Historic Bridge Manual: Alternatives Analysis

Section 3: Alternatives Analysis

Overview

Alternatives are evaluated as to whether they are feasible and prudent using the Standards, Policies, and Guidelines Relating to Highway Bridge Design of the American Association of State Highway and Transportation Officials (AASHTO). Each alternative must be carefully evaluated based on the required structural capacity, the current structural capacity, and the information established in the need and purpose statement before an appropriate alternative can be recommended.

An alternative is deemed not feasible if:

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It can not be built as a matter of sound engineering judgment.

An alternative is deemed not prudent if:

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It results in safety or operational problems.

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It does not effectively address impacts through reasonable mitigation.

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It results in significant additional construction, maintenance, or operational costs.

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It involves multiple factors listed above that, while individually minor, cumulatively cause significant problems.

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Structural Evaluation

Vehicular Service. In order to determine feasibility for vehicular service, each historic bridge must undergo a structural evaluation. For the purposes of the Historic Bridge Team Report, the Bridge Project Manager will perform a preliminary structural evaluation based on the minimum target load rating and information presented in the condition assessment report. This structural evaluation will determine if using the bridge to carry vehicular traffic is a feasible alternative. If the alternative is feasible, an evaluation of construction and maintenance costs will be provided to determine if it is a prudent course of action.

In order to be feasible, the structure must be capable of being rehabilitated to meet the minimum target load rating without losing its historic integrity. TxDOT has established guidelines for rehabilitation for both on- and off-system structures.

On-system structures are those on the designated state highway system, such as interstate highways, US highways, state highways, and farm-to-market roads. In order to be considered for rehabilitation for continued vehicular service, on-system historic bridges must either be rehabilitated or improved to meet applicable design standards, or must be granted a design exception for the deficiency. Design exceptions must be coordinated between TxDOT Design Division (DES), TxDOT Bridge Division (BRG), and TxDOT Environmental Affairs Division (ENV).

In 2005, the Advisory Council of Historic Preservation (ACHP) and FHWA established a program exemption from the Section 106 review for the entire interstate system, essentially stating a 4(f) is not needed for interstate bridges. More information is available on the ACHP website.

Specific bridges in every state were excluded from this exemption, therefore still requiring a Section 106. FHWA maintains the list of Texas interstate bridges that must go through the Section 106 and 4(f) processes.

Off-system structures are those on other public highways, roads, and streets such as city streets and county roads. Off-System historic bridges may be rehabilitated for continued vehicular service provided one of the following conditions is met:

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The historic bridge is rehabilitated or improved to meet applicable design standards;

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The historic bridge is granted a design exception for its deficiency and is able to maintain its historic integrity;

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The historic bridge meets specific geometric, safety, and load capacity criteria that support the retention and preservation of the bridge as defined in the following table, “Minimum Criteria to Support Continued Use by Vehicular Traffic Off-System."

Anchor: #i1060201Minimum Criteria to Support Continued Use by Vehicular Traffic Off-System
Current Average Daily Traffic (ADT)	Minimum Clear Roadway Width¹		Minimum Load-Carrying Capacity (Operating Rating)
	One-Lane, Two-Way Operations²	Two-Lane, Two-Way Operations	Alternate Route Available³	Alternate Route Not Available
ADT 100 or less	10 feet (3.0 m)	18 feet (5.4 m)	HS 5	HS 12⁴
ADT 101 to 250	10 feet (3.0 m)	18 feet (5.4 m)	HS 8	HS 12
ADT 251 to 400	Not applicable⁵	18 feet (5.4 m)	HS 15	HS 15
ADT greater than 400	Not applicable⁵	Not applicable⁶	HS 15	HS 15
¹ For a minimum roadway length of 50 feet (15 meters) adjacent to the bridge end, roadway crown should match clear width across the structure plus additional width to accommodate guard fence if necessary. ² One-Lane, Two-Way operations are assumed to allow for sight distance across the entire length of the structure. In cases where sight distance across the length of the structure is not available, the allowable minimum clear roadway width shall be the allowable minimum for Two-Lane, Two-Way operations. ³ To allow these values, the identified alternate route must add no more than 5 miles (8 kilometers) to a trip for essential services such as school buses, and emergency fire and medical access. Detour route is calculated as the shortest distance from one end of the bridge to the other, if the bridge is out of service. All bridges on the identified alternate route must have a minimum load rating of HS 12. Historic bridges which do not meet the state legal load limit shall be posted. ⁴ HS 12 load rating was selected because it represents a typical minimum value for vehicles essential for educational, medical, and fire suppression services. ⁵ For ADT greater than 250, One-Lane, Two-Way operations on a structure are not permissible. ⁶ For ADT greater than 400, use design standards as appropriate for the class of highway as shown within appropriate sections of the TxDOT Roadway Design Manual.

