SP2 Structural bridge renewals

Introduction

These procedures (SP2) provide a simplified method for appraising the economic efficiency of replacing a bridge for structural reasons. The benefits analysis focuses on the change in heavy commercial vehicle (HCV) users' costs as a result of the project. Guidance on the application of these procedures is found in section 4.2 and through the decision chart on the following page.

If road improvements are being considered in conjunction with the bridge renewal, then the improvements are to be evaluated separately (using SP3, if applicable - refer section 4.2), when it is confirmed that bridge renewal is the preferred option.

The procedure for analysing structural bridge renewals is somewhat different to other projects, in that all options are identified and costed at the outset, including:

• cost of replacement bridge
• average daily traffic
• viability and cost of a concrete ford
• the HCV users of the bridge
• existence of an alternative route, its length and any necessary upgrade costs
• the cost to repair the bridge to a posted limit of 10 tonnes.

Once this has been done, the decision chart on the following page can be used to determine the appropriate course of action and analysis procedure.

The worksheets use a 10% discount rate and 25 year evaluation period. The procedures assume that funded projects will be completed within the first year and will be in service by the start of year 2. Where costs are common to all the options, they are not included in the analysis. All costs are to be exclusive of GST.

Total bridge failure

This procedure does not allow for the possibility of total bridge failure. If this is a real possibility when certain options are chosen, then account should be taken of the extra costs this would impose on road users multiplied by the probability of failure occurring. The calculation of these probabilities should be undertaken by the same engineers who make the decisions regarding posting the bridge.

Decision chart for bridge replacements on low volume roads

Building a ford on a low volume road

Explanation for worksheet 1

Worksheet 1 is used to record and summarise the decision to construct a ford on a low volume road (AADT less than 50 vehicles per day) when the cost of the ford is significantly less than a bridge as specified in section 4.2.

The information required is a subset of the information entered into LTP online.

1. Evaluator(s)/reviewer(s): Enter the full name, contact details, name of organisation, office location, etc, of the evaluator(s) and reviewer(s).
2. Project/package details: Provide a general description of the project and package (where relevant), describe the problem with the existing bridge and the problems to be addressed.
3. Location: A brief description of the project location information (as appropriate) including:
   • a location/route map
   • a layout plan of the project.
4. Alternatives and options: Describe the do minimum. The do minimum should be chosen after analysis of low-cost options. The do minimum will not necessarily maintain the capacity of the bridge to carry 100% Class I loading or even to maintain a crossing at all. Describe the options assessed and how building a ford will affect HCVs (eg, will they use the ford or an alternative route?).
5. Timing: For purposes of the economic efficiency evaluation, the construction start is assumed to be 1 July of the financial year in which the project is submitted for a commitment to funding.
6. Economic efficiency: Enter the timeframe information and road and traffic data for the economic efficiency calculation. Identify the existing route length, the length of any available alternative route(s); the proportion of HCVI and HCVII vehicles using the existing bridge; the load factor of the bridge and the existing bridge posting weight limit. If the bridge is on a route regularly used by HCVs provide a (separate) list of common users together with contact details.
7. PV cost of do minimum: Calculate the PV cost of all possible options and select the least PV cost option as the do minimum. The do minimum will not necessarily maintain the capacity of the bridge to carry 100% Class I loading or even maintain a crossing at all. Worksheet 3 may be helpful to this process.
8. PV cost of building a ford: Enter the PV costs of building a suitable ford.
9. PV cost saving: Calculate the cost saving (in PV terms) for the ford compared with the do minimum.

Worksheet 1 (70.5 KB)

Evaluation summary for bridge renewal

Explanation for worksheet 2

Worksheet 2 provides a summary of the general data used for the evaluation where a decision is for a structural bridge renewal.

