should be noted that the one-time cost of bridge improvements addressed herein could pertain to either superstructure strengthening or superstructure replacement triggered by the need to increase live load capacity. The choice of strengthening vs. replacement would depend on superstructure type and whichever is the more economical alternative. For instance, for a short span concrete slab or T-beam superstructure, replacement may be preferred over strengthening or rehabilitation. For a steel girder bridge, however, strengthening or rehabilitation may be a more economical way to increase load capacity than full superstructure replacement.
It should be noted that the costs presented in this chapter represent the extreme upper bound of possible costs that might result from the introduction of the alternative truck configurations in each scenario. In addition, there are significant cost mitigating factors and neither the actual costs nor the lower bound costs are determinate due to the range of program and policy decisions available to the States. Furthermore, the methodology does not take into account any cost- or budget-driven decisions that may be made by the State DOTs and does not address State DOT policy alternatives that may initiate more refined analysis or load testing options to improve load ratings. Cost estimates are based on total project costs and not just the construction costs.
The study team considered bridge rehabilitation or replacement costs for all alternative truck configuration scenarios on both the Interstate and other NHS roadways, regardless of whether some vehicles (triple trailers for instance) may be excluded for safety or other reasons. As with the structural analysis (Chapter 2) and as outlined in the Modal Shift Comparative Analysis, the parameters for the one-time cost assessment assume the scenario vehicles are able to travel wherever their control vehicles could operate. For analytical purposes triple trailer combinations (Scenarios 5 and 6) are assumed to be restricted to a 74,500 mile network of Interstate and other principal arterial highways. One-time costs assessed in this study take into account the findings related to changes in vehicle use patterns estimated in the Modal Shift Comparative Analysis would result from the travel estimated for each of the alternative vehicle configurations.
The methodology entailed the following nine-step process:
- Determine the distribution of span lengths in the sample database as percentages separately for Interstate bridges and other NHS bridges (see Chapter 2 of this document).
- Calculate the cost of bridge rehabilitations for each span length interval as:
Bridge lengths were taken as the upper limit in the interval (e.g., for 20-40 ft. spans, use 40 ft.). Deck width was taken as 64 ft. (4 x 12 ft. lane + 2 x 8 ft. shoulder) for Interstate bridges and as 48 ft. (3 x 12 ft. lane + 2 x 6 ft. shoulder) for other bridges on the NHS. The unit price, based on total project costs, for replacement or strengthening was taken as $235 per sq. ft. This value is an approximation of the following costs lumped together:
- Construction cost;
- Design (8 percent to 20 percent depending on the project size);
- Construction inspection- and design-related assistance (13 percent);
- Work zone traffic control;
- Substructure rehabilitation;
- Mobilization (4 percent); and
- Other costs, including:
- Railing and transitions;
- Joints and approach pavements; and
- Striping, grooving and sealing the deck, etc.
Based on the parameters given above, approximate bridge rehabilitation costs for Interstate bridges and other NHS bridges are listed in Table 23.
- Determine the percentage of bridges rated less than 1.0 in the structural analysis for each alternative truck configuration scenario for each span interval (See Chapter 2 of this document).
- Determine the total number of Interstate bridges and other NHS bridges in the NBI inventory (See Chapter 2 of this document).
- Estimate the actual number of bridges in each span interval using the distributions observed for the sample database.
- Determine the projected number of bridges with RF < 1.0 for each configuration scenario by multiplying the percentage of bridges rated less than 1.0, calculated in step 3, by the number of bridges in each span interval, calculated in step 5.
- Determine the cost of bridge rehabilitations for each span interval for each truck type, separating Interstate bridges from other bridges on the NHS, by multiplying the cost calculated for a single bridge for that span interval by the projected number of bridges with RF < 1.0 for each truck scenario.
- Add costs from each span interval to determine the total costs for each scenario. The total rehabilitation costs for each alternate truck configuration are listed in Table 24 and Table 25 for Interstate bridges and other bridges on the NHS, respectively.
- Calculate Δcost for each scenario. Δcost is the difference in the cost of rehabilitations due to an alternative truck configuration and that from the related control vehicle. Projected costs are given in Table 26.
For Alternative Trucks Configurations – Scenarios 1, 2 and 3, Δcosts were determined as:
ΔcostScenario 1 = Cost Scenario 1 – Cost3-S2
ΔcostScenario 2 = Cost Scenario 2 – Cost3-S2
ΔcostScenario 3 = Cost Scenario 3 – Cost3-S2
For Alternate Trucks Configurations – Scenarios 4, 5 and 6, Δcosts were determined as:
ΔcostScenario 4 = Cost Scenario 4 – Cost2-S1-2
ΔcostScenario 5 = Cost Scenario 5 – Cost2-S1-2
ΔcostScenario 6= Cost Scenario 6 – Cost2-S1-2
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