Pedestrian Service. Historic bridges being analyzed for possible pedestrian use must meet the design live load as prescribed in the AASHTO Guide Specification for Design of Pedestrian Bridges, Section 3.1. Functional obsolescence is resolved by removing the structure from vehicular service; however, the bridge must be evaluated for ADA compliance and a pedestrian rail must be provided.

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Alternatives

A detailed alternatives analysis must be included in the Historic Bridge Team Report in order to facilitate development of the Section 106 and Section 4(f) documentation. The following alternatives must be addressed in the Alternatives Analysis section of the Historic Bridge Team Report.

No Build. Under the No Build alternative, the historic bridge is left in place without rehabilitation. This option is rarely feasible as most historic structures exhibit some type of structural deficiency. The No Build alternative analysis provides documentation showing whether the No Build alternative is feasible and prudent in terms of meeting the project's overall need and purpose.

For historic bridges that do not meet the project’s need and purpose, the No Build alternative analysis should describe the structural elements causing the low load carrying capacity and any other issues affecting the bridge’s function. The following information should be included in the alternative analysis:

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Detour Length. Discuss the detour length that would be required should the bridge not be in service. Detour length is especially important with regard to essential services vehicles, such as emergency vehicles and school buses, which may exceed the current load carrying capacity of the bridge. Detour length is one of the determining factors for the minimum target load rating, in order to ensure timely access for essential services vehicles.

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Bridge Geometry. Address the bridge geometry and functional obsolescence including deck width, bridge length, and horizontal and vertical clearances. The report should include a discussion of whether or not the geometry meets current federal and state standards, and whether or not it is adequate for traffic volume and vehicular needs at the location.

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Bridge Rail. Describe the bridge rail, if any, and whether or not it meets current standards.

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Structural Adequacy. Discuss the as-built and current load rating capacities, and whether or not these capacities meet federal and state legal requirements. Reference the table above, Minimum Criteria to Support Continued Use by Vehicular Traffic Off-System, for off-system structures. Discuss any damage to the superstructure or substructure.

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Hydraulics. Discuss any scour problems or other hydraulics issues.

The alternatives analysis discussion may be abbreviated for bridges that do not meet the minimum target load rating. For these bridges, only structural adequacy need be discussed in detail.

Rehabilitation for Continued Two-Way Traffic. This alternative references the functional and structural deficiencies discussed in the No Build alternative, and discusses how these deficiencies impact, influence, or relate to the historic bridge being able to remain in vehicular service for two-way traffic. The following items should be included if not previously discussed under the No Build alternative, or if updated or more detailed information is required:

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Detour Length. Discuss the detour length that would be required should the bridge not be in service. Detour length is especially important with regard to essential services vehicles, such as emergency vehicles and school buses, which may exceed the current load carrying capacity of the bridge. Detour length is one of the determining factors for the minimum target load rating, in order to ensure timely access for essential services vehicles.

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Bridge Geometry. Address the bridge geometry and functional obsolescence including deck width, bridge length, and horizontal and vertical clearances.

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Bridge Rail. Describe the bridge rail, if any, and whether or not it meets current standards.

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Structural Adequacy. Discuss the as-built and current load rating capacities, and whether or not these capacities meet federal and state legal requirements for the bridge to carry two-way traffic. Reference Minimum Criteria to Support Continued Use by Vehicular Traffic Off-System, for off-system structures. Discuss any damage to the superstructure or substructure.

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Hydraulics. Discuss any scour problems or other hydraulics issues.

In addition to the items listed above, the discussion for Rehabilitation for Two-Way Traffic must include the following:

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Required Rehabilitation Work. Discuss the efforts required to rehabilitate the historic bridge to carry two-way vehicular traffic and to upgrade the bridge to meet federal and state standards. Required work could include:
- Geometry. Outline any changes to geometry, such as widening, lengthening, or improving vertical clearance.
- Bridge Rail. Describe work that will be required to rehabilitate the existing rail, or to upgrade to a new rail.
- Structural Improvements. Detail any work required to improve the structural capacity of the bridge superstructure or substructure. Required work may include rehabilitating, strengthening or replacing members, or rehabilitating or replacing the entire superstructure or substructure.
- Corrosion Protection Measures. Outline any required work for corrosion protection, including paint or other corrosion protection measures.
- Presence of Hazardous Materials. Discuss the test results of any hazardous materials present on the structure.
Cost Estimate. Prepare a detailed cost estimate for each of the items of work required to rehabilitate the structure to carry two-way traffic if the alternative is feasible. Use the cost estimate to determine whether rehabilitation is a prudent alternative to new construction.