1. Evaluator(s)/reviewer(s): Enter the full name, contact details, name of organisation, office location, etc, of the evaluator(s) and reviewer(s).
2. Project/package details: Provide a general description of the project and package (where relevant), describe the problem with the existing bridge and the problems to be addressed.
3. Location: A brief description of the project location including:
   • a location/route map
   • a layout plan of the project.
4. Alternatives and options: Describe the do minimum. The do minimum should be chosen after analysis of low-cost options. The do minimum will not necessarily maintain the capacity of the bridge to carry 100% Class I loading or even maintain a crossing at all. Describe the options assessed and how the preferred option will affect traffic, particularly HCVs.
5. Timing: For purposes of the economic efficiency evaluation, the construction start is assumed to be 1 July of the financial year in which the project is submitted for a commitment to funding..
6. Economic efficiency: Enter the timeframe information and road and traffic data for the economic efficiency calculation. Identify the existing route length, the length of any available alternative route(s); the proportion of HCVI and HCVII vehicles using the existing bridge; the load factor of the bridge and the existing bridge posting weight limit. If the bridge is on a route regularly used by HCVs provide a (separate) list of common users together with contact details.
7. PV cost of do minimum: Use worksheet 3 to calculate the PV cost of all possible options and select the least cost option as the do minimum.
8. PV cost of preferred option: Use worksheet 3 to estimate the PV cost of the preferred option.
9. Enter the economic evaluation data from worksheet 4 or 5. To convert the road user costs to base date values use the update factors in appendix A12.3. If the road user costs of the do minimum are less than the road user costs of the chosen option, then the option should be abandoned.
10. The national BCR is calculated by dividing the PV of the net benefits (PV benefits of the do minimum subtracted from the PV benefits of the option) by PV of the net costs (PV costs of the do minimum subtracted from the PV costs of the option).
11. First year rate of return (FYRR) is calculated as the benefits in the first full year following completion divided by the project costs. The first year benefits are calculated by dividing the totals at Y and Z by the BDF from table 1 of worksheet 4. Then multiply by 0.91 to get the present value.

Worksheet 2 (79 KB)

Cost of the option(s)

Explanation for worksheet 3

Worksheet 3 is used to calculate the costs of the different options. At the top of the worksheet, circle the option being evaluated. A separate Worksheet 3 is required for each option evaluated. To convert dollar values from different years to base date values, use the update factors in appendix A12.3.

1. Circle the option being considered.
2. Enter the capital cost (including professional services for design and supervision) of the proposed option. The cost is estimated separately on an estimate sheet, which should be attached to this worksheet. Multiply the cost by the discount factor 0.91 and enter at (a).
3. Enter the cost of maintenance for year 1 (b). As this is assumed to be the year that the proposed option works are carried out, this cost will commonly be the same as that for the existing maintenance strategy, as per step 2 on worksheet 2.
4. Enter the cost for annual maintenance and inspections following completion of the works. Multiply by 8.57 to get the PV of annual maintenance costs (c) for years 2 to 25 inclusive.
5. Enter the costs of periodic maintenance. Determine which years this maintenance will be required (if at all) and enter the year, estimated cost and SPPWF (from table 1 below). Calculate the present value (estimated cost ´ SPPWF) for each cost and sum these to obtain the PV of the total periodic maintenance cost (d).
6. Where an alternative route is being considered, the PV cost of additional maintenance required for the road due to extra HCV trips should be calculated. Enter additional annual maintenance costs for the route and multiply by 9.52 to get the PV costs for years 1 to 25 inclusive (e). If the option is to close the existing bridge then (e) will be the cost of the option.
7. Sum all of the PV costs for the option (a) + (b) + (c) + (d) + (e) to determine the PV total costs for each option. Once the costs of all viable options have been calculated, the lowest cost option is chosen as the do minimum. Transfer total to A (if do minimum) or B (if preferred option) on worksheet 1.

Table 1 Single payment present worth factors - for 10 percent discount rate

Single payment present worth factors table - 10% discount rate (18.6 KB)

Worksheet 3 (49.5 KB)

HCV user costs when there is an alternative route

Explanation for worksheet 4

Worksheet 4 is used for calculating the HCV road user costs for the various options when there is an alternative route. If posting the bridge is the do minimum, then the HCV users will either choose to:

1. use the existing bridge with load restriction and make more trips in lightly loaded vehicles, or
2. use the longer alternative route with fully loaded vehicles.

The HCV users operating costs for both alternatives should be calculated and the lowest HCV user cost used in the benefit cost analysis. If bridge is closed and an alternative route takes all diverted traffic, then the additional VOC, travel time and accident costs for the whole traffic stream are calculated. The additional costs per kilometre, including an adjustment for CO₂ emission costs, have been standardised for the HCV user cost calculations. The values are already included in the worksheet.

1. Circle the option being considered.
2. Calculate the HCV user costs for the existing route at ______% Class I loading by entering the information indicated below. Multiply across the lines to get the annual user costs for HCVI and HCVII. Sum these values to get the total HCV user costs (a). Multiply (a) by the appropriate bridge discount factor (BDF in table 1) to give the PV of HCV user costs for 25 years (b).
   