Rehabilitation for Use as Part of a One-Way Pair. The analysis of this alternative is similar to that for the previous alternative, Rehabilitation for Use as Part of a Two-Way Pair. This alternative provides more flexibility in achieving the required minimum target load rating by reducing the traffic to one lane, but poses significant economic and geometric concerns as an additional bridge would be required to accommodate the other lane of traffic.

This alternative should reference the functional and structural deficiencies discussed in the No Build and Rehabilitation for Continued Two-Way Traffic alternatives. The following items should be discussed and updated, if required, for new geometric and loading information based on usage as part of a one-way pair:

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Bridge Geometry. Update geometric information for new geometry resulting from conversion to use as part of a one-way pair.

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Bridge Rail. Describe the bridge rail, if any, and whether or not it meets current standards.

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Structural Adequacy. Discuss the as-built and current load rating capacities, and whether or not these capacities meet federal and state legal requirements for the bridge to carry traffic as part of a one-way pair. Discuss any damage to the superstructure or substructure.

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Hydraulics. Discuss any scour problems or other hydraulics issues.

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Required Rehabilitation Work. Discuss the efforts required to rehabilitate the historic bridge to carry traffic as part of a one-way pair and to upgrade the bridge to meet federal and state standards. Required work could include:
- Geometry. Outline any changes to geometry, such as widening, lengthening, or improving vertical clearance.
- Bridge Rail. Describe work that will be required to rehabilitate the existing rail, or to upgrade to a new rail.
- Structural Improvements. Detail any work required to improve the structural capacity of the bridge superstructure or substructure. Required work may include rehabilitating, strengthening or replacing members, or rehabilitating or replacing the entire superstructure or substructure.
- Corrosion Protection Measures. Outline any required work for corrosion protection, including paint or other corrosion protection measures.
- Presence of Hazardous Materials. Discuss the test results of any hazardous materials present on the structure.

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Cost Estimate. Prepare a detailed cost estimate for each of the items of work required to rehabilitate the structure to carry traffic as part of a one-way pair, if the alternative is feasible. Use the cost estimate to determine whether rehabilitation is a prudent alternative to new construction.

Bypassing the Historic Bridge Using an Alternative Alignment. This alternative references the functional and structural deficiencies discussed in the No Build alternative, and discusses how the deficiencies may be corrected so that the bridge may be left in situ, either as a pedestrian bridge or as a monument.

Converting the historic bridge to pedestrian use provides significantly more flexibility in achieving the required load rating. Pedestrian loading is lighter than vehicular loading for most structures, and can be reduced by narrowing the pathway available to pedestrians through the placement of pedestrian railing. However, this option requires that the existing site be attractive to pedestrians, such as a location in or near a park. Many historic bridges will not meet this requirement, as they are too remote to be an attractive destination for pedestrians. This option also requires realignment of the existing roadway to bypass the historic structure. Realignment may not be geometrically feasible, and may not be prudent due to the expense of realignment and right of way acquisition.

Stabilizing the bridge in place as a monument eliminates the need to consider the load rating since the bridge will not carry either vehicular or pedestrian traffic. The bridge must be stabilized to ensure that it does not pose a risk to the public or to the adjacent structure. The new vehicular bridge should be constructed upstream of the monument if at all possible to ensure no harm to the structure should the existing bridge collapse. As with the previous option, roadway realignment may not be feasible or prudent due to geometric and right of way considerations.

Perform a detailed alternatives analysis to determine if either in-situ preservation option is feasible. This alternatives analysis should reference the functional and structural deficiencies discussed in the previous alternatives. The following items should be discussed and updated, if required, for new geometric and loading information based on restoration in situ as a pedestrian structure or as a monument.

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Bridge Geometry. Discuss realignment constraints for the new bridge.

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Bridge Rail. Describe the pedestrian rail type. Pedestrian rails can be chosen from standards available on the Bridge Division website.

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Structural Adequacy. Discuss the as-built and current load rating capacities, and whether or not these capacities meet federal and state legal requirements for the bridge to carry pedestrian traffic. Discuss any damage to the superstructure or substructure. Discuss repairs that will be required to stabilize the structure as a monument.

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Hydraulics. Discuss any scour problems or other hydraulics issues.

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Required Rehabilitation Work. Discuss the efforts required to rehabilitate the historic bridge to carry pedestrian traffic and to stabilize the bridge as a monument. Required work could include:
- Geometry. Outline changes to bridge geometry, such as lengthening.
- Bridge Rail. Describe rehabilitating the existing rail, if desired.
- Pedestrian Use Improvements. Discuss the pedestrian rail type and placement. Pedestrian rails can be chosen from standards available on the Bridge Division website. Discuss required ADA improvements to the bridge deck, if needed.
- Structural Improvements. Detail any work required to improve the structural capacity of the bridge superstructure or substructure for pedestrian use or stabilization. Required work may include rehabilitating, strengthening, or replacing bridge members or the bridge deck, or rehabilitating or replacing the entire superstructure or substructure. Repairs will be minimal for bridges stabilized in situ as a monument, and limited to those that impact the structure’s ability to sustain its own weight.
- Corrosion Protection Measures. Outline any required work for corrosion protection, including paint or other corrosion protection measures.
- Traffic Regulation Measures. Discuss the need for bollards to restrict the access of vehicular traffic on pedestrian bridges. Discuss removing approach spans or providing fencing to deter pedestrian access on bridges stabilized as a monument.
- Presence of Hazardous Materials. Discuss the test results of any hazardous materials present on the structure. Any local entity owning or receiving a bridge with hazardous material and choosing to not abate the hazard, must sign a special agreement releasing TxDOT’s responsibility of the hazardous material prior to letting the project.

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Cost Estimate. Prepare separate, detailed cost estimates for the work required to rehabilitate the structure to carry pedestrian traffic or to stabilize the bridge as a monument, if feasible. State that the costs provided are based on TxDOT’s average unit costs pricing, therefore, only relevant if TxDOT performs the work. Federal participation is only covered up to the cost of demolition for this alternative. The owner of the bridge will be 100% responsible for all costs over the demolition estimate.

Replacement of the Existing Bridge. This alternative considers two options: rehabilitating the historic bridge for pedestrian use at an alternate location, and demolition. Either option requires that TxDOT make a reasonable effort to identify a new owner for the bridge, as described in Section 4.

The demolition option requires no further analysis. The analysis required for rehabilitating the bridge for pedestrian use at an alternate location is similar to that required for rehabilitating the bridge for pedestrian use in situ. Both options use the same loading, so they will have the same required repairs for the span that will be reused. Repairs to approach spans and substructure that would be required under the in situ option are not required for the relocation option. However, additional costs are incurred for the relocation option in order to move the bridge to its new location. Costs for the new substructure are typically paid by the bridge owner unless the demolition estimate can cover these expenses along with the cost to relocate the structure. Typically, the estimated demolition cost only covers the cost to relocate the bridge. The following items should be discussed as part of the analysis for restoring the bridge as a pedestrian structure at a new location:

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Moving. Discuss the structural feasibility, site constraints, and travel constraints associated with moving the bridge. Bridge type and size may prevent relocation.

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Bridge Rail. Describe the pedestrian rail type. Pedestrian rails should be chosen from standards available on the Bridge Division website.

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Structural Adequacy. Discuss the as-built and current load rating capacities, and whether or not these capacities meet federal and state legal requirements for the bridge to carry pedestrian traffic. Discuss any damage to the superstructure.

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Required Rehabilitation Work. Discuss the work required to rehabilitate the historic bridge to carry pedestrian traffic and to stabilize the bridge as a monument. Required work could include:
- Moving. Discuss the requirements for lifting and moving the bridge.
- Bridge Rail. Describe bridge rail rehabilitation, if desired.
- Pedestrian Use Improvements. Describe the pedestrian rail type and placement. Pedestrian rails can be chosen from standards available on the Bridge Division website. Discuss required ADA improvements to the bridge deck, if needed.
- Structural Improvements. Discuss any work required to improve the structural capacity of the bridge superstructure for pedestrian use. Required work may include rehabilitating, strengthening, or replacing bridge members or the bridge deck.
- Corrosion Protection Measures. Outline any required work for corrosion protection, including paint or other corrosion protection measures.
- Presence of Hazardous Materials. Discuss the test results of any hazardous materials present on the structure. Any local entity owning or receiving a bridge with hazardous material and choosing to not abate the hazard, must sign a special agreement releasing TxDOT’s responsibility of the hazardous material prior to letting the project.
- Traffic Regulation Measures. Discuss the need for bollards to restrict the access of vehicular traffic on pedestrian bridges.

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Cost Estimate. Prepare a cost estimate for the work required to rehabilitate the structure to carry pedestrian traffic, if feasible. State that the costs provided are based on TxDOT’s average unit costs pricing, therefore, only relevant if TxDOT performs the work. Federal participation is only covered up to the cost of demolition for this alternative. The owner of the bridge will be 100% responsible for all costs over the demolition estimate.