   

   Required information:

   L

   length of existing route in kilometres (between intersections with the alternative route). A survey of local transport operators and businesses will provide data to allow an estimate of the trip lengths for HCVs on trips that cross the bridge.

   LA

   length of alternative route in kilometres

   ADT HCVI

   the average daily tally of HCVI on the existing route

   ADT HCVII

   the average daily tally of HCVII on the existing route

   LF

   HCV load factor (% of fully loaded vehicles). Use 70% unless better data is available

   FCF

   freight cost factor (from table 2) used to calculate increased costs due to extra trips required by posting a load restriction on a highway

3. Repeat step 1 for the option of the alternative longer route, to derive values for (c) and (d).
4. Repeat step 1 for the option of the existing route at 100% Class I loading for (e) and (f).
5. Where the option to maintain the existing route at 100% Class I loading requires downgrading the bridge and constructing a ford, the additional user costs of a ford must be calculated and added to (f). Repeat the process for step 1 for the option of a ford, to derive the values for (g) and (h). To get the total HCV user costs for the option add (f) and (h) to get (j).
6. Transfer the HCV user costs for the do minimum to A and preferred option to B on worksheet 2.

Table 1 Bridge renewal discount factors (BDF) for years 1 to 25 inclusive

Table 2 Freight cost factors

Table 1(BDF) and table 2(freight cost factors) (74 KB)

Worksheet 4 (74 KB)

HCV user costs when there is no alternative route

Explanation for worksheet 5

Worksheet 5 provides a method for calculating HCV road user costs when no alternative route is available. In this situation, the HCV user costs for bridge crossings should be calculated for the distance between the origin and destination of the trips.

1. Circle the option being considered.
2. Calculate the HCV user costs for the existing route at ______% Class I loading as follows by entering the information indicated below. Multiply across the lines to get the annual user costs for HCVI and HCVII. Sum these two values to get the total HCV user costs (a). Multiply the total in (a) by the appropriate bridge discount factor (BDF in table 1 of worksheet 4) to give the PV of HCV user costs for 25 years (b).
   
   

   Required information:

   L

   length of existing route in kilometres (between intersections with the alternative route). A survey of local transport operators and businesses will provide data to allow an estimate of the trip lengths for HCVs on trips that cross the bridge.

   ADT HCVI

   the average daily tally of HCVI on the existing route

   ADT HCVII

   the average daily tally of HCVII on the existing route

   LF

   load factor (the percentage of fully loaded vehicles) for HCVs. Use 0.7 (70%) unless better data is available

   FCF

   freight cost factor (from table 2 of worksheet 4) used to calculate increased costs due to extra trips required by posting a load restriction on a highway

3. Repeat step 1 for the option of the existing route at 100% Class I loading (bridge retained), to derive the values for (c) and (d).
4. Where the option to maintain the existing route at 100% Class I loading requires downgrading the bridge and constructing a ford, the additional user costs of a ford must be calculated and added to (d) to get the total HCV user costs for the option (g).
5. Transfer the HCV user costs for the selected do minimum to C and preferred option to D on worksheet 2.

Worksheet 5 (60 KB)

BCR and incremental analysis

Explanation for worksheet 6

Cost benefit analysis

1. Under benefits, enter the PVs for the benefits for the do minimum and for each option. Then subtract the benefits for the options from the benefits for the do minimum to get the net benefits of each option.
2. Under costs, enter the PVs of the capital and maintenance costs for the do minimum and each option. Subtract the PV costs for the do minimum from the costs for the options to get the net costs of each option.
3. Calculate the national BCR by dividing the net benefits by the net costs.

Incremental analysis

1. Select the appropriate target incremental BCR from appendix A12.4.
2. Rank the options in order of increasing cost.
3. Compare the lowest cost option with the next higher cost option to calculate the incremental BCR.
4. If the incremental BCR is less than the target incremental BCR, discard the second option in favour of the first and compare the first option with the next higher cost option.
5. If the incremental BCR is greater than the target incremental BCR, the second option becomes the basis for comparison against the next higher cost option.
6. Repeat the procedure until no higher cost options are available that have an incremental BCR greater than the target incremental BCR. The highest cost option with an incremental BCR greater than the target incremental BCR is generally considered as the preferred option.

Worksheet 6 (62.5 KB)